Matthew Podgorsak - Academia.edu (original) (raw)

Papers by Matthew Podgorsak

Research paper thumbnail of Brachytherapy Seed Localization Using Geometric and Linear Programming Techniques

Ieee Transactions on Medical Imaging, Sep 1, 2007

We propose an optimization algorithm to solve the brachytherapy seed localization problem in pros... more We propose an optimization algorithm to solve the brachytherapy seed localization problem in prostate brachytherapy. Our algorithm is based on novel geometric approaches to exploit the special structure of the problem and relies on a number of key observations which help us formulate the optimization problem as a minimization integer program (IP). Our IP model precisely defines the feasibility polyhedron for this problem using a polynomial number of half-spaces; the solution to its corresponding linear program is rounded to yield an integral solution to our task of determining correspondences between seeds in multiple projection images. The algorithm is efficient in theory as well as in practice and performs well on simulation data ( 98% accuracy) and real X-ray images ( 95% accuracy).

Research paper thumbnail of Breast-conserving therapy (BCT): radiating the entire breast may not be necessary

Research paper thumbnail of Performance evaluation of a plastic scintillation detector for the measurement of electron dose distributions near an inhomogeneity

International Journal of Radiation Oncologybiologyphysics, 1992

Research paper thumbnail of The MapCHECK Measurement Uncertainty function and its effect on planar dose pass rates

Journal of applied clinical medical physics / American College of Medical Physics, 2016

Our study aimed to quantify the effect of the Measurement Uncertainty function on planar dosimetr... more Our study aimed to quantify the effect of the Measurement Uncertainty function on planar dosimetry pass rates, as measured and analyzed with the Sun Nuclear Corporation MapCHECK 2 array and its associated software. This optional func-tion is toggled in the program preferences of the software (though turned on by default upon installation), and automatically increases the dose difference toler-ance defined by the user for each planar dose comparison. Dose planes from 109 static-gantry IMRT fields and 40 VMAT arcs, of varying modulation complexity, were measured at 5 cm water-equivalent depth in the MapCHECK 2 diode array, and respective calculated dose planes were exported from a commercial treatment planning system. Planar dose comparison pass rates were calculated within the Sun Nuclear Corporation analytic software using a number of calculation parameters, including Measurement Uncertainty on and off. By varying the percent difference (%Diff) criterion for similar analyses perform...

Research paper thumbnail of VMAT for the treatment of gynecologic malignancies for patients unable to receive HDR brachytherapy

Journal of Applied Clinical Medical Physics, 2014

This investigation studies the use of volumetric-modulated arc therapy (VMAT) to deliver the foll... more This investigation studies the use of volumetric-modulated arc therapy (VMAT) to deliver the following conceptual gynecological brachytherapy (BT) dose distributions: Type 1, traditional pear-shaped dose distribution with substantial dose gradients; Type 2, homogeneous dose distribution throughout PTV (BT prescription volume); and Type 3, increased dose to PTV without organ-at-risk (OAR) overdose. A tandem and ovoid BT treatment plan, with the prescription dose of 6 Gy to point A, was exported into the VMAT treatment planning system (TPS) and became the baseline for comparative analysis. The 200%, 150%, 130%, 100%, 75%, and 50% dose volumes were converted into structures for optimization and evaluation purposes. The 100% dose volume was chosen to be the PTV. Five VMAT plans (Type 1) were created to duplicate the Ir-192 tandem and ovoid inhomogeneous dose distribution. Another five VMAT plans (Type 2) were generated to deliver a homogeneous dose of 6 Gy to the PTV. An additional five VMAT plans (Type 3) were created to increase the dose to the PTV with a homogeneous dose distribution. In the first set of plans, the dose given to 99% of the 200%-100% dose volumes agreed within 2% of the BT plan on average. Additionally, it was found that the 75% dose volumes agreed within 5% of the BT plan and the 50% dose volumes agreed within 6.4% of the BT plan. In the second set of comparative analyses, the 100% dose volume was found to be within 1% of the original plan. Furthermore, the maximum increase of dose to the PTV in the last set of comparative analyses was 8 Gy with similar doses to OARs as the other VMAT plans. The maximum increase of dose was 2.50 Gy to the rectum and the maximum decrease of dose was 0.70 Gy to the bladder. Henceforth, VMAT was successful at reproducing brachytherapy dose distributions demonstrating that alternative dose distributions have the potential to be used in lieu of brachytherapy. It should also be noted that differences in radiobiology need to be further investigated.

