Hossein Afsharpour

ALGEBRA: ALgorithm for the heterogeneous dosimetry based on GEANT4 for BRAchytherapy

Task group 43 (TG43)-based dosimetry algorithms are efficient for brachytherapy dose calculation in water. However, human tissues have chemical compositions and densities different than water. Moreover, the mutual shielding effect of seeds on each other (interseed attenuation) is neglected in the TG43-based dosimetry platforms. The scientific community has expressed the need for an accurate dosimetry platform in brachytherapy. The purpose of this paper is to present ALGEBRA, a Monte Carlo platform for dosimetry in brachytherapy which is sufficiently fast and accurate for clinical and research purposes. ALGEBRA is based on the GEANT4 Monte Carlo code and is capable of handling the DICOM RT standard to recreate a virtual model of the treated site. Here, the performance of ALGEBRA is presented for the special case of LDR brachytherapy in permanent prostate and breast seed implants. However, the algorithm is also capable of handling other treatments such as HDR brachytherapy.

Influence of breast composition and interseed attenuation in dose calculations for post-implant assessment of permanent breast 103Pd seed implant.

The impact of tissue heterogeneity and interseed attenuation is studied in post-implant evaluation of five clinical permanent breast (103)Pd seed implants using the Monte Carlo (MC) dose calculation method. Dose metrics for the target (PTV) as well as an organ at risk (skin) are used to visualize the differences between a TG43-like MC method and more accurate MC methods capable of considering the breast tissue heterogeneity as well as the interseed attenuation. PTV dose is reduced when using a breast tissue model instead of water in MC calculations while the dose to the skin is increased. Furthermore, we investigate the effect of varying the glandular/adipose proportion of the breast tissue on dose distributions. The dose to the PTV (skin) decreases (increases) with the increasing adipose proportion inside the breast. In a complete geometry and compared to a TG43-like situation, the average PTV D(90) reduction varies from 3.9% in a glandular breast to 35.5% when the breast consists entirely of adipose. The skin D(10) increases by 28.2% in an entirely adipose breast. The results of this work show the importance of an accurate and patient-dependent breast tissue model to be used in the dosimetry for this kind of low energy implant.

Tissue modeling schemes in low energy breast brachytherapy

Breast tissue is heterogeneous and is mainly composed of glandular (G) and adipose (A) tissues. The proportion of G versus A varies considerably among the population. The absorbed dose distributions in accelerated partial breast irradiation therapy with low energy photon brachytherapy sources are very sensitive to tissue heterogeneities. Current clinical algorithms use the recommendations of the AAPM TG43 report which approximates the human tissues by unit density water. The aim of this study is to investigate various breast tissue modeling schemes for low energy brachytherapy. A special case of breast permanent seed implant is considered here. Six modeling schemes are considered. Uniform and non-uniform water breast (UWB and NUWB) consider the density but neglect the effect of the composition of tissues. The uniform and the non-uniform G/A breast (UGAB and NUGAB) as well the age-dependent breast (ADB) models consider the effect of the composition. The segmented breast tissue (SBT) method uses a density threshold to distinguish between G and A tissues. The PTV D(90) metric is used for the analysis and is based on the dose to water (D(90(w,m))). D(90(m,m)) is also reported for comparison to D(90(w,m)). The two-month post-implant D(90(w,m)) averaged over 38 patients is smaller in NUWB than in UWB by about 4.6% on average (ranging from 5% to 13%). Large average differences of G/A breast models with TG43 (17% and 26% in UGAB and NUGAB, respectively) show that the effect of the chemical composition dominates the effect of the density on dose distributions. D(90(w,m)) is 12% larger in SBT than in TG43 when averaged. These differences can be as low as 4% or as high as 20% when the individual patients are considered. The high sensitivity of dosimetry on the modeling scheme argues in favor of an agreement on a standard tissue modeling approach to be used in low energy breast brachytherapy. SBT appears to generate the most geometrically reliable breast tissue models in this report.

