Dose Distributions Of Treatment Plans Research Articles

The Impact of Dose Calibration Point Differences Between the Treatment Plan and Patient-specific Quality Assurance in Passive Scattering Proton Beam Therapy.

Passive scattering proton beam therapy (PSPT) is performed by taking actual measurements of all pre-designated fields in a treatment plan followed by appropriate adjustments to the prescribed dose. For this reason, it is necessary to ensure precision management of the measurements (patient-specific calibration) in the administration of a planned dose. Therefore, this study investigated the impact on dose distribution in treatment planning when the patient calibration point differs from the normalized point in a treatment plan. A total of 16 cases were selected, where the patient calibration point and normalized point did not match, and the normalized point used in the treatment plan was changed to the patient calibration point using a treatment planning system (VQA ver. 2.01, HITACHI). At this point, the displacement of the relative dose at the isocenter was estimated as an error owing to the difference compared to the patient calibration point. Overall, the error was within the range of ±1.5%, with the exception of orbit cases. Calibrated points also tended to be lower than the normalized points in the treatment plan. In terms of treatment sites, a greater deviation was observed for head cases. Cases with a large deviation in sites other than the head were attributed to poor flatness within the radiation field owing to a narrower opening of the patient collimator. Dose measurement errors in PSPT due to differing calibration points were generally within ±1.5%, with higher deviations observed in head treatments because of complex structures and narrow collimator openings. A γ analysis for significant deviations showed a 98.7% passing rate, suggesting limited overall impact. It is important to select stable calibration points in dosimetry to ensure high precision in dose administration, particularly in complex treatment areas.

Geometric and dosimetric evaluation of deep learning based auto-segmentation for clinical target volume on breast cancer.

Recently, target auto-segmentation techniques based on deep learning (DL) have shown promising results. However, inaccurate target delineation will directly affect the treatment planning dose distribution and the effect of subsequent radiotherapy work. Evaluation based on geometric metrics alone may not be sufficient for target delineation accuracy assessment. The purpose of this paper is to validate the performance of automatic segmentation with dosimetric metrics and try to construct new evaluation geometric metrics to comprehensively understand the dose-response relationship from the perspective of clinical application. A DL-based target segmentation model was developed by using 186 manual delineation modified radical mastectomy breast cancer cases. The resulting DL model were used to generate alternative target contours in a new set of 48 patients. The Auto-plan was reoptimized to ensure the same optimized parameters as the reference Manual-plan. To assess the dosimetric impact of target auto-segmentation, not only common geometric metrics but also new spatial parameters with distance and relative volume ( ) to target were used. Correlations were performed using Spearman's correlation between segmentation evaluation metrics and dosimetric changes. Only strong (|R2 |>0.6, p<0.01) or moderate (|R2 |>0.4, p<0.01) Pearson correlation was established between the traditional geometric metric and three dosimetric evaluation indices to target (conformity index, homogeneity index, and mean dose). For organs at risk (OARs), inferior or no significant relationship was found between geometric parameters and dosimetric differences. Furthermore, we found that OARs dose distribution was affected by boundary error of target segmentation instead of distance and to target. Current geometric metrics could reflect a certain degree of dose effect of target variation. To find target contour variations that do lead to OARs dosimetry changes, clinically oriented metrics that more accurately reflect how segmentation quality affects dosimetry should be constructed.

Association between incidental dose outside the prostate and tumor control after modern image-guided radiotherapy.