Research paper thumbnail of Quality assurance for variable-length catheters with an afterloading brachytherapy device

Research paper thumbnail of Validity of stereotactic frame localization during radiosurgery after one fixation pin removal

Journal of Neurosurgery, 2002

This study was designed to examine the effect on target localization of removing one fixation pin... more This study was designed to examine the effect on target localization of removing one fixation pin or post. A stereotactic frame was applied to a head phantom by using four fixation pins. Contiguous axial computerized tomography (CT) slices (1 mm thick) were obtained through the head phantom. Using clinical treatment planning software, a marker was identified and its coordinates were determined. The imaging procedure and point localization were repeated independently seven times in the control configuration, after four-pin fixation, to study reproducibility. Standard deviations in marker coordinates were 0.013, 0.046, and 0.039 mm along the x, y, and z axes, respectively, indicating excellent reproducibility. Each of the four pins was then removed separately, leaving three pins providing fixation to the skull. Imaging was repeated for each three-pin configuration. To simulate the forces at each pin-skull interface, a lever arm was connected to the head phantom allowing application of variable torque to the system. The CT scans were obtained for each torque strength and pin removal combination. Marker coordinates were compared with the control. In most cases, it was found that accurate target positioning could be achieved after removal of a single pin and/or post. When high torque was used, however, removal of a pin resulted in up to a 1.2-mm error. The findings may be significant for clinical practice, depending on the condition being treated.

Research paper thumbnail of TBI lung dose comparisons using bilateral and anteroposterior delivery techniques and tissue density corrections

Journal of applied clinical medical physics / American College of Medical Physics, 2015

This study compares lung dose distributions for two common techniques of total body photon irradi... more This study compares lung dose distributions for two common techniques of total body photon irradiation (TBI) at extended source-to-surface distance calculated with, and without, tissue density correction (TDC). Lung dose correction factors as a function of lateral thorax separation are approximated for bilateral opposed TBI (supine), similar to those published for anteroposterior-posteroanterior (AP-PA) techniques in AAPM Report 17 (i.e., Task Group 29). 3D treatment plans were created retrospectively for 24 patients treated with bilateral TBI, and for whom CT data had been acquired from the head to the lower leg. These plans included bilateral opposed and AP-PA techniques- each with and without - TDC, using source-to-axis distance of 377 cm and largest possible field size. On average, bilateral TBI requires 40% more monitor units than AP-PA TBI due to increased separation (26% more for 23 MV). Calculation of midline thorax dose without TDC leads to dose underestimation of 17% on av...

Research paper thumbnail of Duplicating a tandem and ovoids distribution with intensity-modulated radiotherapy: a feasibility study

Journal of applied clinical medical physics / American College of Medical Physics, 2007

Brachytherapy plays an important role in the definitive treatment of cervical cancers by radiothe... more Brachytherapy plays an important role in the definitive treatment of cervical cancers by radiotherapy. In the present study, we investigated whether sliding-window intensity-modulated radiation therapy (IMRT) can achieve a pear-shaped distribution with a similar sharp dose falloff identical to that of brachytherapy. The computed tomography scans of a tandem and ovoid patient were pushed to both a high dose rate (HDR) and an IMRT treatment planning system (TPS) after the rectum, bladder, and left and right femoral heads had been outlined, ensuring identical structures in both planning systems. A conventional plan (7 Gy in 5 fractions, defined as the average dose to the left and right point A) was generated for HDR treatment. The 150%, 125%, 100%, 75%, 50%, and 25% isodose curves were drawn on each slice and then transferred to the IMRT TPS. The 100% isodose envelope from the HDR plan was the target for IMRT planning. A 7-field IMRT plan using 6-MV X-ray beams was generated and compar...

Research paper thumbnail of Validity of stereotactic frame localization during radiosurgery after one fixation pin removal

Journal of neurosurgery, 2002

This study was designed to examine the effect on target localization of removing one fixation pin... more This study was designed to examine the effect on target localization of removing one fixation pin or post. A stereotactic frame was applied to a head phantom by using four fixation pins. Contiguous axial computerized tomography (CT) slices (1 mm thick) were obtained through the head phantom. Using clinical treatment planning software, a marker was identified and its coordinates were determined. The imaging procedure and point localization were repeated independently seven times in the control configuration, after four-pin fixation, to study reproducibility. Standard deviations in marker coordinates were 0.013, 0.046, and 0.039 mm along the x, y, and z axes, respectively, indicating excellent reproducibility. Each of the four pins was then removed separately, leaving three pins providing fixation to the skull. Imaging was repeated for each three-pin configuration. To simulate the forces at each pin-skull interface, a lever arm was connected to the head phantom allowing application of...

Research paper thumbnail of A fully electronic intensity-modulated radiation therapy quality assurance (IMRT QA) process implemented in a network comprised of independent treatment planning, record and verify, and delivery systems

Radiology and Oncology, 2010

Disclosure: No potential conflicts of interest were disclosed.

Research paper thumbnail of EPID dosimetry for pretreatment quality assurance with two commercial systems

Journal of applied clinical medical physics / American College of Medical Physics, 2012

This study compares the EPID dosimetry algorithms of two commercial systems for pretreatment QA, ... more This study compares the EPID dosimetry algorithms of two commercial systems for pretreatment QA, and analyzes dosimetric measurements made with each system alongside the results obtained with a standard diode array. 126 IMRT fields are examined with both EPID dosimetry systems (EPIDose by Sun Nuclear Corporation, Melbourne FL, and Portal Dosimetry by Varian Medical Systems, Palo Alto CA) and the diode array, MapCHECK (also by Sun Nuclear Corporation). Twenty-six VMAT arcs of varying modulation complexity are examined with the EPIDose and MapCHECK systems. Optimization and commissioning testing of the EPIDose physics model is detailed. Each EPID IMRT QA system is tested for sensitivity to critical TPS beam model errors. Absolute dose gamma evaluation (3%, 3 mm, 10% threshold, global normalization to the maximum measured dose) yields similar results (within 1%-2%) for all three dosimetry modalities, except in the case of off-axis breast tangents. For these off-axis fields, the Portal ...

