Is a quasi-3D dosimeter better than a 2D dosimeter for Tomotherapy delivery quality assurance? (original) (raw)

A Quality Assurance Method that Utilizes 3D Dosimetry and Facilitates Clinical Interpretation

International Journal of Radiation Oncology*Biology*Physics, 2012

A new 3D quality-assurance (QA) method is presented that facilitates evaluation of the clinical significance of QA data. Two advantages accrue: treatment verification is comprehensive throughout the whole treated volume; and the clinical significance of deviations can be assessed through the generation of doseevolume histogram curves and dose overlays on the patient's anatomy. Both steps represent developments that advance the clinical relevance of complex treatment QA. Purpose: To demonstrate a new three-dimensional (3D) quality assurance (QA) method that provides comprehensive dosimetry verification and facilitates evaluation of the clinical significance of QA data acquired in a phantom. Also to apply the method to investigate the dosimetric efficacy of base-of-skull (BOS) intensity-modulated radiotherapy (IMRT) treatment. Methods and Materials: Two types of IMRT QA verification plans were created for 6 patients who received BOS IMRT. The first plan enabled conventional 2D planar IMRT QA using the Varian portal dosimetry system. The second plan enabled 3D verification using an anthropomorphic head phantom. In the latter, the 3D dose distribution was measured using the DLOS/ Presage dosimetry system (DLOS Z Duke Large-field-of-view Optical-CT System, Presage Heuris Pharma, Skillman, NJ), which yielded isotropic 2-mm data throughout the treated volume. In a novel step, measured 3D dose distributions were transformed back to the patient's CT to enable calculation of doseevolume histograms (DVH) and dose overlays. Measured and planned patient DVHs were compared to investigate clinical significance. Results: Close agreement between measured and calculated dose distributions was observed for all 6 cases. For gamma criteria of 3%, 2 mm, the mean passing rate for portal dosimetry was 96.8% (range, 92.0%e98.9%), compared to 94.9% (range, 90.1%e98.9%) for 3D. There was no clear correlation between 2D and 3D passing rates. Planned and measured dose distributions were evaluated on the patient's anatomy, using DVH and dose overlays. Minor deviations were detected, and the clinical significance of these are presented and discussed. Conclusions: Two advantages accrue to the methods presented here. First, treatment accuracy is evaluated throughout the whole treated volume, yielding comprehensive verification. Second, the clinical significance of any deviations can be assessed through the generation of DVH curves and dose overlays on the patient's anatomy. The latter step represents an important development that advances the clinical relevance of complex treatment QA.

Clinical Implementation of a 3D Dosimeter for Accurate IMRT and VMAT Patient Specific QA

Open Journal of Biophysics, 2013

The Delta4 3D dose verification device was commissioned in the current work for pre-treatment quality assurance (QA) of Intensity Modulated Radiotherapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT) plans. The cross calibration and relative array calibration were performed to enable absolute dose comparison. The linearity of response with dose and temperature sensitivity tests were also conducted to investigate dosimetric properties of the Delta4 device. The need to modify the original CT image of the Delta4 phantom for accurate dose calculation and comparison is addressed in this work, applying a CT extension algorithm. A number of test plans varying from simple 4-field conformal to IMRT and VMAT plans were measured to evaluate the accuracy of this device. It was found that the Delta4 device measured dose accurately to within ±1%. In order to maintain this level of accuracy the machine output fluctuations need to be corrected prior to each measurement and the relative array calibration needs to be performed every six months.