Monte Carlo dosimetry of high dose rate gynecologic interstitial brachytherapy.

PURPOSE To assess the dosimetric effects of the presence of the applicator, air pockets in clinical target volume (CTV) and OARs along with tissue heterogeneities using the Monte Carlo (MC) method in high dose rate (HDR) gynecologic interstitial brachytherapy with a Syed-Neblett template. METHODS AND MATERIALS The CT based dosimetry has been achieved with the Geant4 MC toolkit version 9.2. DICOM-RT files of 38 patients were imported into our own platform for MC simulations. The dose distributions were then compared to those obtained with a conventional TG-43 calculation. RESULTS Taking account of heterogeneities has effects of the order of 1% on the HDR gynecological dose distributions. However, the exclusion of air pockets and applicator from the DVH calculation can lower the CTV D90 and V100 by as much as 8.7% and 5.0% in comparison with TG-43. Rectum dosimetric indices can also be lowered by approximately 3% compared with TG-43 for most cases. Differences for urethra and bladder are for most cases below 1%. CONCLUSIONS Exclusion of non-biological material such as air pockets and applicator volume from the CTV is important for both TG-43 and MC calculations. It could be easily implemented and automated in treatment planning systems without affecting computation times.

Dose reduction in LDR brachytherapy by implanted prostate gold fiducial markers.

PURPOSE The dosimetric impact of gold fiducial markers (FM) implanted prior to external beam radiotherapy of prostate cancer on low dose rate (LDR) brachytherapy seed implants performed in the context of combined therapy was investigated. METHODS A virtual water phantom was designed containing a single FM. Single and multi source scenarios were investigated by performing Monte Carlo dose calculations, along with the influence of varying orientation and distance of the FM with respect to the sources. Three prostate cancer patients treated with LDR brachytherapy for a recurrence following external beam radiotherapy with implanted FM were studied as surrogate cases to combined therapy. FM and brachytherapy seeds were identified on post implant CT scans and Monte Carlo dose calculations were performed with and without FM. The dosimetric impact of the FM was evaluated by quantifying the amplitude of dose shadows and the volume of cold spots. D(90) was reported based on the post implant CT prostate contour. RESULTS Large shadows are observed in the single source-FM scenarios. As expected from geometric considerations, the shadows are dependent on source-FM distance and orientation. Large dose reductions are observed at the distal side of FM, while at the proximal side a dose enhancement is observed. In multisource scenarios, the importance of shadows appears mitigated, although FM at the periphery of the seed distribution caused underdosage (<prescription dose). In clinical cases, the FM reduced the dose to some voxels by up to 50% and generated shadows with extents of the order of 4 mm. Within the prostate contour, cold spots (<95% prescription dose) of the order of 20 mm(3) were observed. D(90) proved insensitive to the presence of FM for the cases selected. CONCLUSIONS There is a major local impact of FM present in LDR brachytherapy seed implant dose distributions. Therefore, reduced tumor control could be expected from FM implanted in tumors, although our results are too limited to draw conclusions regarding clinical significance.

SU‐FF‐T‐406: Toward a More Accurate Dose Calculation Technique Using a Semiautomatic Organ Contouring in Monte Carlo Post‐Implant Assessment of Breast LDR Brachytherapy