Research paper thumbnail of Target and peripheral dose during patient repositioning with the Gamma Knife automatic positioning system (APS) device

Journal of applied clinical medical physics / American College of Medical Physics, 2010

The GammaPlan treatment planning system does not account for the leakage and scatter dose during ... more The GammaPlan treatment planning system does not account for the leakage and scatter dose during APS repositioning. In this study, the dose delivered to the target site and its periphery from the defocus stage and intershot couch transit (couch motion from the focus to defocus position and back) associated with APS repositioning are measured for the Gamma Knife model 4C. A stereotactic head-frame was attached to a Leksell 16 cm diameter spherical phantom with a calibrated ion chamber at its center. Using a fiducial box, CT images of the phantom were acquired and registered in the GammaPlan treatment planning system to determine the coordinates of the target (center of the phantom). An absorbed dose of 10 Gy to the 50% isodose line was prescribed to the target site for all measurements. Plans were generated for the 8, 14 and 18 mm collimator helmets to determine the relationship of measured dose to the number of repositions of the APS system and to the helmet size. The target coordin...

Research paper thumbnail of Technical and Dosimetric Considerations in IMRT Treatment Planning for Large Target Volumes

Journal of Applied Clinical Medical Physics, 2005

The maximum width of an intensity-modulated radiotherapy (IMRT) treatment field is usually smalle... more The maximum width of an intensity-modulated radiotherapy (IMRT) treatment field is usually smaller than the conventional maximum collimator opening because of design limitations inherent in some multileaf collimators (MLCs). To increase the effective field width, IMRT fluences can be split and delivered with multiple carriage positions. However, not all treatment-planning systems and MLCs support this technique, and if they do, the maximum field width in multiple carriage position delivery is still significantly less than the maximum collimator opening. For target volumes with dimensions exceeding the field size limit for multiple carriage position delivery, such as liver tumors or other malignancies in the abdominal cavity, IMRT treatment can be accomplished with multiple isocenters or with an extended treatment distance. To study dosimetric statistics of large field IMRT planning, an elliptical volume was chosen as a target within a cubic phantom centered at a depth of 7.5 cm. Multiple three-field plans (one AP and two oblique beams with 160° between them to avoid parallel opposed geometry) with constraints designed to give 100% dose to the elliptical target were developed. Plans were designed with a single anterior field with dual carriage positions, or with the anterior field split into two fields with separate isocenters 8 cm apart with the beams being forcibly matched at the isocenter or with a 1 cm, 2 cm, 3 cm, and 4 cm overlap. The oblique beams were planned with a single carriage position in all cases. All beams had a nominal energy of 6 MV. In the dual isocenter plans, jaws were manually fixed and dose constraints remained unaltered. Dosimetric statistics were studied for plans developed for treatment delivery using both dynamic leaf motion (sliding window) and multiple static segments (step and shoot) with the number of segments varying from 5 to 30. All plans were analyzed based on the dose homogeneity in the isocenter plane, 2 cm anterior and 2 cm posterior to it, along with their corresponding dose-volume histograms (DVHs). All the dual isocenter plans had slight underdosage anterior to the match point and slight overdosage posterior to it, while the dual carriage plan had a nice blending of the dose distribution without the accompanying hot or cold spots. Based on the dose statistics, it was noted that the dual isocenter plans can be clinically acceptable if they have at least a 3-cm overlap. In the case of step and shoot IMRT, the number of segments used in a dual carriage plan was found to affect the overall plan dosimetric indices.

Research paper thumbnail of Spatial variation of dosimetric leaf gap and its impact on dose delivery

Medical physics, 2014

During dose calculation, the Eclipse treatment planning system (TPS) retracts the multileaf colli... more During dose calculation, the Eclipse treatment planning system (TPS) retracts the multileaf collimator (MLC) leaf positions by half of the dosimetric leaf gap (DLG) value (measured at central axis) for all leaf positions in a dynamic MLC plan to accurately model the rounded leaf ends. The aim of this study is to map the variation of DLG along the travel path of each MLC leaf pair and quantify how this variation impacts delivered dose. 6 MV DLG values were measured for all MLC leaf pairs in increments of 1.0 cm (from the line intersecting the CAX and perpendicular to MLC motion) to 13.0 cm off axis distance at dmax. The measurements were performed on two Varian linear accelerators, both employing the Millennium 120-leaf MLCs. The measurements were performed at several locations in the beam with both a Sun Nuclear MapCHECK device and a PTW pinpoint ion chamber. The measured DLGs for the middle 40 MLC leaf pairs (each 0.5 cm width) at positions along a line through the CAX and perpendi...

