Development of advanced geometric models and acceleration techniques for Monte Carlo simulation in Medical Physics

Els programes de simulació Monte Carlo de caràcter general s'utilitzen actualment en una gran varietat d'aplicacions.Tot i això, els models geomètrics implementats en la majoria de programes imposen certes limitacions a la forma dels objectes que es poden definir. Aquests models no són adequats per descriure les superfícies arbitràries que es troben en estructures anatòmiques o en certs aparells mèdics i, conseqüentment, algunes aplicacions que requereixen l'ús de models geomètrics molt detallats no poden ser acuradament estudiades amb aquests programes.L'objectiu d'aquesta tesi doctoral és el desenvolupament de models geomètrics i computacionals que facilitin la descripció dels objectes complexes que es troben en aplicacions de física mèdica. Concretament, dos nous programes de simulació Monte Carlo basats en PENELOPE han sigut desenvolupats. El primer programa, penEasy, utilitza un algoritme de caràcter general estructurat i inclou diversos models de fonts de radiació i detectors que permeten simular fàcilment un gran nombre d'aplicacions. Les noves rutines geomètriques utilitzades per aquest programa, penVox, extenen el model geomètric estàndard de PENELOPE, basat en superfícices quàdriques, per permetre la utilització d'objectes voxelitzats. Aquests objectes poden ser creats utilitzant la informació anatòmica obtinguda amb una tomografia computeritzada i, per tant, aquest model geomètric és útil per simular aplicacions que requereixen l'ús de l'anatomia de pacients reals (per exemple, la planificació radioterapèutica). El segon programa, penMesh, utilitza malles de triangles per definir la forma dels objectes simulats. Aquesta tècnica, que s'utilitza freqüentment en el camp del disseny per ordinador, permet representar superfícies arbitràries i és útil per simulacions que requereixen un gran detall en la descripció de la geometria, com per exemple l'obtenció d'imatges de raig x del cos humà.Per reduir els inconvenients causats pels llargs temps d'execució, els algoritmes implementats en els nous programes s'han accelerat utilitzant tècniques sofisticades, com per exemple una estructura octree. També s'ha desenvolupat un paquet de programari per a la paral·lelització de simulacions Monte Carlo, anomentat clonEasy, que redueix el temps real de càlcul de forma proporcional al nombre de processadors que s'utilitzen.Els programes de simulació que es presenten en aquesta tesi són gratuïts i de codi lliures. Aquests programes s'han provat en aplicacions realistes de física mèdica i s'han comparat amb altres programes i amb mesures experimentals.Per tant, actualment ja estan llestos per la seva distribució pública i per la seva aplicació a problemes reals.Monte Carlo simulation of radiation transport is currently applied in a large variety of areas. However, the geometric models implemented in most general-purpose codes impose limitations on the shape of the objects that can be defined. These models are not well suited to represent the free-form (i.e., arbitrary) shapes found in anatomic structures or complex medical devices. As a result, some clinical applications that require the use of highly detailed phantoms can not be properly addressed.This thesis is devoted to the development of advanced geometric models and accelration techniques that facilitate the use of state-of-the-art Monte Carlo simulation in medical physics applications involving detailed anatomical phantoms. To this end, two new codes, based on the PENELOPE package, have been developed. The first code, penEasy, implements a modular, general-purpose main program and provides various source models and tallies that can be readily used to simulate a wide spectrum of problems. Its associated geometry routines, penVox, extend the standard PENELOPE geometry, based on quadric surfaces, to allow the definition of voxelised phantoms. This kind of phantoms can be generated using the information provided by a computed tomography and, therefore, penVox is convenient for simulating problems that require the use of the anatomy of real patients (e.g., radiotherapy treatment planning). The second code, penMesh, utilises closed triangle meshes to define the boundary of each simulated object. This approach, which is frequently used in computer graphics and computer-aided design, makes it possible to represent arbitrary surfaces and it is suitable for simulations requiring a high anatomical detail (e.g., medical imaging).A set of software tools for the parallelisation of Monte Carlo simulations, clonEasy, has also been developed. These tools can reduce the simulation time by a factor that is roughly proportional to the number of processors available and, therefore, facilitate the study of complex settings that may require unaffordable execution times in a sequential simulation.The computer codes presented in this thesis have been tested in realistic medical physics applications and compared with other Monte Carlo codes and experimental data. Therefore, these codes are ready to be publicly distributed as free and open software and applied to real-life problems.Postprint (published version