Background and purposeExternal beam radiotherapy for prostate cancer deposits incidental dose to a region surrounding the target volume. Previously, an association was identified between tumor control and incidental dose for patients treated with conventional radiotherapy. We investigated whether such an association exists for patients treated using intensity modulated radiotherapy (IMRT) and tighter margins.Materials and methodsComputed tomography scans and three-dimensional treatment planning dose distributions were available from the Dutch randomized HYPRO trial for 397 patients in the standard fractionation arm (39 × 2 Gy) and 407 patients in the hypofractionation arm (19 × 3.4 Gy), mainly delivered using online image-guided IMRT. Endpoint was any treatment failure within 5 years. A mapping of 3D dose distributions between anatomies was performed based on distance to the surface of the prostate delineation. Mean mapped dose distributions were computed for patient groups with and without failure, obtaining dose difference distributions. Random patient permutations were performed to derive p values and to identify relevant regions.ResultsFor high-risk patients treated in the conventional arm, higher incidental dose was significantly associated with a higher probability of tumor control in both univariate and multivariate analysis. The locations of the excess dose mainly overlapped with the position of obturator internus muscles at about 2.5 cm from the prostate surface. No such relationship could be established for intermediate-risk patients.ConclusionsAn association was established between reduced treatment failure and the delivery of incidental dose outside the prostate for high-risk patients treated using conventionally fractionated IMRT.

Effects of dose calculation algorithms on the estimate NTCP for thoracic cancer

The study aims to investigate the effect of dose calculation algorithms on the estimate normal tissue complication probability (NTCP) of thoracic cancer. Dose distributions of treatment plans are calculated by using Collapsed Cone (CC) and Pencil Beam (PB) algorithms from Oncentra Masterplan treatment planning system. Four treatment plans of thoracic cancers are created and dose volume histograms (DVHs) from each planning were evaluated based on criteria of acceptance level. The DVH data of the treatment planning were exported to Biosuite Software to compute the NTCP values. The NTCP were calculated accounting for the CC and PB algorithm using the Lyman-Kutcher-Burman (LKB) and Relative Seriality (RS) model. The NTCP values of the LKB model are found to have slightly higher values compare to RS model for all treatment planning using both algorithms. This study also demonstrates that the dose calculation algorithms of CC and PB did not significantly influence the computed NTCP value in the present treatment planning of thoracic cancer.

Volume-dose indexes and dose prescription descriptive review of radiosurgery planning

In Stereotactic radiosurgery (SRS), indexes are used to compare plans, comparing treatment techniques and evaluate clinical complications. However, they have some limitations and dependencies that need to be well known. Therefore, the analysis using indices is better suited for comparison of plans for the same patient (lesion). The evaluation of stereotactic plans must be undertaken with great care and criteria because there is a huge amount of information between different dose distributions of treatment plans. The objectives are to show some tools for planning analysis in SRS their limitations, some indexes descriptive review in the literature that seek to quantify the three properties mentioned and discuss the concepts involved in SRS dose prescription.

Investigating the effects of different kernels used for CT image reconstruction on dose distributions in treatment planning of kidney cancer radiotherapy

Introduction: The quality of CT images used for treatment planning of cancer patients is an important issue in accurate outlining of the tumor volume and organs at risk. Different kernels in CT scanner systems are available for improving the image quality. Applying these kernels on CT images will change the CT numbers and electron density of tissues, consequently. Therefore, the aim of this study was to assess effects of different CT kernels on Hounsfield unit variations, the related computed electron densities and the calculated dose distributions in radiation treatment planning system. Materials and Methods: The 16 slice Siemens CT scanner used in this work. The B30, B35, B41 and B50 kernels applied on abdomen CT images for a kidney cancer patient. The ISOgray treatment planning system was used for radiation treatment planning and calculating dose of 6MV photon beam energy. The dose volume histogram (DVH) of left kidney as target volume and spinal cord as organ at risk were calculated. Results: The B50 kernel, had the greatest effect on calculated CT numbers for considered reference points relative to standard kernel B30, among the applied kernels for image reconstruction (16-19 HU variations). The average of calculated percent dose in target volume for 3 reference points obtained %99.19, %100.36T %99.89 and %100.87 for standard kernel B30 and other B35, B41 and B50 kernels, respectively. Mean dose in DVH was 2.03 for B30 and 2.05 for other kernels. Conclusion: The Hounsfield units of the selected reference points, in the target volume of reconstructed CT images by various kernels had relatively high variations respect to B30 kernel. Despite these variations, electron density and consequently the average of calculated percent dose in the target volume did not show considerable changes. Hence, it can be concluded that the use of image reconstruction kernels to improve the quality of CT images will help to determine the edges and outlines of tumors and organs at risk more precisely. While, applying these kernels does not significantly affect the amount of calculated doses in the treatment planning system