Research paper thumbnail of SU-FF-T-170: Dose Delivery to Curved Structures in Intra-Operative High Dose Rate (IOHDR) Brachytherapy

Research paper thumbnail of Duplicating a tandem and ovoid distribution with IMRT: A feasibility study

Journal of Applied Clinical Medical Physics, 2007

Brachytherapy plays an important role in the definitive treatment of cervical cancers by radiothe... more Brachytherapy plays an important role in the definitive treatment of cervical cancers by radiotherapy. In the present study, we investigated whether slidingwindow intensity-modulated radiation therapy (IMRT) can achieve a pear-shaped distribution with a similar sharp dose falloff identical to that of brachytherapy. The computed tomography scans of a tandem and ovoid patient were pushed to both a high dose rate (HDR) and an IMRT treatment planning system (TPS) after the rectum, bladder, and left and right femoral heads had been outlined, ensuring identical structures in both planning systems. A conventional plan (7 Gy in 5 fractions, defined as the average dose to the left and right point A) was generated for HDR treatment. The 150%, 125%, 100%, 75%, 50%, and 25% isodose curves were drawn on each slice and then transferred to the IMRT TPS. The 100% isodose envelope from the HDR plan was the target for IMRT planning. A 7-field IMRT plan using 6-MV X-ray beams was generated and compared with the HDR plan using isodose conformity to the target and 125% volume, dosevolume histograms, and integral dose. The resulting isodose distribution demonstrated good agreement between the HDR and IMRT plans in the 100% and 125% isodose range. The dose falloff in the HDR plan was much steeper than that in the IMRT plan, but it also had a substantially higher maximum dose. Integral dose for the target, rectum, and bladder were found to be 6.69 J, 1.07 J, and 1.02 J in the HDR plan; the respective values for IMRT were 3.47 J, 1.79 J, and 1.34 J. Our preliminary results indicate that the HDR dose distribution can be replicated using a standard sliding-window IMRT dose delivery technique for points lying closer to the three-dimensional isodose envelope surrounding point A. Differences in radiobiology and patient positioning between the two techniques merit further consideration. PACS: 87.53.Jw

Research paper thumbnail of Variability of Marker-Based Rectal Dose Evaluation in HDR Cervical Brachytherapy

Medical Dosimetry, 2010

In film-based intracavitary brachytherapy for cervical cancer, position of the rectal markers may... more In film-based intracavitary brachytherapy for cervical cancer, position of the rectal markers may not accurately represent the anterior rectal wall. This study was aimed at analyzing the variability of rectal dose estimation as a result of interfractional variation of marker placement. A cohort of five patients treated with multiple-fraction tandem and ovoid high-dose-rate (HDR) brachytherapy was studied. The cervical os point and the orientation of the applicators were matched among all fractional plans for each patient. Rectal points obtained from all fractions were then input into each clinical treated plan. New fractional rectal doses were obtained and a new cumulative rectal dose for each patient was calculated. The maximum interfractional variation of distances between rectal dose points and the closest source positions was 1.1 cm. The corresponding maximum variability of fractional rectal dose was 65.5%. The percentage difference in cumulative rectal dose estimation for each patient was 5.4%, 19.6%, 34.6%, 23.4%, and 13.9%, respectively. In conclusion, care should be taken when using rectal markers as reference points for estimating rectal dose in HDR cervical brachytherapy. The best estimate of true rectal dose for each fraction should be determined by the most anterior point among all fractions.

Research paper thumbnail of Retrospective analysis of dose delivery in intra-operative high dose rate brachytherapy

Radiology and Oncology, 2007

Background. This study was performed to quantify the inaccuracy in clinical dose delivery due to ... more Background. This study was performed to quantify the inaccuracy in clinical dose delivery due to the incomplete scatter conditions inherent in intra-operative high dose rate (IOHDR) brachytherapy. Methods. Treatment plans of 10 patients previously treated in our facility, which had irregular shapes of treated areas, were used. Treatment geometries reflecting each clinical case were simulated using a phantom assembly with no added build-up on top of the applicator. The treatment planning geometry (full scatter surrounding the applicator) was subsequently simulated for each case by adding bolus on top of the applicator. Results. For geometries representing the clinical IOHDR incomplete scatter environment, measured doses at the 5 mm and 10 mm prescription depths were lower than the corresponding prescribed doses by about 7.7% and 11.1%, respectively. Also, for the two prescription methods, an analysis of the measured dose distributions and their corresponding treatment plans showed average decreases of 1.2 mm and 2.2 mm in depth of prescription dose, respectively. Conclusions. Dosimetric calculations with the assumption of an infinite scatter environment around the applicator and target volume have shown to result in dose delivery errors that significantly decrease the prescription depth for IOHDR treatment.

Research paper thumbnail of Practical issues in treating heavy patients on a linac treatment couch

Journal of Applied Clinical Medical Physics, 2005

Safe delivery of external beam radiation therapy to patients whose weight approaches the toleranc... more Safe delivery of external beam radiation therapy to patients whose weight approaches the tolerance of a treatment couch presents some challenges, particularly if the couch has been in use for several years and has seen significant wear and tear. An analysis of treatment couch design can identify locations that become stressed and could potentially fail when supporting a heavy patient, leading to serious injury. Some practical methods to decrease the likelihood of couch failure are presented in this work. The design and implementation of a lifting apparatus to support the treatment couch is also described.