Quantitative characterization of tomotherapy MVCT dosimetry

Medical Dosimetry, 2013

Megavoltage computed tomography (MVCT) is used as image guidance for patient setup in almost every tomotherapy treatment. Frequent use of ionizing radiation for image guidance has raised concern of imaging dose. The purpose of this work is to quantify and characterize tomotherapy MVCT dosimetry. Our dose calculation was based on a commissioned dose engine, and the calculation result was compared with film measurement. We studied dose profiles, center dose, maximal dose, surface dose, and mean dose on homogeneous cylindrical water phantoms of various diameters for various scanning parameters, including 3 different jaw openings (of nominal value J4, J1, and J0.1) and couch speeds (fine, normal, and coarse). The comparison between calculation and film measurement showed good agreement. In particular, the thread pattern on the film of the helical delivery matched very well with calculation. For the J1 jaw and coarse imaging mode, the maximum difference between calculation and measurement was about 6% of the center dose. Calculation on various sizes of synthesized phantoms showed that the center dose decreases almost linearly as the phantom diameter increases, and that the fine mode (couch speed of 4 mm/rotation) received twice the dose of the normal mode (couch speed of 8 mm/rotation) and 3 times that of the coarse mode (couch speed of 12 mm/rotation) as expected. The maximal dose ranged from 100% to $ 200% of the center dose, with increasing ratios for larger phantoms, smaller jaws, and faster couch speed. For all jaw settings and couch speeds, the mean dose and average surface dose vary from 95% to 125% of the center dose with increasing ratios for larger phantoms. We present a quantitative dosimetric characterization of the tomotherapy MVCT in terms of scanning parameters, phantom size, center dose, maximal dose, surface dose, and mean dose. The results can provide an overall picture of dose distribution and a reference data set that enables estimation of CT dose index for the tomotherapy MVCT.

3D Dose Verification Using Tomotherapy CT Detector Array

Fuel and Energy Abstracts

and y TomoTherapy, Inc., Madison, WI Purpose: To evaluate a three-dimensional dose verification method based on the exit dose using the onboard detector of tomotherapy. Methods and Materials: The study included 347 treatment fractions from 24 patients, including 10 prostate, 5 head and neck (HN), and 9 spinal stereotactic body radiation therapy (SBRT) cases. Detector sonograms were retrieved and back-projected to calculate entrance fluence, which was then forward-projected on the CT images to calculate the verification dose, which was compared with ion chamber and film measurement in the QA plans and with the planning dose in patient plans. Results: Root mean square (RMS) errors of 2.0%, 2.2%, and 2.0% were observed comparing the dose verification (DV) and the ion chamber measured point dose in the phantom plans for HN, prostate, and spinal SBRT patients, respectively. When cumulative dose in the entire treatment is considered, for HN patients, the error of the mean dose to the planning target volume (PTV) varied from 1.47% to 5.62% with a RMS error of 3.55%. For prostate patients, the error of the mean dose to the prostate target volume varied from -5.11% to 3.29%, with a RMS error of 2.49%. The RMS error of maximum doses to the bladder and the rectum were 2.34% (-4.17% to 2.61%) and 2.64% (-4.54% to 3.94%), respectively. For the nine spinal SBRT patients, the RMS error of the minimum dose to the PTV was 2.43% (-5.39% to 2.48%). The RMS error of maximum dose to the spinal cord was 1.05% (-2.86% to 0.89%). Conclusions: An excellent agreement was observed between the measurement and the verification dose. In the patient treatments, the agreement in doses to the majority of PTVs and organs at risk is within 5% for the cumulative treatment course doses. The dosimetric error strongly depends on the error in multileaf collimator leaf opening time with a sensitivity correlating to the gantry rotation period. Ó 2011 Elsevier Inc.

Clinical applications of 3-D dosimeters

Journal of Physics: Conference Series, 2015

Both 3-D gels and radiochromic plastic dosimeters, in conjunction with dose image readout systems (MRI or optical-CT), have been employed to measure 3-D dose distributions in many clinical applications. The 3-D dose maps obtained from these systems can provide a useful tool for clinical dose verification for complex treatment techniques such as IMRT, SRS/SBRT, brachytherapy, and proton beam therapy. These complex treatments present high dose gradient regions in the boundaries between the target and surrounding critical organs. Dose accuracy in these areas can be critical, and may affect treatment outcome. In this review, applications of 3-D gels and PRESAGE dosimeter are reviewed and evaluated in terms of their performance in providing information on clinical dose verification as well as commissioning of various treatment modalities. Future interests and clinical needs on studies of 3-D dosimetry are also discussed.