Purpose: We assess the impact of the uncertainty of glandular/adipose tissue proportion on dose distribution inside the breast for LDR brachytherapy. Furthermore, we propose a semiautomatic tissue segmentation method to be used in Monte Carlo(MC) calculations for breast LDR patients. Methods & materials: Post‐implant CT exams of five breast brachytherapy patients are imported into our Geant4 MC platform. The CTV and the PTV were contoured by the oncologist; the external contours of the breast, skin,lungs and the ribs were made manually by a physicist. In a first step, the breast is assigned with different proportions of glandular/adipose tissue (ρ = 100/0, 75/25, 50/50, 25/75, 0/100) in order to study the impact of this variation on clinical dosimetries. In a second step, a density‐based semiautomatic segmentation of breast tissue is used to identify glandular from adipose regions inside the breast and to create a more representative breast anatomy with appropriate chemical compositions. A TG43‐like simulation (SMC: Superposition MC) is also performed for each patient. Does‐Volume‐Histograms are used to visualize the effect of dose reduction. Results: In all patients, the glandular/adipose proportion, affects the dose distribution across the breast. The higher the adipose proportion in the breast tissue, the larger is the dose reduction across the organ. The D90 clinical parameter is reduced by up to 22% when the breast is entirely made of adipose. The semiautomatic contouring enabled a patient‐dependent segmentation of the glandular and adipose regions leading to more accurate dose calculations for each patient. Conclusion: Using realistic chemical compositions in MC simulations is achievable for MC calculations. Unlike external‐beam radiotherapy, the low‐energy emission of 103Pd is strongly affected by the heterogeneities adipose proportion in breast. A patient‐dependant glandular/adipose segmentation in breast is important for accurate dose determinations in MC, especially for breast LDR brachytherapy.

TH‐C‐213AB‐12: On the Importance of Heterogeneous Calculation in Brachytherapy: A Radiobiological Point of View

Purpose: The purpose of this work is to investigate the importance of heterogeneity corrections for dosimetry in brachytherapy. In particular, we are interested to see how the estimations of the a/b parameter for prostate cancer may change if accurate dose calculations are implemented in brachytherapy.Methods: A Monte Carlodose engine called ALGEBRA (Based on GEANT4) is used to accurately calculate dose distributions for 30 prostate cancer patients treated with brachytherapy at our institution. The equivalent uniform BED (EUBED) is used to take the high spatial dose heterogeneity of BT into account for estimating the biological efficiency of treatments. For the same level of clinical outcome, the EUBED of BT can be assumed is‐effective with BED of external beam radiotherapy to extract the a/b value for prostate cancer.Results: When the heterogeneities are neglected, a/b value equals 3.1 Gy as reported in the literature. When heterogeneities are considered in BT, an a/b value of 5.4 Gy is predicted for prostate cancer.Conclusions: The importance of a precise dosimetry method in plan evaluation has been recognized but its importance on the radiobiological evaluations is usually neglected. This study shows that by using an accurate dosimetry method in BT, the estimation of a/b can change considerably with regard to the reported value of 3.1 Gy in the literature.

SU‐D‐BRB‐06: G4DBR: A Fast Geant4‐Based Monte Carlo Dosimetry Platform for Brachytherapy

Purpose: To present, G4DBR, a fast Geant4‐based Monte Carlo (MC)dosimetry platform for brachytherapy. The special case of low dose rate (LDR) brachytherapy is considered here. Methods: Geant4 9.3 has been used for designing a new MC platform for calculating the dose distribution in brachytherapy called G4DBR. This code is capable of dealing with the DICOM RT format to build a virtual representation of each patient with the full multi‐seed configuration. The dose is scored both to medium and to water using track‐length estimator. The dose distributions are extracted in 3ddose format for visualization or to calculate the DVHs. Results: One prostate permanent 125I seed (PPSI) and one breast permanent 103Pd seed implant (BSPI) patient have been selected for evaluating the performance of G4DBR on a 2.93 GHz Intel Xeon Nehalem single core. Post‐implant dosimetry of those cases are performed in a 2 mm3 mesh for comparison with BrachyDose and MCPI. 45 seconds were required for G4DBR to reach a statistical uncertainty of 2% on PTV dose in PPSI. Note that for a similar precision, BrachyDose requires 30 seconds on 3 GHz Woodcrest (Thomson et al. Med. Phys. 2010) while MCPI needs 59 seconds on a single 2.4 GHz Pentium 4 CPU (Chibani et al. Med. Phys. 2005). G4DBR takes 114 seconds to attain a statistical uncertainty of less than 2% in the BPSI case. Conclusions: G4DBR is accurate and fast enough for clinical purposes. G4DBR is able to achieve good calculation speeds comparable with BrachyDose and MCPI. Indeed, a statistical uncertainty of less than 2% is attained in 45 seconds in a prostate case while 118 seconds were needed in BPSI to achieve 0.5% of uncertainty. Further developments will include the incorporation of high dose rate (HDR) dosimetry and a user‐friendly GUI for G4DBR.

