Jean-François Carrier

Validation of GEANT4, an object‐oriented Monte Carlo toolkit, for simulations in medical physics

GEANT4 (GEometry ANd Tracking 4) is an object-oriented Monte Carlo simulation toolkit that has been developed by a worldwide collaboration of scientists. It simulates the passage of particles through matter. In order to validate GEANT4 for medical physics applications, different simulations are conducted. The results are compared to published results based on three Monte Carlo codes widely used in medical physics: MCNP, EGS4, and EGSnrc. When possible, the simulation results are also compared to experimental data. Different geometries are tested (multilayer and homogeneous phantoms), different sources considered (point-source and broad parallel beam), and different primary particles simulated (photons and electrons) at different energies. For the heterogeneous media, there are notable differences between the Monte Carlo codes reaching up to over 5% in relative difference. For the monoenergetic electrons in a homogeneous medium, the difference between GEANT4 and the experimental measurements is similar to the difference between EGSnrc and the experimental measurements; for the depth-dose curves, the difference expressed as a fraction of the peak dose is always smaller than 4%. We conclude that GEANT4 is a promising Monte Carlo simulation toolkit for low-energy medical applications.

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.

Poor Predictive Value of Intraoperative Real-Time Dosimetry for Prostate Seed Brachytherapy

PURPOSE To identify dosimetric parameters predictive of a good prostate seed I(125) quality implant. We analyzed preimplant and postimplant realtime dosimetry in patients treated with intraoperative (IO) inverse planning. METHODS AND MATERIALS We analyzed 127 consecutively treated patients with primarily low-risk prostate carcinoma who underwent prostate permanent seed I(125) brachytherapy using an IO planning approach. The implant was done using the three-dimensional transrectal ultrasound (PRE-TRUS)-guided IO interactive inverse preplanning system. The TRUS was repeated in the operating room after the implant procedure was complete (POST-TRUS). The prostate was recontoured and postimplant dosimetry was calculated. Each patient underwent computed tomography scan on Day 28 (CT-D28) to evaluate implant quality. Area under the receiver operating characteristic curves (AUROC) was evaluated for models predictive of a V100 of > or =90% and a D90 of > or =140 Gy on the basis of CT-D28 values. RESULTS On CT-D28, 72.4% of patients had a V100 of > or =90% and 74.8% had a D90 of > or =140 Gy. AUROC for a V100 of > or =90% was 0.665 (p = 0.004) on PRE-TRUS and 0.619 (p = 0.039) on POST-TRUS. AUROC for D90 of > or =140 Gy was 0.602 (p = 0.086) on PRE-TRUS and 0.614 (p = 0.054) on POST-TRUS. Using PRE-TRUS V100 cutoff of >97% gives sensitivity of 88% and a false-positive rate of 63%. A POST-TRUS D90 cutoff of >170 Gy resulted in a sensitivity of 62% and a false-positive rate of 34%. CONCLUSIONS Because of unacceptably high false-positive rates, IO preimplant and postimplant TRUS-based dosimetry are not accurate tools to predict for postimplant computed tomography-based dosimetry.

SU‐E‐J‐87: Lung Deformable Image Registration Using Surface Mesh Deformation for Dose Distribution Combination

PURPOSE To allow a reliable deformable image registration (DIR) method for dose calculation in radiation therapy. This work proposes a performance assessment of a morphological segmentation algorithm that generates a deformation field from lung surface displacements with 4DCT datasets. METHODS From the 4DCT scans of 15 selected patients, the deep exhale phase of the breathing cycle is identified as the reference scan. Varian TPS EclipseTM is used to draw lung contours, which are given as input to the morphological segmentation algorithm. Voxelized contours are smoothed by a Gaussian filter and then transformed into a surface mesh representation. Such mesh is adapted by rigid and elastic deformations to match each subsequent lung volumes. The segmentation efficiency is assessed by comparing the segmented lung contour and the TPS contour considering two volume metrics, defined as Volumetric Overlap Error (VOE) [%] and Relative Volume Difference (RVD) [%] and three surface metrics, defined as Average Symmetric Surface Distance (ASSD) [mm], Root Mean Square Symmetric Surface Distance (RMSSD) [mm] and Maximum Symmetric Surface Distance (MSSD) [mm]. Then, the surface deformation between two breathing phases is determined by the displacement of corresponding vertices in each deformed surface. The lung surface deformation is linearly propagated in the lung volume to generate 3D deformation fields for each breathing phase. RESULTS The metrics were averaged over the 15 patients and calculated with the same segmentation parameters. The volume metrics obtained are a VOE of 5.2% and a RVD of 2.6%. The surface metrics computed are an ASSD of 0.5 mm, a RMSSD of 0.8 mm and a MSSD of 6.9 mm. CONCLUSION This study shows that the morphological segmentation algorithm can provide an automatic method to capture an organ motion from 4DCT scans and translate it into a volume deformation grid needed by DIR method for dose distribution combination.

Monte Carlo iodine brachytherapy dosimetry: study for a clinical application

At present, all clinical algorithms used in brachytherapy are based on the TG-43 algorithm, which has the advantage to offer very fast calculation time. However, this formalism has many simplifications, assuming for example the patient tissue composition equivalent to water. For low energy brachytherapy seeds such as iodine seeds, it is of interest to evaluate the dosimetric differences between calculations based on Monte Carlo simulations (considered the gold standard) and the TG-43 formalism. For a 6711 model 125I seed calculated photon spectra were compared to spectra measured with a CdTe spectrometer. Good agreement was found except for the lowest energy peak which seems to be over-estimated by the experiment due to the contribution of the spectrometer CdTe diode to the measurement. Dose distributions in water are measured with EBT Gafchromic film and compared to the Monte Carlo calculation. A very good agreement is found. Finally, the method to create a MCNPX input file from computed tomography (CT) scanner images is explained and some preliminary isodose distributions are presented.