P269] Absorbed dose calculation based on CBCT data for head and neck cancer patients

Purpose In a radiotherapy magnetic resonance imaging (MRI) only workflow, a synthetic CT (sCT) data set is generated from the MRI data as a substitute to the computed tomography (CT). Since no CT scan will be acquired, no such data is available for quality control (QC) of the sCT. It has been suggested to use kV-cone beam CT (CBCT) data for QC in the head region [1] . The aim of this study is to evaluate the possibility to use CBCT for QC of the sCT for head and neck (H&N) cancer by comparing H&N treatment plans calculated using the CT and CBCT data, respectively. Methods Five patients with one CT and two CBCT data sets each, were evaluated. All CBCT data sets were deformably registered to the CT data, remediating errors originating from repositioning and/or anatomical changes between the imaging sessions. The CBCT and CT data were adjusted to the same field of view (FOV) in the longitudinal direction. A clinical treatment plan was created based on the CT data for each patient. Treatment volumes, organs-at-risk (OAR) and treatment plan were copied from the CT to the deformable registered CBCT. The treatment plan was then recalculated, with the standard CT HU-RED conversion curve. Dosimetric differences between the dose distributions for the treatment plans were calculated and evaluated by comparing dose-volume constrains. Results The mean local difference for the mean absorbed dose to the clinical target volume (CTV) was 0.41% with a maximum and minimum value of 1.37% and 0.15% respectively. The CTV D 98 % criteria had a difference of 0.19% and CTV D 2 % criteria a difference of 0.38%. The planning OAR volume (PRV) of the spinal cord had a difference of 0.39% and the parotid right and left a difference of 0.36% and 0.08%, respectively. Conclusions The dosimetric difference between the dose distributions for treatment plans calculated based on CT and CBCT data were minor. This indicates that CBCT data could be a good candidate for QC of sCT data in an MRI only workflow for H&N cancer patients.

Verification of high-dose-rate brachytherapy treatment planning dose distribution using liquid-filled ionization chamber array.

PurposeThis study aims to investigate the dosimetric performance of a liquid-filled ionization chamber array in high-dose-rate (HDR) brachytherapy dosimetry. A comparative study was carried out with air-filled ionization chamber array and EBT3 Gafchromic films to demonstrate its suitability in brachytherapy.Material and methodsThe PTW OCTAVIUS detector 1000 SRS (IA 2.5-5 mm) is a liquid-filled ionization chamber array of area 11 x 11 cm2 and chamber spacing of 2.5-5 mm, whereas the PTW OCTAVIUS detector 729 (IA 10 mm) is an air vented ionization chamber array of area 27 x 27 cm2 and chamber spacing of 10 mm. EBT3 films were exposed to doses up to a maximum of 6 Gy and evaluated using multi-channel analysis. The detectors were evaluated using test plans to mimic a HDR intracavitary gynecological treatment. The plan was calculated and delivered with the applicator plane placed 20 mm from the detector plane. The acquired measurements were compared to the treatment plan. In addition to point dose measurement, profile/isodose, gamma analysis, and uncertainty analysis were performed. Detector sensitivity was evaluated by introducing simulated errors to the test plans.ResultsThe mean point dose differences between measured and calculated plans were 0.2% ± 1.6%, 1.8% ± 1.0%, and 1.5% ± 0.81% for film, IA 10 mm, and IA 2.5-5 mm, respectively. The average percentage of passed gamma (global/local) values using 3%/3 mm criteria was above 99.8% for all three detectors on the original plan. For IA 2.5-5 mm, local gamma criteria of 2%/1 mm with a passing rate of at least 95% was found to be sensitive when simulated positional errors of 1 mm was introduced.ConclusionThe dosimetric properties of IA 2.5-5 mm showed the applicability of liquid-filled ionization chamber array as a potential QA device for HDR brachytherapy treatment planning systems.