Quality assurance in 3D dosimetry by optical-CT

Journal of Physics: Conference Series, 2006

A promising new system for 3D dosimetry combines a radiochromic plastic dosimeter, PRESAGE [1-3], with an optical-computed-tomography system (optical-CT) capable of reading the dose recorded in the dosimeter [4,5]. Optical-CT is also the method of choice for reading out the dose recorded in the more established polymer gel dosimetry system, for many applications [6-9]. Achieving accurate dosimetry depends critically on the performance characteristics of the optical-CT imaging system. Several systems have been developed [6,10-12], and two are commercially available at the present time (MGS Research Inc, and Modus Medical Devices Inc). Common issues of quality assurance (QA) become significant for all these systems to ensure correct initial commissioning of optical-CT 3D dosimetry and correct continued functioning. Here we present a QA phantom and procedure designed for efficient evaluation of the basic imaging performance of any optical-CT scanning system. Example results are presented from two optical-CT systems, an inhouse CCD based system and the OCTOPUS™ system from MGS Research.

Making and assessing 3D dosimeters

Journal of Physics: Conference Series

Several 3D dosimeters are commercially available. However, there are many circumstances that require a customized 3D dosimeter. Examples include feasibility tests of nonstandard treatment modalities, inhomogeneous tissue configurations, unique shapes and sizes and teaching. In this session, general approaches for preparing radiochromic dosimeters, Fricke and polymer gel dosimeters, micelle gel and silicone dosimeters were presented. Advise will be given to developers of new 3D dosimeters. For optical readout, light absorption and scatter can limit the practical size of dosimeters. Specifically, increasing from 5 to 15 cm diameter dosimeters is optically challenging. Strategies to maximize initial optical transmission were presented. For MRI readout, the dose resolution is determined by both the dosimeter sensitivity and the pulse sequence parameters and the accuracy is determined by the sensitivity of the dosimeter to temperature and dose rate, next to imaging performance. For X-ray CT imaging, the dose resolution is determined by the sensitivity of the dosimeter which largely depends on the polymer density that can be achieved. The importance of characterizing the dosimeter in terms of dose sensitivity and stability, spatial integrity, dose rate and fractionation dependence, oxygen and ambient light sensitivity, temperature sensitivity and thermal history were emphasized. The dosimeter requirements also dictate the types of vessels and scanners appropriate for readout. For example, the preferred dosimeter formulation may include a compound that is incompatible with the preferred vessel.

Development of a Dose Calculation Model as a Supplemental Quality Assurance Tool for TomoTherapy

2009

Helical TomoTherapy is a state-of-the-art delivery technique used in photon radiotherapy. This system relies on the superposition of many small fields to precisely administer the prescribed dose to the patient. This thesis presents a new model that predicts dose distributions delivered by TomoTherapy. The model may be used within a dose verification tool for quality assurance purposes. It is based on so-called "energy deposition kernels" and can accurately predict dose distributions in a homogeneous medium for a broad range of field sizes, down to 2 mm in diameter. The model takes into account the two main effects that influence the dose distribution in small fields: (i) the spatial extension of the radiation source and (ii) the loss of charged particle equilibrium (CPE) within the field. The shape of the source is determined by a combination of a "slit-method" reconstruction and a collimator factor fitting procedure, whereas the loss of CPE is taken into account...

