OC-0349: Prediction of GTV median dose differences benefit Monte Carlo re-prescription in lung SBRT (original) (raw)
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Radiotherapy and Oncology, 2010
Purpose: To define a method of dose prescription employing Monte Carlo (MC) dose calculation in stereotactic body radiotherapy (SBRT) for lung tumours aiming at a dose as low as possible outside of the PTV. Methods and materials: Six typical T1 lung tumours -three small, three large -were constructed centrally, peripherally in the lung, and nearby the thoracic wall, respectively. For each of these, five treatment plans employing dynamic conformal arc technique were made in which the dose was prescribed to encompass the PTV with the prescription isodose level (PIL) set in a range between 50% and 80% of the isocenter dose. Three shells of respectively 10 mm thickness around the PTV were constructed to assess the dose in the tissues directly adjacent to the PTV. Results: The PTV was nicely covered (mean 98.8% ± 0.9%) with favourable conformity indices (mean 1.09 ± 0.1). Mean doses around the PTVs were 73% (±1.3%), 76% (±3.5%), and 85% (±5.1%) of the prescribed dose in shell 1 for PIL50%, PIL65%, and PIL80%, respectively; 40% (±2.6%), 44% (±5.1%), 54% (±9.3%) in shell 2; and 24% (±1.9%), 26% (±3.6%), 33% (±6.8%) in shell 3. All normal tissue doses including the integral dose were also consistently worst for PIL80%. Monitor units were 30% higher for PIL65%, and 70% higher for PIL50%, compared with PIL80%. Conclusions: To improve normal tissue sparing the dose should be prescribed at an isodose lower than 80% of the isocenter dose in SBRT when using conformal arc technique with MC dose calculation.
Calculation of the absorbed dose delivered to a patient during radiotherapy treatment is extremely important and has a direct impact on the treatment outcome. The calculation of the dose to tumour and normal tissues is particularly challenging for lung cancer treatments where large density variations can exist. Previous studies have compared different algorithms used for dose calculation in the treatment planning system (TPS). However, the impact of dose calculation accuracy on treatment outcomes prediction has not been widely studied, especially in regards to lung stereotactic body radiotherapy treatment (SBRT). This research aims to investigate the accuracy of the collapsed cone convolution algorithm employed in the Pinnacle 3 TPS for dose calculation of lung SBRT plans and the potential impact of any dose uncertainties on treatment outcomes prediction. For this purpose, a EGSnrc/BEAMnrc Monte Carlo model of an Elekta Axesse linear accelerator equipped with the Beam Modulator collimation system was developed and commissioned. The commissioned model was used to perform Monte Carlo simulations of the dose distribution of twenty early stage non-small cell lung cancer patient plans. The dosimetric parameters of the planning treatment volume (PTV) and organs at risk (OARs) were evaluated and compared with the TPS calculation. The effects of dose calculation uncertainties to the tumour control probability (TCP) and normal tissue complication probability (NTCP) were modelled using the Linear Quadratic Poisson TCP model and the Lyman-Kutcher-Burman NTCP model. The study found that no significant difference was observed in the PTV dose parameters between the TPS and Monte Carlo calculations. An agreement of ±6% was observed for the PTV coverage of the prescribed isodose, and even greater agreement of ±2% for the coverage of the 90% prescribed isodose. The TPS algorithm tended to overestimate the dose to OARs, with the exception of normal lung tissue, brachial plexus, and pericardium. A significant difference was mostly observed for the maximum point dose parameter. However, most dose parameters to OARs were still below the dose constraints outlined in the RTOG 1021 protocol for both the TPS and Monte Carlo plans. The only significant dose constraint violation was observed for the maximum point dose to the ribs, occurring in plans with a tumour located closest to the chest wall. The radiobiological analysis showed that the TCP parameters were more sensitive to dose calculation uncertainties than NTCP