Evaluating Which Dose-Function Metrics Are Most Critical for Functional Guided Radiation Therapy with CT Ventilation Imaging

4DCT-ventilation imaging is increasingly being used to calculate lung ventilation and implement functional guided radiotherapy in clinical trials. There has been little exhaustive work evaluating which dose-function metrics should be used for treatment planning and plan evaluation for functional guided radiotherapy. Our study evaluated which dose-function metrics are most critical in assessing the risk of radiation pneumonitis (RP). Seventy lung cancer patients with 4DCT imaging and RP grading were used. Pre-treatment 4DCTs of each patient were used to calculate ventilation images. We evaluated 3 different types of dose function metrics that combined that patient’s 4DCT-ventilation image and treatment planning dose distribution: 1) structure-based approaches 2) image-based approaches using the dose-function histogram (DFH) and 3) non-linear weighting schemes. Structure-based approaches used a binary approximation in which the lung was binned to either functional or non-functional status. A full range of cutoff values to delineate between functional and non-functional and the volume of functional lung receiving ≥ 5Gy, 10Gy, 20Gy, and 30Gy was investigated. Imaging-based and non-linear approaches investigated the fraction of lung function receiving ≥ 5Gy, 10Gy, 20Gy, and 30Gy. Maximum likelihood methods were used to generate normal tissue complication probability (NTCP) models predicting grade 3+ RP for the dose-function schemes and all tested dose-function metrics. The area under the curve (AUC) for each model was used to quantify the ability to predict for toxicity. All techniques were compared to NTCP models based on traditional, total lung dose metrics. The most predictive models for grade 3+ RP were structure-based approaches that focused on the volume of functional lung receiving ≥20Gy (AUC=0.70) where functional lung was treated as regions with ventilation values ≥84th percentile. Imaging-based analysis with the DFH and non-linear weighted ventilation values yielded AUCs of 0.66 and 0.67, respectively, when evaluating the fraction of functionality receiving ≥20Gy. All dose-function metrics outperformed the traditional lung dose metrics (mean lung dose, AUC = 0.55). A full range of dose-function metrics and functional thresholds were examined. The AUC values for the most predictive functional models occupied a narrow range (0.66-0.70) and all demonstrated improvements over AUC from traditional lung dose metrics (0.55). Identifying the dose-function parameters most predictive of grade 3+ RP provides valuable data for treatment planning and plan evaluation parameters. With prospective clinical trials of functional guided radiotherapy using 4DCT-ventilation imaging underway, this work provides seminal data to help establish guidelines for the implementation of functional guided radiotherapy.

Dosimetric effect of internal metallic ports in temporary tissue expanders on postmastectomy radiation therapy: a Monte Carlo study

To investigate the dosimetric effect of the internal metallic port (IMP) in a tissue expander (TE) on the dose distribution of postmastectomy radiation therapy (PMRT). A total of 10 patients who have received PMRT with a TE were selected retrospectively. For each patient, the dose distributions of treatment plans with a 10 MV photon beam were calculated using the Monte Carlo (MC) method with CT images. The dose distributions without the TE were also calculated by designating the mass densities of the TE including the IMP as those of tissue. From the MC calculations, the dose-volumetric parameters were calculated and analyzed for several structures: the planning target volume (PTV) including the TE, the PTV excluding the TE (PTVreal), the TE alone, heart, and lungs. For the PTV and PTVreal, dose-volumetric parameters did not appear to depend on the IMP. Within the TE volume, the maximum dose and D1% were higher with the IMP than without the IMP (62.8 ± 1.4 Gy versus 57.9 ± 1.3 Gy with p < 0.001 and 58.6 ± 1.6 Gy versus 57.0 ± 1.2 Gy with p = 0.035). The values of V100% and V95% were lower with the IMP than without the IMP (77.9% ± 7.6% versus 87.2% ± 5.3% with p = 0.008 and 89.5% ± 5.6% versus 94.6% ± 2.9% with p = 0.027). The IMP did not affect dose-volumetric parameters of heart and lungs. Dosimetric changes due to the IMP occurred mainly within the TE, and not in the target volume, heart, and lungs.