115. Feasibility study for In-Vivo Dosimetry procedure in routine clinical practice

Physica Medica, 2018

The aim of the in vivo dosimetry, during the fractionated radiation therapy, is the verification of the correct dose delivery to patient. Nowadays, in vivo dosimetry procedures for photon beams are based on the use of the electronic portal imaging device and dedicated software to elaborate electronic portal imaging device images. Methods: In total, 8474 in vivo dosimetry tests were carried out for 386 patients treated with 3-dimensional conformal radiotherapy, intensity-modulated radiotherapy, and volumetric modulated arc therapy techniques, using the SOFTDISO. SOFTDISO is a dedicated software that uses electronic portal imaging device images in order to (1) calculate the R index, that is, the ratio between daily reconstructed dose and the planned one at isocenter and (2) perform a g-like analysis between the signals, S, of a reference electronic portal imaging device image and that obtained in a daily fraction. It supplies 2 indexes, the percentage g% of points with g < 1 and the mean g value, g mean. In g-like analysis, the pass criteria for the signals agreement DS% and distance to agreement Dd have been selected based on the clinical experience and technology used. The adopted tolerance levels for the 3 indexes were fixed in 0.95 R 1.05, g% ! 90%, and g mean 0.5. Results: The results of R ratio, g-like, and a visual inspection of these data reported on a monitor screen permitted to individuate 2 classes of errors (1) class 1 that included errors due to inadequate standard quality controls and (2) class 2, due to patient morphological changes. Depending on the technique and anatomical site, a maximum of 18% of tests had at least 1 index out of tolerance; once removed the causes of class-1 errors, almost all patients (except patients with 4 lung and 2 breast cancer treated with 3-dimensional conformal radiotherapy) presented mean indexes values (R, g%, and g mean) within tolerance at the end of treatment course. Class-2 errors were found in some patients. Conclusions: The in vivo dosimetry procedure with SOFTDISO resulted easily implementable, able to individuate errors with a limited workload.

A comprehensive evaluation of the PRESAGE/optical-CT 3D dosimetry system

Medical Physics, 2009

This work presents extensive investigations to evaluate the robustness ͑intradosimeter consistency and temporal stability of response͒, reproducibility, precision, and accuracy of a relatively new 3D dosimetry system comprising a leuco-dye doped plastic 3D dosimeter ͑PRESAGE͒ and a commercial optical-CT scanner ͑OCTOPUS 5ϫ scanner from MGS Research, Inc͒. Four identical PRESAGE 3D dosimeters were created such that they were compatible with the Radiologic Physics Center ͑RPC͒ head-and-neck ͑H&N͒ IMRT credentialing phantom. Each dosimeter was irradiated with a rotationally symmetric arrangement of nine identical small fields ͑1 ϫ 3 cm 2 ͒ impinging on the flat circular face of the dosimeter. A repetitious sequence of three dose levels ͑4, 2.88, and 1.28 Gy͒ was delivered. The rotationally symmetric treatment resulted in a dose distribution with high spatial variation in axial planes but only gradual variation with depth along the long axis of the dosimeter. The significance of this treatment was that it facilitated accurate film dosimetry in the axial plane, for independent verification. Also, it enabled rigorous evaluation of robustness, reproducibility and accuracy of response, at the three dose levels. The OCTOPUS 5ϫ commercial scanner was used for dose readout from the dosimeters at daily time intervals. The use of improved optics and acquisition technique yielded substantially improved noise characteristics ͑reduced to ϳ2%͒ than has been achieved previously. Intradosimeter uniformity of radiochromic response was evaluated by calculating a 3D gamma comparison between each dosimeter and axially rotated copies of the same dosimeter. This convenient technique exploits the rotational symmetry of the distribution. All points in the gamma comparison passed a 2% difference, 1 mm distance-toagreement criteria indicating excellent intradosimeter uniformity even at low dose levels. Postirradiation, the dosimeters were all found to exhibit a slight increase in opaqueness with time. However, the relative dose distribution was found to be extremely stable up to 90 h postirradiation indicating excellent temporal stability. Excellent interdosimeter reproducibility was also observed between the four dosimeters. Gamma comparison maps between each dosimeter and the average distribution of all four dosimeters showed full agreement at the 2% difference, 2 mm distance-toagreement level. Dose readout from the 3D dosimetry system was found to agree better with independent film measurement than with treatment planning system calculations in penumbral regions and was generally accurate to within 2% dose difference and 2 mm distance-to-agreement. In conclusion, these studies demonstrate excellent precision, accuracy, robustness, and reproducibility of the PRESAGE/optical-CT system for relative 3D dosimetry and support its potential integration with the RPC H&N credentialing phantom for IMRT verification.