TU‐D‐224C‐04: A Monte Carlo Post‐Implant Prostate Dosimetry Study : Interseed Attenuation, Tissue Composition, and Calcification

Purpose: To use an automated Monte Carlo (MC) code to perform a clinical dosimetry study that includes 18 post‐implant prostate brachytherapy cases. The interseed attenuation and the impact of tissue compositions are specifically investigated. Method and Materials: A MC technique is used to simulate the post‐implant dosimetry for voxelized patients. For a typical patient, 200,000 voxels are stacked to generate the realistic anatomy. For each voxel, the density is set by the Hounsfield Units (HU) from the computed tomography data and the elemental composition is set according to the radiation oncologists contours. Between 180 and 320 different materials (prostate tissue,muscle, rectum tissue, bladder tissue, calcification, adipose tissue, and various mixtures) are utilized for each patient. Data (HU, seed positions, and contours) is input into the MC program through the DICOM‐RT protocol. Several MC simulations are performed in order to study different elements: interseed attenuation, impact of inter‐organ tissue composition, prostate composition, and specifically calcifications inside the prostate. Results: The interseed attenuation level is 3.9±1.5% for the D90 CTV parameter. The effect on organs at risk is an average decrease of 0.2 cm3 for the V100 parameter in both rectum and bladder. For the 18 clinical cases, the comparison between the TG‐43 based dosimetry and the complete MC simulation leads to average differences of 12±3 Gy for the D90 CTV parameter. If localized and/or diffuse calcifications are incorporated into the prostate tissue, differences of up to 14 Gy are added. Conclusion: The overall difference between the clinically approved TG‐43 based calculations and MC simulations can reach non‐negligible levels. Including the calcifications in the prostate can lead to important dosimetry differences. However, the presence of calcification is difficult to establish with traditional CT data.

Fusion of Intraoperative Transrectal Ultrasound Images with Post-implant Computed Tomography and Magnetic Resonance Imaging

Purpose To compare the impact of the fusion of intraoperative transrectal ultrasound (TRUS) images with day 30 computed tomography (CT) and magnetic resonance imaging (MRI) on prostate volume and dosimetry. Methods and materials Seventy-five consecutive patients with CT and MRI obtained on day 30 with a Fast Spin Echo T2-weighted magnetic resonance (MR) sequence were analyzed. A rigid manual registration was performed between the intraoperative TRUS and day-30 CT based on the prostate volume. A second manual rigid registration was performed between the intraoperative TRUS and the day-30 MRI. The prostate contours were manually modified on CT and MRI. The difference in prostate volume and dosimetry between CT and MRI were compared. Results Prostate volume was on average 8% (standard deviation (SD) ± 16%) larger on intraoperative TRUS than on CT and 6% (18%) larger than on MRI. In 48% of the cases, the difference in volume on CT was > 10% compared to MRI. The difference in prostate volume between CT and MRI was inversely correlated to the difference in D90 (minimum dose that covers 90% of the prostate volume) between CT and MRI (r = -0.58, P < .001). A D90 < 90% was found in 5% (n = 4) on MRI and in 10% (n = 7) on CT (Fisher exact test one-sided P = .59), but in no patient was the D90 < 90% on both MRI and CT. Conclusions When fusing TRUS images with CT and MRI, the differences in prostate volume between those modalities remain clinically important in nearly half of the patients, and this has a direct influence on how implant quality is evaluated.

Sci‐YIS Fri ‐ 04: Clinical impact of seed density and prostate elemental composition on permanent seed implant dosimetry

Purpose: To evaluated the impact of inter‐seed attenuation and prostate elemental composition in clinical prostate treatment plans with 6711 125I permanent seed implants using the Monte Carlo(MC) method. The effect of seed density (number of seeds per prostate unit volume) is specifically investigated. Method and Materials: The MC toolkit Geant4 is used to perform the simulations. The study focuses on treatment plans that were generated for clinical cases. For each plan, four different dose calculation techniques are compared: TG43‐based calculation, superposition MC (SMC), full MC with water prostate (MCW), and full MC with realistic prostate tissue (MCP). The SMC method is a technique for which a shifted one‐source MC distribution is added to the total dose distribution for each source position. The realistic prostate composition includes the ten most abundant elements in prostate tissue based on ICRP23. Results: Seed density has a definite influence on inter‐seed attenuation. A typical low seed density (42 0.6 mCi seeds in a 26 cc prostate) corresponds to a 0.7% drop in the CTV D90 value when comparing SMC to MCW while a drop of 2.5% is calculated for a higher seed density (75 0.3 mCi seeds, same prostate). The influence of the prostate elemental composition is similar for all plans. When comparing MCW to MCP, the difference in total dose deposited in the CTV is 3.0 ±/− 0.2%, while it is 3.6 +/− 0.3% for the D90 parameter. When considering all effects, the variation on the CTV D90 value is ranging from 3.8% to 9.3% when comparing TG43 to MCP, depending on the seed density. Conclusion: The MC study establishes the dependence of inter‐seed attenuation on seed density and reveals a 3% overdose for a water prostate compared to a realistic composition. Overall, the effect on the CTV dosimetry is clinically significant.