Evaluating Which Dose-Function Metrics Are Most Critical for Functional-Guided Radiation Therapy

Four-dimensional (4D) computed tomography (CT) ventilation imaging is increasingly being used to calculate lung ventilation and implement functional-guided radiation therapy in clinical trials.There has been little exhaustive work evaluating which dose-function metrics should be used for treatment planning and plan evaluation. The purpose of our study was to evaluate which dose-function metrics best predict for radiation pneumonitis (RP). Seventy lung cancer patients who underwent 4D CT imaging and pneumonitis grading were assessed. Pretreatment 4D CT scans of each patient were used to calculate ventilation images. We evaluated 3 types of dose-function metrics that combined the patient's 4D CT ventilation image and treatment planning dose distribution: (1) structure-based approaches; (2) image-based approaches using the dose-function histogram; and (3) nonlinear weighting schemes. Log-likelihood methods were used to generate normal tissue complication probability models predicting grade 3 or higher (ie, grade 3+) pneumonitis for all dose-function schemes. The area under the curve (AUC) was used to assess the predictive power of the models. All techniques were compared with normal tissue complication probability models based on traditional, total lung dose metrics. The most predictive models were structure-based approaches that focused on the volume of functional lung receiving ≥20Gy (AUC, 0.70). Probabilities of grade 3+RP of 20% and 10% correspond to V20 (percentage of volume receiving ≥20Gy) to the functional subvolumes of 26.8% and 9.3%, respectively.Imaging-based analysis with the dose-function histogram and nonlinear weighted ventilation values yielded AUCs of 0.66 and 0.67, respectively, when we evaluated the percentage of functionality receiving ≥20Gy.All dose-function metrics outperformed the traditional dose metrics (mean lung dose, AUC of 0.55). A full range of dose-function metrics and functional thresholds was examined. The calculated AUC values for the most predictive functional models occupied a narrow range (0.66-0.70), and all showed notable improvements over AUC from traditional lung dose metrics (0.55). Identifying the combinations most predictive of grade 3+RP provides valuable data to inform the functional-guided radiation therapy process.

Tumor control probability reduction in gated radiotherapy of non-small cell lung cancers: a feasibility study.

We studied the feasibility of evaluating tumor control probability (TCP) reductions for tumor motion beyond planned gated radiotherapy margins. Tumor motion was determined from cone‐beam CT projections acquired for patient setup, intrafraction respiratory traces, and 4D CTs for five non‐small cell lung cancer (NSCLC) patients treated with gated radiotherapy. Tumors were subdivided into 1 mm sections whose positions and doses were determined for each beam‐on time point. (The dose calculation model was verified with motion phantom measurements.) The calculated dose distributions were used to generate the treatment TCPs for each patient. The plan TCPs were calculated from the treatment planning dose distributions. The treatment TCPs were compared to the plan TCPs for various models and parameters. Calculated doses matched phantom measurements within 0.3% for up to 3 cm of motion. TCP reductions for excess motion greater than 5 mm ranged from 1.7% to 11.9%, depending on model parameters, and were as high as 48.6% for model parameters that simulated an individual patient. Repeating the worst case motion for all fractions increased TCP reductions by a factor of 2 to 3, while hypofractionation decreased these reductions by as much as a factor of 3. Treatment motion exceeding gating margins by more than 5 mm can lead to considerable TCP reductions. Appropriate margins for excess motion are recommended, unless applying daily tumor motion verification and adjusting the gating window.PACS numbers: 87.55.dk, 87.57.Q‐