Treatment delivery verification using a pelvic anthropormophic phantom

Aim: A series of measurements in a pelvic anthropomorphic phantom was performed as part of the validation of the entire radiotherapy treatment chain. Two treatment planning calculation algorithms were used: the Pencil Beam (PB) and the Collapsed Cone (CC). The dose calculation algorithms of Radiotherapy Treatment Planning Systems (RTPS) were validated to ensure that the dose delivered to a treatment target was accurately predicted. An anthropomorphic phantom, designed and manufactured locally at the Pretoria Academic Hospital, was employed in this study. The phantom was fabricated with locally available materials as human tissue substitutes based on the attenuation coefficients, electron densities and effective atomic numbers. Materials: Pelvic anthropomorphic phantom, Thermoluminescent Dosimeters (TLD_100 chips), X-Omat V film, film processors, a densitometer and 6 MV and 15 MV linear accelerator photon beams with beam quality (TPR20,10) of 0.674 and 0.763 respectively. Results: Two treatment planning techniques were studied, a four field “box” and parallel opposed beams using a local cancer of the cervix protocol. Point doses calculated by the RTPS were compared with equivalent point dose values measured with thermoluminescent dosimeters (TLDs). Three dose regions emerged for the four field technique, those of low, intermediate and high dose gradient. The four field technique for 6 MV gave a dose deviation from -4.9% to -32.1% and at 15 MV from -1.5% to -20.6%. For 6 MV and 15 MV parallel opposed beams, percentage dose deviations from -2.7% to -11.8% and from +0.2% to -11.5 were observed. The mean value of the ratios of measured to calculated dose values was 0.91±0.05 for the four field technique and 0.94±0.02 for the AP/PA. Radiographic film was used to compare the predicted 2D isodose distributions to the actual dose distribution in the phantom. The 2D isodose distributions obtained were not meaningful in comparing the doses predicted by the planning system. A smaller field size of 7 cm x 7 cm was also employed and results of both TLD and film obtained were comparable to those predicted by the planning system. iv Conclusion: The stated goal of dose delivery accuracy (ICRU, 1987) to within 5% was not generally met in this study. On average the measured doses using TLDs and film at a field size of 7 cm x 7 cm were lower than the point doses predicted by the RTPS dose calculation algorithms whereas the film over-responded when a local cancer of the cervix protocol was employed. At a field size of 7 cm x 7 cm, film dosimetry was comparable to the TLD results. Film and TLDs were calibrated perpendicularly and exposed parallel. The phantom is unsuitable for film dosimetry studies at field sizes more than 14 cm x 14 cm

The effects of anatomic resolution, respiratory variations and dose calculation methods on lung dosimetry

The goal of this thesis was to explore the effects of dose resolution, respiratory variation and dose calculation method on dose accuracy. To achieve this, two models of lung were created. The first model, called TISSUE, approximated the connective alveolar tissues of the lung. The second model, called BRANCH, approximated the lungs bronchial, arterial and venous branching networks. Both models were varied to represent the full inhalation, full exhalation and midbreath phases of the respiration cycle. To explore the effects of dose resolution and respiratory variation on dose accuracy, each model was converted into a CT dataset and imported into a Monte Carlo simulation. The resulting dose distributions were compared and contrasted against dose distributions from Monte Carlo simulations which included the explicit model geometries. It was concluded that, regardless of respiratory phase, the exclusion of the connective tissue structures in the CT representation did not significantly effect the accuracy of dose calculations. However, the exclusion of the BRANCH structures resulted in dose underestimations as high as 14\% local to the branching structures. As lung density decreased, the overall dose accuracy marginally decreased. To explore the effects of dose calculation method on dose accuracy, CT representations of the lung models were imported into the Pinnacle$^3$ treatment planning system. Dose distributions were calculated using the collapsed cone convolution method and compared to those derived using the Monte Carlo method. For both lung models, it was concluded that the accuracy of the collapsed cone algorithm decreased with decreasing density. At full inhalation lung density, the collapsed cone algorithm underestimated dose by as much as 15\%. Also, the accuracy of the CCC method decreased with decreasing field size. Further work is needed to determine the source of the discrepancy

VOXEL-LEVEL ABSORBED DOSE CALCULATIONS WITH A DETERMINISTIC GRID-BASED BOLTZMANN SOLVER FOR NUCLEAR MEDICINE AND THE CLINICAL VALUE OF VOXEL-LEVEL CALCULATIONS