Research paper thumbnail of Brachytherapy Seed Localization Using Geometric and Linear Programming Techniques

Ieee Transactions on Medical Imaging, Sep 1, 2007

We propose an optimization algorithm to solve the brachytherapy seed localization problem in pros... more We propose an optimization algorithm to solve the brachytherapy seed localization problem in prostate brachytherapy. Our algorithm is based on novel geometric approaches to exploit the special structure of the problem and relies on a number of key observations which help us formulate the optimization problem as a minimization integer program (IP). Our IP model precisely defines the feasibility polyhedron for this problem using a polynomial number of half-spaces; the solution to its corresponding linear program is rounded to yield an integral solution to our task of determining correspondences between seeds in multiple projection images. The algorithm is efficient in theory as well as in practice and performs well on simulation data ( 98% accuracy) and real X-ray images ( 95% accuracy).

Research paper thumbnail of Breast-conserving therapy (BCT): radiating the entire breast may not be necessary

Research paper thumbnail of Performance evaluation of a plastic scintillation detector for the measurement of electron dose distributions near an inhomogeneity

International Journal of Radiation Oncologybiologyphysics, 1992

Research paper thumbnail of The MapCHECK Measurement Uncertainty function and its effect on planar dose pass rates

Journal of applied clinical medical physics / American College of Medical Physics, 2016

Our study aimed to quantify the effect of the Measurement Uncertainty function on planar dosimetr... more Our study aimed to quantify the effect of the Measurement Uncertainty function on planar dosimetry pass rates, as measured and analyzed with the Sun Nuclear Corporation MapCHECK 2 array and its associated software. This optional func-tion is toggled in the program preferences of the software (though turned on by default upon installation), and automatically increases the dose difference toler-ance defined by the user for each planar dose comparison. Dose planes from 109 static-gantry IMRT fields and 40 VMAT arcs, of varying modulation complexity, were measured at 5 cm water-equivalent depth in the MapCHECK 2 diode array, and respective calculated dose planes were exported from a commercial treatment planning system. Planar dose comparison pass rates were calculated within the Sun Nuclear Corporation analytic software using a number of calculation parameters, including Measurement Uncertainty on and off. By varying the percent difference (%Diff) criterion for similar analyses perform...

Research paper thumbnail of VMAT for the treatment of gynecologic malignancies for patients unable to receive HDR brachytherapy

Journal of Applied Clinical Medical Physics, 2014

This investigation studies the use of volumetric-modulated arc therapy (VMAT) to deliver the foll... more This investigation studies the use of volumetric-modulated arc therapy (VMAT) to deliver the following conceptual gynecological brachytherapy (BT) dose distributions: Type 1, traditional pear-shaped dose distribution with substantial dose gradients; Type 2, homogeneous dose distribution throughout PTV (BT prescription volume); and Type 3, increased dose to PTV without organ-at-risk (OAR) overdose. A tandem and ovoid BT treatment plan, with the prescription dose of 6 Gy to point A, was exported into the VMAT treatment planning system (TPS) and became the baseline for comparative analysis. The 200%, 150%, 130%, 100%, 75%, and 50% dose volumes were converted into structures for optimization and evaluation purposes. The 100% dose volume was chosen to be the PTV. Five VMAT plans (Type 1) were created to duplicate the Ir-192 tandem and ovoid inhomogeneous dose distribution. Another five VMAT plans (Type 2) were generated to deliver a homogeneous dose of 6 Gy to the PTV. An additional five VMAT plans (Type 3) were created to increase the dose to the PTV with a homogeneous dose distribution. In the first set of plans, the dose given to 99% of the 200%-100% dose volumes agreed within 2% of the BT plan on average. Additionally, it was found that the 75% dose volumes agreed within 5% of the BT plan and the 50% dose volumes agreed within 6.4% of the BT plan. In the second set of comparative analyses, the 100% dose volume was found to be within 1% of the original plan. Furthermore, the maximum increase of dose to the PTV in the last set of comparative analyses was 8 Gy with similar doses to OARs as the other VMAT plans. The maximum increase of dose was 2.50 Gy to the rectum and the maximum decrease of dose was 0.70 Gy to the bladder. Henceforth, VMAT was successful at reproducing brachytherapy dose distributions demonstrating that alternative dose distributions have the potential to be used in lieu of brachytherapy. It should also be noted that differences in radiobiology need to be further investigated.

Research paper thumbnail of Quality assurance for variable-length catheters with an afterloading brachytherapy device

Research paper thumbnail of Validity of stereotactic frame localization during radiosurgery after one fixation pin removal

Journal of Neurosurgery, 2002

This study was designed to examine the effect on target localization of removing one fixation pin... more This study was designed to examine the effect on target localization of removing one fixation pin or post. A stereotactic frame was applied to a head phantom by using four fixation pins. Contiguous axial computerized tomography (CT) slices (1 mm thick) were obtained through the head phantom. Using clinical treatment planning software, a marker was identified and its coordinates were determined. The imaging procedure and point localization were repeated independently seven times in the control configuration, after four-pin fixation, to study reproducibility. Standard deviations in marker coordinates were 0.013, 0.046, and 0.039 mm along the x, y, and z axes, respectively, indicating excellent reproducibility. Each of the four pins was then removed separately, leaving three pins providing fixation to the skull. Imaging was repeated for each three-pin configuration. To simulate the forces at each pin-skull interface, a lever arm was connected to the head phantom allowing application of variable torque to the system. The CT scans were obtained for each torque strength and pin removal combination. Marker coordinates were compared with the control. In most cases, it was found that accurate target positioning could be achieved after removal of a single pin and/or post. When high torque was used, however, removal of a pin resulted in up to a 1.2-mm error. The findings may be significant for clinical practice, depending on the condition being treated.