Dosimetric comparison of absolute and relative dose distributions between tissue maximum ratio and convolution algorithms for acoustic neurinoma plans in Gamma Knife radiosurgery

The treatment planning for Gamma Knife (GK) stereotactic radiosurgery (SRS) that performs dose calculations based on tissue maximum ratio (TMR) algorithm has disadvantages in predicting dose in tissue heterogeneity. The latest version of the planning software is equipped with a convolution dose algorithm as an optional extra and the new algorithm is able to compensate for head inhomogeneity. However, the effect of this improved calculation method requires detailed validation in clinical cases. In this study, we compared absolute and relative dose distributions of treatment plans for acoustic neurinoma between TMR and the convolution calculation. Twenty-nine clinically used plans created by TMR algorithm were recalculated by convolution method. Differences between TMR and convolution were evaluated in terms of absolute dose (beam-on time), dosimetric parameters including target coverage, selectivity, conformity index, gradient index, radical homogeneity index and the dose-volume relationship. The discrepancy in estimated absolute dose to the target ranged from 1 to 7 % between TMR and convolution. In addition, dosimetric parameters of the two methods achieved statistical significance. However, it was difficult to see the change of relative dose distribution by visual assessment on a monitor. Convolution, heterogeneity correction calculation, and the algorithm are necessary to reduce the dosimetric uncertainty of each case in GK SRS.

Monte Carlo implementation, validation, and characterization of a 120 leaf MLC

Recently, the new high definition multileaf collimator (HD120 MLC) was commercialized by Varian Medical Systems providing high resolution in the center section of the treatment field. The aim of this work is to investigate the characteristics of the HD120 MLC using Monte Carlo (MC) methods. Based on the information of the manufacturer, the HD120 MLC was implemented into the already existing Swiss MC Plan (SMCP). The implementation has been configured by adjusting the physical density and the air gap between adjacent leaves in order to match transmission profile measurements for 6 and 15 MV beams of a Novalis TX. These measurements have been performed in water using gafchromic films and an ionization chamber at an SSD of 95 cm and a depth of 5 cm. The implementation was validated by comparing diamond measured and calculated penumbra values (80%-20%) for different field sizes and water depths. Additionally, measured and calculated dose distributions for a head and neck IMRT case using the DELTA(4) phantom have been compared. The validated HD120 MLC implementation has been used for its physical characterization. For this purpose, phase space (PS) files have been generated below the fully closed multileaf collimator (MLC) of a 40 × 22 cm(2) field size for 6 and 15 MV. The PS files have been analyzed in terms of energy spectra, mean energy, fluence, and energy fluence in the direction perpendicular to the MLC leaves and have been compared with the corresponding data using the well established Varian 80 leaf (MLC80) and Millennium M120 (M120 MLC) MLCs. Additionally, the impact of the tongue and groove design of the MLCs on dose has been characterized. Calculated transmission values for the HD120 MLC are 1.25% and 1.34% in the central part of the field for the 6 and 15 MV beam, respectively. The corresponding ionization chamber measurements result in a transmission of 1.20% and 1.35%. Good agreement has been found for the comparison between transmission profiles resulting from MC simulations and film measurements. The simulated and measured values for the penumbra agreed within <0.5 mm for all field sizes, depths, and beam energies, and a good agreement has been found between the measured and the calculated dose distributions for the IMRT case. The total energy spectra are almost identical for the three MLCs. However, the mean energy, fluence and energy fluence are significantly different. Due to the different leaf widths of the MLCs, the shape of these distributions is different, each representing its leave structure. Due to the increase in width from the inner to the outer HD120 MLC leaves, the fluence and energy fluence clearly decrease below the outer leaves. The MLC80 and the M120 MLC resulted in an increase of the fluence and energy fluence compared with those resulted for the HD120 MLC. The dose reduction can exceed 20% compared with the dose of the open field due to the tongue and groove design of the HD120 MLC. The HD120 MLC has been successfully implemented into the SMCP. Comparisons between MC calculations and measurements show very good agreement. The SMCP is now able to calculate accurate dose distributions for treatment plans using the HD120 MLC.