Voxel-level absorbed dose (VLAD) is rarely calculated for nuclear medicine (NM) procedures involving unsealed sources or 90Y microspheres (YM). The current standard of practice for absorbed dose calculations in NM utilizes MIRD S-values, which 1) assume a uniform distribution in organs, 2) do not use patient specific geometry, and 3) lack a tumor model. VLADs overcome these limitations. One reason VLADs are not routinely performed is the difficulty in obtaining accurate absorbed doses in a clinically acceptable time. The deterministic grid-based Boltzmann solver (GBBS) was recently applied to radiation oncology where it was reported as fast and accurate for both megavoltage photons and high dose rate nuclide-based photon brachytherapy. This dissertation had two goals. The first was to demonstrate that the general GBBS code ATTILA™ can be used for VLADs in NM, where primary photon and electron sources are distributed throughout a patient. The GBBS was evaluated in voxel-S-value geometries where agreement with Monte Carlo (MC) in the source voxel was 6% for 90Y and 131I; 20% differences were seen for mono-energetic 10 keV photons in bone. An adaptive tetrahedral mesh (ATM) generation procedure was developed using information from both the SPECT and CT for 90Y and 131I patients. The ATM with increased energy transport cutoffs, enabled GBBS transport to execute in under 2 (90Y) and 10 minutes (131I). GBBS absorbed doses to tumors and organs were within 4.5% of MC. Dose volume histograms were indistinguishable from MC. The second goal was to demonstrate VLAD value using 21 YM patients. Package insert dosimetry was not able to predict mean VLAD tumor absorbed doses. Partition model had large bias (factor of 0.39) and uncertainty (±128 Gy). Dose-response curves for hepatocellular carcinoma tumors were generated using logistic regression. The dose covering 70% of volume (D70) predicted binary modified RECIST response with an area under the curve of 80.3%. A D70 88 Gy threshold yielded 89% specificity and 69% sensitivity. The GBBS was shown to be fast and accurate, flaws in clinical dosimetry models were highlighted, and dose-response curves were generated. The findings in this dissertation support the adoption of VLADs in NM

Advanced dose calculations and imaging in prostate brachytherapy treatment planning

Brachytherapy using low dose rate (LDR) permanent seed implant or high dose rate (HDR) temporary implant is a well established treatment for prostate cancer. This study investigates the use of advanced dose calculation and imaging techniques to improve clinical prostate brachytherapy treatments. Monte Carlo (MC) simulations are used to assess the impact of source interactions and tissue composition effects that are ignored by the TG-43U1 dose calculation algorithm used in clinical practice. MC simulation results are validated using experimental phantom measurements. The development of prostate cancer may be driven by a dominant intra-prostatic lesion (DIL) but standard brachytherapy treatments prescribe the same dose level to the whole prostate. This study assesses the feasibility of multi-parametric (mp-MRI) guided focal boost treatments that escalate dose to the DIL to improve tumour control and of focal treatments that target the DIL to reduce treatment related side effects. Source interactions and tissue effects are shown to reduce the dose that is delivered to patients in LDR treatments, particularly for patients with calcifications, however the dosimetric impact is small compared to other uncertainties in LDR seed implant brachytherapy. For HDR treatments attenuation by steel catheters has only a small impact on dose distributions. Feasibility of mp-MRI guided focal boost HDR prostate brachytherapy is demonstrated in terms of tumour delineation and the ability to dose escalate the DIL without increased dose to normal tissues. The dosimetric feasibility of LDR and HDR focal therapy treatments is demonstrated. Focal therapy treatments are shown to be more sensitive to source position errors than whole gland treatments. MC simulations of focal therapy treatments show that there are no additional concerns in terms of dosimetric accuracy compared to standard whole gland treatments. Advanced dose calculation and imaging techniques can improve clinical prostate brachytherapy treatments

Development of techniques for verification of advanced radiotherapy by portal dosimetry

This research work is related to the development of an enhanced method for the treatment verification of Intensity Modulated Radiotherapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT). Such advanced treatment techniques require accurate verification procedures to ensure treatments are delivered as correctly as possible. This work focused on the use of the Varian aS1000 Electronic Portal Imaging Device (EPID) with Dosimetry Check software-based verification system. This EPID-based patient dose verification had been widely discussed and proposed as a way to achieve treatment delivery accuracy and patient safety, and as an ‘in vivo’ verification technique that helps to avoid or minimise dosimetric errors. In this work, a novel matrix-based software method to correct for backscatter effects from the Varian aS1000 EPID support arm has been developed. The methodology allows a reliable quantification of the backscatter effect to be applied directly to the Dosimetry Check calibration and verification system. This process includes the use of a clinical treatment planning system (Oncentra MasterPlan, Nucletron) to calculate predicted dose distribution within a phantom or patient, which may be compared to the dose reconstructed by Dosimetry Check. It has been demonstrated that the developed method can be applied to both ‘pre-treatment’ and ‘on treatment’ portal dosimetry for IMRT Head-and-Neck. The Gamma Index Method confirmed excellent validation rates of 97% (3%/3mm) and 95% (5%/3mm) for the ‘pre-treatment’ and ‘on treatment’ approach respectively. Pre-treatment verification of VMAT Head-and Neck treatment also reported excellent validation rates of 96% (3%/5mm). In addition, a convenient way to use the developed methodology within Dosimetry Check software was also piloted and tested. This presents an opportunity of future clinical implementation of the techniques developed in this investigation