Research paper thumbnail of TBI lung dose comparisons using bilateral and anteroposterior delivery techniques and tissue density corrections

Journal of applied clinical medical physics / American College of Medical Physics, 2015

This study compares lung dose distributions for two common techniques of total body photon irradi... more This study compares lung dose distributions for two common techniques of total body photon irradiation (TBI) at extended source-to-surface distance calculated with, and without, tissue density correction (TDC). Lung dose correction factors as a function of lateral thorax separation are approximated for bilateral opposed TBI (supine), similar to those published for anteroposterior-posteroanterior (AP-PA) techniques in AAPM Report 17 (i.e., Task Group 29). 3D treatment plans were created retrospectively for 24 patients treated with bilateral TBI, and for whom CT data had been acquired from the head to the lower leg. These plans included bilateral opposed and AP-PA techniques- each with and without - TDC, using source-to-axis distance of 377 cm and largest possible field size. On average, bilateral TBI requires 40% more monitor units than AP-PA TBI due to increased separation (26% more for 23 MV). Calculation of midline thorax dose without TDC leads to dose underestimation of 17% on av...

Research paper thumbnail of Duplicating a tandem and ovoids distribution with intensity-modulated radiotherapy: a feasibility study

Journal of applied clinical medical physics / American College of Medical Physics, 2007

Brachytherapy plays an important role in the definitive treatment of cervical cancers by radiothe... more Brachytherapy plays an important role in the definitive treatment of cervical cancers by radiotherapy. In the present study, we investigated whether sliding-window intensity-modulated radiation therapy (IMRT) can achieve a pear-shaped distribution with a similar sharp dose falloff identical to that of brachytherapy. The computed tomography scans of a tandem and ovoid patient were pushed to both a high dose rate (HDR) and an IMRT treatment planning system (TPS) after the rectum, bladder, and left and right femoral heads had been outlined, ensuring identical structures in both planning systems. A conventional plan (7 Gy in 5 fractions, defined as the average dose to the left and right point A) was generated for HDR treatment. The 150%, 125%, 100%, 75%, 50%, and 25% isodose curves were drawn on each slice and then transferred to the IMRT TPS. The 100% isodose envelope from the HDR plan was the target for IMRT planning. A 7-field IMRT plan using 6-MV X-ray beams was generated and compar...

Research paper thumbnail of Validity of stereotactic frame localization during radiosurgery after one fixation pin removal

Journal of neurosurgery, 2002

This study was designed to examine the effect on target localization of removing one fixation pin... more This study was designed to examine the effect on target localization of removing one fixation pin or post. A stereotactic frame was applied to a head phantom by using four fixation pins. Contiguous axial computerized tomography (CT) slices (1 mm thick) were obtained through the head phantom. Using clinical treatment planning software, a marker was identified and its coordinates were determined. The imaging procedure and point localization were repeated independently seven times in the control configuration, after four-pin fixation, to study reproducibility. Standard deviations in marker coordinates were 0.013, 0.046, and 0.039 mm along the x, y, and z axes, respectively, indicating excellent reproducibility. Each of the four pins was then removed separately, leaving three pins providing fixation to the skull. Imaging was repeated for each three-pin configuration. To simulate the forces at each pin-skull interface, a lever arm was connected to the head phantom allowing application of...

Research paper thumbnail of A fully electronic intensity-modulated radiation therapy quality assurance (IMRT QA) process implemented in a network comprised of independent treatment planning, record and verify, and delivery systems

Radiology and Oncology, 2010

Disclosure: No potential conflicts of interest were disclosed.

Research paper thumbnail of EPID dosimetry for pretreatment quality assurance with two commercial systems

Journal of applied clinical medical physics / American College of Medical Physics, 2012

This study compares the EPID dosimetry algorithms of two commercial systems for pretreatment QA, ... more This study compares the EPID dosimetry algorithms of two commercial systems for pretreatment QA, and analyzes dosimetric measurements made with each system alongside the results obtained with a standard diode array. 126 IMRT fields are examined with both EPID dosimetry systems (EPIDose by Sun Nuclear Corporation, Melbourne FL, and Portal Dosimetry by Varian Medical Systems, Palo Alto CA) and the diode array, MapCHECK (also by Sun Nuclear Corporation). Twenty-six VMAT arcs of varying modulation complexity are examined with the EPIDose and MapCHECK systems. Optimization and commissioning testing of the EPIDose physics model is detailed. Each EPID IMRT QA system is tested for sensitivity to critical TPS beam model errors. Absolute dose gamma evaluation (3%, 3 mm, 10% threshold, global normalization to the maximum measured dose) yields similar results (within 1%-2%) for all three dosimetry modalities, except in the case of off-axis breast tangents. For these off-axis fields, the Portal ...