Position‐probability‐sampled Monte Carlo calculation of VMAT, 3DCRT, step‐shoot IMRT, and helical tomotherapy dose distributions using BEAMnrc/DOSXYZnrc

The commercial release of volumetric modulated arc therapy techniques using a conventional linear accelerator and the growing number of helical tomotherapy users have triggered renewed interest in dose verification methods, and also in tools for exploring the impact of machine tolerance and patient motion on dose distributions without the need to approximate time-varying parameters such as gantry position, MLC leaf motion, or patient motion. To this end we have developed a Monte Carlo-based calculation method capable of simulating a wide variety of treatment techniques without the need to resort to discretization approximations. The ability to perform complete position-probability-sampled Monte Carlo dose calculations was implemented in the BEAMnrc/DOSXZYnrc user codes of EGSnrc. The method includes full accelerator head simulations of our tomotherapy and Elekta linacs, and a realistic representation of continous motion via the sampling of a time variable. The functionality of this algorithm was tested via comparisons with both measurements and treatment planning dose distributions for four types of treatment techniques: 3D conformal, step-shoot intensity modulated radiation therapy, helical tomotherapy, and volumetric modulated are therapy. For static fields, the absolute dose agreement between the EGSnrc Monte Carlo calculations and measurements is within 2%/1 mm. Absolute dose agreement between Monte Carlo calculations and treatment planning system for the four different treatment techniques is within 3%/3 mm. Discrepancies with the tomotherapy TPS on the order of 10%/5 mm were observed for the extreme example of a small target located 15 cm off-axis and planned with a low modulation factor. The increase in simulation time associated with using position-probability sampling, as opposed to the discretization approach, was less than 2% in most cases. A single Monte Carlo simulation method can be used to calculate patient dose distribution for various types of treatment techniques delivered with either tomotherapy or a conventional linac. The method simplifies the simulation process, improves dose calculation accuracy, and involves an acceptably small change in computation time.

Sci-Fri PM: Delivery - 02: Position-Probability-Sampled Monte Carlo Calculation of VMAT, 3DCRT, Step-Shoot IMRT and Helical Tomotherapy Dose Distributions Using BEAMnrc / DOSXYZnrc

The commercial release of volumetric modulated arc therapy techniques using a conventional linear accelerator and the growing number of helical tomotherapy users has triggered renewed interest in both MC dose verification methods and tools for doing treatment planning studies and exploring the impact of machine tolerance and patient motion on dose distributions. We report here the development of a method for performing complete position-probability-sampled Monte Carlo dose calculation using the BEAMnrc / DOSXYZnrc code. The method includes full accelerator head simulation and a realistic representation of continuous motion, with no resort to discretization approximations or analytical procedure. Accelerator head models were defined and validated against measurements for a conventional accelerator (Elekta Synergy ™) and a helical tomotherapy accelerator (TomoTherapy Hi-Art®). The method functionality was tested by comparing simulated and treatment planning dose distributions for four types of treatment techniques: 3D-conformal, step-shoot intensity modulated radiation therapy, helical tomotherapy and volumetric modulated arc therapy. Absolute dose agreement for static fields between MC and measurements is within 2% / 2mm. Absolute dose agreement between MC and TPS was determined to be 3% / 3mm. The method simplifies the simulation process, improves the dose calculation accuracy and involves a minimal change in computation time.