Sci—Sat AM(2): Brachy — 02: The Sensitivity of BED and TCP Parameters to Dose Heterogeneity in Brachytherapy Treatments

Purpose: To investigate the sensitivity of radiobiological parameters to dose heterogeneity in brachytherapy.Materials and Methods: Biological effective dose (BED) is calculated using a homogeneous and a heterogeneous model. The homogeneous model is based on the minimal peripheral dose to the target and does not take into account the heterogeneity of the dose distribution in brachytherapy. The heterogeneous model is capable of taking the dose heterogeneity into account. Different Monte Carlo calculation protocols were used for five breast permanent seed implant patients. The tumor control probability (TCP) was calculated for both the homogeneous and the heterogeneous models. Results: The homogeneous model is overestimating the BED in all calculation cases. This overestimation is higher for plans with higher D 90 values. For example, by about 10% for a plan with D 90 =40 Gy, this overestimation increases to 40% when the D 90 =90 Gy. Depending on the D 90 , the homogeneous model overestimates the TCP vis‐a‐vis the heterogeneous model. In this case the difference is large for intermediate D 90 , however, they agree well for higher D 90 doses. The 50% control dose is higher by 13 Gy while the γ50 parameter is lower by 60% in the heterogeneous compared to the homogeneous model. Conclusion: The BED is always overestimated by the homogeneous model for all brachytherapy plans. The difference between the homogeneous and the heterogeneous TCP models is large at intermediate D 90 but they agree at higher D 90 . Because of the high dose heterogeneities, the homogeneous model is not suitable for brachytherapy.

SU-GG-T-496: The Sensitivity of BED and TCP Parameters to Dose Heterogeneity in Brachytherapy Treatments

Purpose: To investigate the sensitivity of radiobiological parameters to dose heterogeneity in brachytherapy.Materials and Methods: Biological effective dose (BED) is calculated using a homogeneous and a heterogeneous model. The homogeneous model is based on the minimal peripheral dose to the target and does not take into account the heterogeneity of the dose distribution in brachytherapy. The heterogeneous model (recommended by the AAPM TG137) is capable of taking the dose heterogeneity into account. Different Monte Carlo calculation protocols were used for five breast permanent seed implant patients. The tumor control probability (TCP) was calculated for both the homogeneous and the heterogeneous models. Results: The homogeneous model is overestimating the BED in all calculation cases. This overestimation is higher for plans with higher D 90 values. For example, by about 10% for a plan with D 90=40 Gy, this overestimation increases to 30% when the D 90=90 Gy. Depending on the D 90, the homogeneous model overestimates the TCP vis‐a‐vis the heterogeneous model. Although the difference between the two models is large for intermediate D 90 (between 40 Gy and 80 Gy), however, they agree well for higher D 90doses above 80 Gy. The 50% control dose is higher by 13 Gy while the γ50 parameter is lower by 60% in the heterogeneous compared to the homogeneous model. Conclusion: The BED is always overestimated by the homogeneous model for all brachytherapy plans. The difference between the homogeneous and the heterogeneous TCP models is large at intermediate D 90 (40Gy to 80Gy) but they agree at higher D 90. Because of the elevated dose heterogeneities, the homogeneous model is not suitable for brachytherapy. A more accurate estimation of the BED and the TCP will improve outcome predictions for brachytherapy treatments.