Research paper thumbnail of Target and peripheral dose during patient repositioning with the Gamma Knife automatic positioning system (APS) device

Journal of applied clinical medical physics / American College of Medical Physics, 2010

The GammaPlan treatment planning system does not account for the leakage and scatter dose during ... more The GammaPlan treatment planning system does not account for the leakage and scatter dose during APS repositioning. In this study, the dose delivered to the target site and its periphery from the defocus stage and intershot couch transit (couch motion from the focus to defocus position and back) associated with APS repositioning are measured for the Gamma Knife model 4C. A stereotactic head-frame was attached to a Leksell 16 cm diameter spherical phantom with a calibrated ion chamber at its center. Using a fiducial box, CT images of the phantom were acquired and registered in the GammaPlan treatment planning system to determine the coordinates of the target (center of the phantom). An absorbed dose of 10 Gy to the 50% isodose line was prescribed to the target site for all measurements. Plans were generated for the 8, 14 and 18 mm collimator helmets to determine the relationship of measured dose to the number of repositions of the APS system and to the helmet size. The target coordin...

Research paper thumbnail of Technical and Dosimetric Considerations in IMRT Treatment Planning for Large Target Volumes

Journal of Applied Clinical Medical Physics, 2005

The maximum width of an intensity-modulated radiotherapy (IMRT) treatment field is usually smalle... more The maximum width of an intensity-modulated radiotherapy (IMRT) treatment field is usually smaller than the conventional maximum collimator opening because of design limitations inherent in some multileaf collimators (MLCs). To increase the effective field width, IMRT fluences can be split and delivered with multiple carriage positions. However, not all treatment-planning systems and MLCs support this technique, and if they do, the maximum field width in multiple carriage position delivery is still significantly less than the maximum collimator opening. For target volumes with dimensions exceeding the field size limit for multiple carriage position delivery, such as liver tumors or other malignancies in the abdominal cavity, IMRT treatment can be accomplished with multiple isocenters or with an extended treatment distance. To study dosimetric statistics of large field IMRT planning, an elliptical volume was chosen as a target within a cubic phantom centered at a depth of 7.5 cm. Multiple three-field plans (one AP and two oblique beams with 160° between them to avoid parallel opposed geometry) with constraints designed to give 100% dose to the elliptical target were developed. Plans were designed with a single anterior field with dual carriage positions, or with the anterior field split into two fields with separate isocenters 8 cm apart with the beams being forcibly matched at the isocenter or with a 1 cm, 2 cm, 3 cm, and 4 cm overlap. The oblique beams were planned with a single carriage position in all cases. All beams had a nominal energy of 6 MV. In the dual isocenter plans, jaws were manually fixed and dose constraints remained unaltered. Dosimetric statistics were studied for plans developed for treatment delivery using both dynamic leaf motion (sliding window) and multiple static segments (step and shoot) with the number of segments varying from 5 to 30. All plans were analyzed based on the dose homogeneity in the isocenter plane, 2 cm anterior and 2 cm posterior to it, along with their corresponding dose-volume histograms (DVHs). All the dual isocenter plans had slight underdosage anterior to the match point and slight overdosage posterior to it, while the dual carriage plan had a nice blending of the dose distribution without the accompanying hot or cold spots. Based on the dose statistics, it was noted that the dual isocenter plans can be clinically acceptable if they have at least a 3-cm overlap. In the case of step and shoot IMRT, the number of segments used in a dual carriage plan was found to affect the overall plan dosimetric indices.

Research paper thumbnail of Spatial variation of dosimetric leaf gap and its impact on dose delivery

Medical physics, 2014

During dose calculation, the Eclipse treatment planning system (TPS) retracts the multileaf colli... more During dose calculation, the Eclipse treatment planning system (TPS) retracts the multileaf collimator (MLC) leaf positions by half of the dosimetric leaf gap (DLG) value (measured at central axis) for all leaf positions in a dynamic MLC plan to accurately model the rounded leaf ends. The aim of this study is to map the variation of DLG along the travel path of each MLC leaf pair and quantify how this variation impacts delivered dose. 6 MV DLG values were measured for all MLC leaf pairs in increments of 1.0 cm (from the line intersecting the CAX and perpendicular to MLC motion) to 13.0 cm off axis distance at dmax. The measurements were performed on two Varian linear accelerators, both employing the Millennium 120-leaf MLCs. The measurements were performed at several locations in the beam with both a Sun Nuclear MapCHECK device and a PTW pinpoint ion chamber. The measured DLGs for the middle 40 MLC leaf pairs (each 0.5 cm width) at positions along a line through the CAX and perpendi...