SU-FF-J-119: Accuracy of Using Deformable Image Registration to Map Computed Dose Distributions From Treatment Planning CT to Individual Phase CTs

Purpose: Current clinical practice assumes the treatment planningdose distribution is representative of the delivered dose distribution despite organ motion. We investigate the effect of respiratory motion on calculated four‐dimensional dose distributions, accuracy of only computing dose on a reference free‐breathing (FB) CT, and whether dose distributions mapped through deformable image registration (DIR) using a reference computed dose distribution can replace individually calculated dose distributions. Method and Materials: Data for this study was chosen from those patients undergoing radiation therapy for lungcancer, and accompanying 10‐phase 4DCT datasets. Two patients exhibited tumor motion >0.5cm, while two exhibited motion <0.5cm. Dose distributions were calculated on each phase of the 4D dataset. The Juggler DIR algorithm was used to calculate deformation vectors between the FBCT and each phase of the 4DCT for each patient. This vector field was then applied to the FB dose distribution to deform it onto each of the phases. Results: For individual phase dose distributions compared to their respective FB dose distributions, average agreement between dose voxels of 54.6%, 73.6%, and 84.7% were observed for 2%, 5%, and 10% dose difference thresholds. Differences between average calculated dose and average deformed dose ranged from 73.6%–78.5% dose voxels agreement for 5% dose difference, and 83.8–87.5% for 10% dose difference. Differences between average calculated dose and FB calculated dose ranged from 66.7%–79.6% for 5% dose difference, and 77.1%–89.1% for 10% dose difference. Conclusion: The accuracy of using the dose distribution computed on the FB CT only, which represents the standard clinical practice, as opposed to the ideal case of using the dose calculated on each individual phase is of the same accuracy as using DIR to remap the FB dose distribution onto individual phases. Accuracy of DIR for dose mapping was insensitive to range of tumor motion.

The relevance of very low energy ions for heavy-ion therapy

Heavy-ion radiotherapy exploits the high biological effectiveness of localized energy deposition delivered by so-called Bragg-peak particles. Recent publications have challenged the established procedures to calculate biological effective dose distributions in treatment planning. They emphasize the importance of very low energy (<500 keV amu−1) ions, either as primary particles or originating from molecular and nuclear fragmentations. We show, however, that slow heavy ions with energies below 500 keV amu−1 only play a negligible role in cancer treatments for several reasons. Their residual range is very small compared to the relevant length scale of treatment planning. Moreover, their relative frequency and also their relative dose distribution are insignificant, since energy loss and range straggling in ion slowing down processes as well as the necessary superposition of Bragg peaks wash out small-scale special effects. Additionally, we show that even a 1000 times larger biological damage of such slow ions would not result in a clinically relevant increase of the photon-equivalent dose. Therefore, neither a more precise physical description of ions in the very distal part of the Bragg peak nor the consideration of radiation damage induced by hyperthermal ions would result in a meaningful improvement of current models for heavy-ion treatment planning.

A comparison of dose distributions of HDR intracavitary brachytherapy using different sources and treatment planning systems

To evaluate the influence of different sources and treatment planning systems (TPSs), we calculated the dose distributions of 10 patients treated by HDR intracavitary brachytherapy. The dose distributions of treatment plans were compared to the points A, B and ICRU bladder and rectum reference points located in the relevant clinical planes. The results show that the dose discrepancies between the two treatment plans are mainly affected by the differences in the physical characteristics of source and the positioning method in TPSs.

Dosimetric evaluation of nuclear interaction models in the Geant4 Monte Carlo simulation toolkit for carbon-ion radiotherapy

We tested the ability of two separate nuclear reaction models, the binary cascade and JQMD (Jaeri version of Quantum Molecular Dynamics), to predict the dose distribution in carbon-ion radiotherapy. This was done by use of a realistic simulation of the experimental irradiation of a water target. Comparison with measurement shows that the binary cascade model does a good job reproducing the spread-out Bragg peak in depth-dose distributions in water irradiated with a 290 MeV/u (per nucleon) beam. However, it significantly overestimates the peak dose for a 400 MeV/u beam. JQMD underestimates the overall dose because of a tendency to break a nucleus into lower-Z fragments than does the binary cascade model. As far as shape of the dose distribution is concerned, JQMD shows fairly good agreement with measurement for both beam energies of 290 and 400 MeV/u, which favors JQMD over the binary cascade model for the calculation of the relative dose distribution in treatment planning.