Journal of medical imaging and radiation oncology, 2018
Introduction: Concerns were raised about the accuracy of pencil beam (PB) calculation and potential underdosing of medically inoperable non-small cell lung cancer (NSCLC) treated with stereotactic body radiation therapy (SBRT). From our institutional series, we designed a matched-pair study where each local failure and controlled patient was matched based upon several clinical factors, to investigate the dose difference between the matched-pair. Methods: Eighteen pairs of NSCLC patients, treated with 50 Gy in five fractions, were selected. These patients were matched based on treatment intent, tumour size, histology and clinical follow-up. All PB calculated clinical plans were retrospectively recalculated with a MC algorithm. The D 99 and D Mean of the gross tumour volume (GTV) and D 95 and D Mean of the planning tumour volume (PTV) from PB and Monte Carlo (MC) calculation were compared between local failures and controls using the Mann-Whitney test. Results: The mean PB calculated D95 of PTV was 50.4 Gy for both failures and controls (P = 0.85), indicating no planning differences between the groups. From MC calculations, the mean (AESD) of GTV D 99 , GTV D Mean , PTV D 95 , PTV D Mean were 47.6 AE 2.6/46.3 AE 2.4, 50.4 AE 2.1/49.8 AE 1.6, 44.4 AE 2.7/ 43.6 AE 3.1, 48.7 AE 2.4/48.2 AE 2.4 Gy for failure/controlled groups, respectively, and there was no significant difference between two groups (all P > 0.1). The dose differences between MC and PB calculations were in agreement with other literatures and there was no significant difference between two groups. Conclusions: While PB algorithms may overestimate tumour doses relative to MC algorithms, our matched-pair study did not find dose differences between local failure and local controlled cases.
International Journal of Radiation Oncology*Biology*Physics, 2007
Purpose: To formulate uncertainty-based stopping criteria for Monte Carlo (MC) calculations of intensity-modulated radiotherapy and intensity-modulated arc therapy patient dose distributions and evaluate their influence on MC simulation times and dose characteristics. Methods and Materials: For each structure of interest, stopping criteria were formulated as follows: s rel # s rel,tol or Ds rel # D lim s rel,tol within $95% of the voxels, where s rel represents the relative statistical uncertainty on the estimated dose, D. The tolerated uncertainty (s rel,tol ) was 2%. The dose limit (D lim ) equaled the planning target volume (PTV) prescription dose or a dose value related to the organ at risk (OAR) planning constraints. An intensity-modulated radiotherapy-lung, intensity-modulated radiotherapy-ethmoid sinus, and intensity-modulated arc therapy-rectum patient case were studied. The PTV-stopping criteria-based calculations were compared with the PTV+OAR-stopping criteria-based calculations. Results: The MC dose distributions complied with the PTV-stopping criteria after 14% (lung), 21% (ethmoid), and 12% (rectum) of the simulation times of a 100 million histories reference calculation, and increased to 29%, 44%, and 51%, respectively, by the addition of the OAR-stopping criteria. Dose-volume histograms corresponding to the PTV-stopping criteria, PTV+OAR-stopping criteria, and reference dose calculations were indiscernible. The median local dose differences between the PTV-stopping criteria and the reference calculations amounted to 1.4% (lung), 2.1% (ethmoid), and 2.5% (rectum). Conclusions: For the patient cases studied, the MC calculations using PTV-stopping criteria only allowed accurate treatment plan evaluation. The proposed stopping criteria provided a flexible tool to assist MC patient dose calculations. The structures of interest and appropriate values of s rel,tol and D lim should be selected for each patient individually according to the clinical treatment planning goals. Ó 2007 Elsevier Inc.