Research paper thumbnail of SU-FF-T-170: Dose Delivery to Curved Structures in Intra-Operative High Dose Rate (IOHDR) Brachytherapy

Research paper thumbnail of Duplicating a tandem and ovoid distribution with IMRT: A feasibility study

Journal of Applied Clinical Medical Physics, 2007

Brachytherapy plays an important role in the definitive treatment of cervical cancers by radiothe... more Brachytherapy plays an important role in the definitive treatment of cervical cancers by radiotherapy. In the present study, we investigated whether slidingwindow intensity-modulated radiation therapy (IMRT) can achieve a pear-shaped distribution with a similar sharp dose falloff identical to that of brachytherapy. The computed tomography scans of a tandem and ovoid patient were pushed to both a high dose rate (HDR) and an IMRT treatment planning system (TPS) after the rectum, bladder, and left and right femoral heads had been outlined, ensuring identical structures in both planning systems. A conventional plan (7 Gy in 5 fractions, defined as the average dose to the left and right point A) was generated for HDR treatment. The 150%, 125%, 100%, 75%, 50%, and 25% isodose curves were drawn on each slice and then transferred to the IMRT TPS. The 100% isodose envelope from the HDR plan was the target for IMRT planning. A 7-field IMRT plan using 6-MV X-ray beams was generated and compared with the HDR plan using isodose conformity to the target and 125% volume, dosevolume histograms, and integral dose. The resulting isodose distribution demonstrated good agreement between the HDR and IMRT plans in the 100% and 125% isodose range. The dose falloff in the HDR plan was much steeper than that in the IMRT plan, but it also had a substantially higher maximum dose. Integral dose for the target, rectum, and bladder were found to be 6.69 J, 1.07 J, and 1.02 J in the HDR plan; the respective values for IMRT were 3.47 J, 1.79 J, and 1.34 J. Our preliminary results indicate that the HDR dose distribution can be replicated using a standard sliding-window IMRT dose delivery technique for points lying closer to the three-dimensional isodose envelope surrounding point A. Differences in radiobiology and patient positioning between the two techniques merit further consideration. PACS: 87.53.Jw

Research paper thumbnail of Variability of Marker-Based Rectal Dose Evaluation in HDR Cervical Brachytherapy

Medical Dosimetry, 2010

In film-based intracavitary brachytherapy for cervical cancer, position of the rectal markers may... more In film-based intracavitary brachytherapy for cervical cancer, position of the rectal markers may not accurately represent the anterior rectal wall. This study was aimed at analyzing the variability of rectal dose estimation as a result of interfractional variation of marker placement. A cohort of five patients treated with multiple-fraction tandem and ovoid high-dose-rate (HDR) brachytherapy was studied. The cervical os point and the orientation of the applicators were matched among all fractional plans for each patient. Rectal points obtained from all fractions were then input into each clinical treated plan. New fractional rectal doses were obtained and a new cumulative rectal dose for each patient was calculated. The maximum interfractional variation of distances between rectal dose points and the closest source positions was 1.1 cm. The corresponding maximum variability of fractional rectal dose was 65.5%. The percentage difference in cumulative rectal dose estimation for each patient was 5.4%, 19.6%, 34.6%, 23.4%, and 13.9%, respectively. In conclusion, care should be taken when using rectal markers as reference points for estimating rectal dose in HDR cervical brachytherapy. The best estimate of true rectal dose for each fraction should be determined by the most anterior point among all fractions.

Research paper thumbnail of Retrospective analysis of dose delivery in intra-operative high dose rate brachytherapy

Radiology and Oncology, 2007

Background. This study was performed to quantify the inaccuracy in clinical dose delivery due to ... more Background. This study was performed to quantify the inaccuracy in clinical dose delivery due to the incomplete scatter conditions inherent in intra-operative high dose rate (IOHDR) brachytherapy. Methods. Treatment plans of 10 patients previously treated in our facility, which had irregular shapes of treated areas, were used. Treatment geometries reflecting each clinical case were simulated using a phantom assembly with no added build-up on top of the applicator. The treatment planning geometry (full scatter surrounding the applicator) was subsequently simulated for each case by adding bolus on top of the applicator. Results. For geometries representing the clinical IOHDR incomplete scatter environment, measured doses at the 5 mm and 10 mm prescription depths were lower than the corresponding prescribed doses by about 7.7% and 11.1%, respectively. Also, for the two prescription methods, an analysis of the measured dose distributions and their corresponding treatment plans showed average decreases of 1.2 mm and 2.2 mm in depth of prescription dose, respectively. Conclusions. Dosimetric calculations with the assumption of an infinite scatter environment around the applicator and target volume have shown to result in dose delivery errors that significantly decrease the prescription depth for IOHDR treatment.

Research paper thumbnail of Practical issues in treating heavy patients on a linac treatment couch

Journal of Applied Clinical Medical Physics, 2005

Safe delivery of external beam radiation therapy to patients whose weight approaches the toleranc... more Safe delivery of external beam radiation therapy to patients whose weight approaches the tolerance of a treatment couch presents some challenges, particularly if the couch has been in use for several years and has seen significant wear and tear. An analysis of treatment couch design can identify locations that become stressed and could potentially fail when supporting a heavy patient, leading to serious injury. Some practical methods to decrease the likelihood of couch failure are presented in this work. The design and implementation of a lifting apparatus to support the treatment couch is also described.