Journal of Nuclear Medicine, 2009
Dosimetric calculations are performed with an increasing frequency before or after treatment in targeted radionuclide therapy, as well as for radiation protection purposes in diagnostic nuclear medicine. According to the MIRD committee formalism, the mean absorbed dose to a target is given by the product of the cumulated activity and a dose-conversion factor, known as the S factor. Standard S factors have been published for mathematic phantoms and for unit-density spheres. The accuracy of the results from the use of these S factors is questionable, because patient morphology can vary significantly. The aim of this work was to investigate differences between patient-specific dosimetric results obtained using Monte Carlo methodology and results obtained using S factors calculated on standard models. Methods: The CT images of 9 patients, who ranged in size, were used. Patient-specific S factors for 131 I were calculated with the MCNPX2.5.0 Monte Carlo code using a tool for personalized internal dose assessment, OEDIPE; standard S factors from OLINDA/EXM were compared against the patient-specific S factors. Furthermore, realistic biodistributions and cumulated activities for normal organs and tumors were used, and mean organ-and tumor-absorbed doses calculated with OEDIPE and OLINDA/EXM were compared. Results: The ratio of the standard and the patient-specific S factors were between 0.49 and 1.84 for a target distant from the source for 4 organs and 2 tumors studied as source and targets. For the case of self-irradiation, the equivalent ratio ranged between 0.45 and 2.47 and between 1.00 and 1.06 when mass correction was applied. Differences in mean absorbed doses were as high as 140% when realistic cumulated activity values were used. These values decreased to less than 26% in all cases studied when mass correction was applied to the self-irradiation given by OLINDA/EXM. Conclusion: Standard S factors can yield mean absorbed doses for normal organs or tumors with a reasonable accuracy (26% for the cases studied) as compared with absorbed doses calculated with Monte Carlo, provided that they have been corrected for mass.
Radiotherapy and Oncology, 2009
Purpose: To evaluate against Monte-Carlo the performance of various dose calculations algorithms regarding lung tumour coverage in stereotactic body radiotherapy (SBRT) conditions. Materials and methods: Dose distributions in virtual lung phantoms have been calculated using four commercial Treatment Planning System (TPS) algorithms and one Monte Carlo (MC) system (EGSnrc). We compared the performance of the algorithms in calculating the target dose for different degrees of lung inflation. The phantoms had a cubic 'body' and 'lung' and a central 2-cm diameter spherical 'tumour' (the body and tumour have unit density). The lung tissue was assigned five densities (q lung ): 0.01, 0.1, 0.2, 0.4 and 1 g=cm 3 . Four-field treatment plans were calculated with 6-and 18 MV narrow beams for each value of q lung . We considered the Pencil Beam Convolution (PBC Ecl ) and the Analytical Anisotropic Algorithm (AAA Ecl ) from Varian Eclipse and the Pencil Beam Convolution (PBC OMP ) and the Collapsed Cone Convolution (CCC OMP ) algorithms from Oncentra MasterPlan.
International Journal of Radiation Oncology*Biology*Physics, 2010
Purpose: Dose calculation based on pencil beam (PB) algorithms has its shortcomings predicting dose in tissue heterogeneities. The aim of this study was to compare dose distributions of clinically applied non-intensity-modulated radiotherapy 15-MV plans for stereotactic body radiotherapy between voxel Monte Carlo (XVMC) calculation and PB calculation for lung lesions. Methods and Materials: To validate XVMC, one treatment plan was verified in an inhomogeneous thorax phantom with EDR2 film (Eastman Kodak, Rochester, NY). Both measured and calculated (PB and XVMC) dose distributions were compared regarding profiles and isodoses. Then, 35 lung plans originally created for clinical treatment by PB calculation with the Eclipse planning system (Varian Medical Systems, Palo Alto, CA) were recalculated by XVMC (investigational implementation in PrecisePLAN [Elekta AB, Stockholm, Sweden]). Clinically relevant dose-volume parameters for target and lung tissue were compared and analyzed statistically. Results: The XVMC calculation agreed well with film measurements (<1% difference in lateral profile), whereas the deviation between PB calculation and film measurements was up to +15%. On analysis of 35 clinical cases, the mean dose, minimal dose and coverage dose value for 95% volume of gross tumor volume were 1.14 ± 1.72 Gy, 1.68 ± 1.47 Gy, and 1.24 ± 1.04 Gy lower by XVMC compared with PB, respectively (prescription dose, 30 Gy). The volume covered by the 9 Gy isodose of lung was 2.73% ± 3.12% higher when calculated by XVMC compared with PB. The largest differences were observed for small lesions circumferentially encompassed by lung tissue. Conclusions: Pencil beam dose calculation overestimates dose to the tumor and underestimates lung volumes exposed to a given dose consistently for 15-MV photons. The degree of difference between XVMC and PB is tumor size and location dependent. Therefore XVMC calculation is helpful to further optimize treatment planning. Ó 2010 Elsevier Inc.
Medical Dosimetry, 2014
To increase the efficacy of radiotherapy for non-small cell lung cancer (NSCLC), many schemes of dose fractionation were assessed by a new "toxicity index" (I), which allows one to choose the fractionation schedules that produce less toxic treatments. Thirty-two patients affected by non resectable NSCLC were treated by standard 3-dimensional conformal radiotherapy (3DCRT) with a strategy of limited treated volume. Computed tomography datasets were employed to re plan by simultaneous integrated boost intensity-modulated radiotherapy (IMRT). The dose distributions from plans were used to test various schemes of dose fractionation, in 3DCRT as well as in IMRT, by transforming the dose-volume histogram (DVH) into a biological equivalent DVH (BDVH) and by varying the overall treatment time. The BDVHs were obtained through the toxicity index, which was defined for each of the organs at risk (OAR) by a linear quadratic model keeping an equivalent radiobiological effect on the target volume. The less toxic fractionation consisted in a severe/moderate hyper fractionation for the volume including the primary tumor and lymph nodes, followed by a hypofractionation for the reduced volume of the primary tumor. The 3DCRT and IMRT resulted, respectively, in 4.7% and 4.3% of dose sparing for the spinal cord, without significant changes for the combined-lungs toxicity (p o 0.001). Schedules with reduced overall treatment time (accelerated fractionations) led to a 12.5% dose sparing for the spinal cord (7.5% in IMRT), 8.3% dose sparing for V 20 in the combined lungs (5.5% in IMRT), and also significant dose sparing for all the other OARs (p o 0.001). The toxicity index allows to choose fractionation schedules with reduced toxicity for all the OARs and equivalent radiobiological effect for the tumor in 3DCRT, as well as in IMRT, treatments of NSCLC.
International Journal of Medical Physics, Clinical Engineering and Radiation Oncology, 2018
Aim: Dosimetric analysis of three different Radiotherapy techniques in patients with Breast Cancer and their impact on OAR's. Materials and Methods: 12 patients of Carcinoma Breast who received breast radiotherapy were selected for analysis. Computed tomography (CT) simulation image data sets were retrieved. Planning target Volume (PTV), heart and ipsilateral lung were contoured for planning and analysis of doses. Three different plans using conventional bi-tangential fields were prepared, Plan 1 with SAD full beam with wedge (SAD-FBW), Plan 2 with SSD half beam with wedge (SSD-HBW) and plan 3 with SSD half beam without wedge techniques (SSD-HBO) in CMS Xio TPS. Statistical analysis was done using SPSS version 16.0. Results: The PTV coverage was significantly better in SAD-FBW techniques when compared with the other two techniques i.e. SSD-HBW (mean = 92.33, SD = 4.69, p = 0.005) and SSD-HBO (mean = 75.05, SD = 11.92, p = 0.002). The mean heart doses were significantly better in SAD-FBW compared to SSD-HBW (mean = 3.75, SD = 2.27, p = 0.017) but in SSD-HBO technique mean heart doses were better than SAD-FBW technique (mean = 3.27, SD = 1.94, p = 0.004). Similarly, the left lung V20 values were significantly better in SSD-HBO technique than SAD-FBW technique (mean = 11.75, SD = 4.34, p = 0.004) but there was statistically insignificant difference between the SAD-FBW and SSD-HBW techniques.