Paula Cristina Guimarães Antunes

The accuracy required in small-field dosimetry for stereotactic radiosurgery imposes limitations on the conventional dosimetric calibration protocols used in radiotherapy. This study presents the development and validation of a computational model of the CyberKnife® system using the Monte Carlo method with the MCNP6 code. The main objective is to establish a validated computational model that enables future research and clinical applications in stereotactic radiosurgery. The model was developed to validate the 60 mm conical collimator, and the results were compared with experimental data from percentage depth dose (PDD) and lateral dose profile (LDP) curves at depths of 10 cm and 20 cm. The analyses revealed the influence of field size and photon source characteristics on the dose distribution, showing good agreement between the simulated and measured data within the established tolerance limits. These findings demonstrate that the proposed model accurately reproduces the CyberKnife® beam behavior and serves as a robust and promising tool for the investigation of treatment variables, support of therapeutic planning, and optimization of stereotactic radiosurgery protocols.

The Nuclear and Energy Research Institute and the National Nuclear Energy Commission (IPEN-CNEN/SP) are developing a new Iodine-125 seed for use in brachytherapy. The manufacturing process includes cutting the titanium tube, welding the ends, and performing leak tests. Seed approval is based on a microscopic analysis of its external geometry. The dosimetric methodology follows Task Group 43 (TG-43), which adopts the Monte Carlo method as the standard simulation technique. However, TG-43 does not account for geometric variations that may occur during the manufacturing process. This study investigated two types of variation: (1) the seed width after welding, and (2) the random positions that the core may assume within the seed. A total of 100 simulations were performed for each variation. The width was measured experimentally using a caliper, after welding, while the core position was modeled using a random number generator, since it is not feasible to measure it directly. These values were used to generate 201 input files for simulations in MCNP 6.2: 100 for each investigated parameter and one for the reference geometry (IPEN-CNEN/SP seed). The results were analyzed using MATLAB through bidimensional matrices of 101 × 101 points. For each point in the matrix, the mean, standard deviation, relative difference (compared to the reference geometry), and Type A uncertainty were calculated. The main results were: a standard deviation of 0.6% and a relative difference of 11% for the seed width; and a standard deviation of 50% and a relative difference of 25% for the core position. The Type A uncertainty was below 0.3% in all cases.

Objective. To estimate dose rates delivered by using radioactive ¹⁹⁸Au nanoparticles for prostate cancer nanobrachytherapy, identifying contribution by photons and electrons emmited from the source. Approach. Utilizing in silico models, two different anatomical representations were compared: a mathematical model and a unstructured mesh model based on the International Commission on Radiological Protection (ICRP) Publication 145 phantom. Dose rates by activity were calculated to the tumor and nearby healthy tissues, including healthy prostate tissue, urinary bladder wall and rectum, using Monte Carlo code MCNP6.2. Main results. Results indicate that both models provide dose rate estimates within the same order of magnitude, with the mathematical model overestimating doses to the prostate and bladder by approximately 20% compared to the unstructured mesh model. The discrepancies for the tumor and rectum were below 4%. Photons emmited from the source were defined as the primary contributors to dose to other organs, while 97.9% of the dose to the tumor was due to electrons emmited from the source. Significance. Our findings emphasize the importance of model selection in dosimetry, particularly the advantages of using realistic anatomical phantoms for accurate dose calculations. The study demonstrates the feasibility and effectiveness of ¹⁹⁸Au nanoparticles in achieving high dose concentrations in tumor regions while minimizing exposure to surrounding healthy tissues. Beta emissions were found to be predominantly responsible for tumor dose delivery, reinforcing the potential of ¹⁹⁸Au nanoparticles in localized radiation therapy. We advocate for using realistic body phantoms in further research to enhance reliability in dosimetry for nanobrachytherapy, as the field still lacks dedicated protocols.

A eficiência de procedimentos radioterápicos depende do equilíbrio entre o fornecimento de altas doses conformadas ao volume tumoral e a restrição das doses recebidas pelos tecidos e órgãos saudáveis circundantes. Sendo uma modalidade de radioterapia, a protonterapia destaca-se neste cenário por possuir vantagens dosimétricas, que, quando combinadas com avanços tecnológicos, permitem que um grande potencial na conformidade da distribuição de dose. Este trabalho visa contribuir em um estudo dosimétrico, especificamente considerando os efeitos da heterogeneidade devido à presença de materiais tecido equivalentes com diferentes densidades e composições químicas, de modo a analisar qual destes parâmetros exerce maior influência na distribuição de dose longitudinal. A metodologia desenvolvida neste trabalho foi baseada em simulações de Monte Carlo com o código GEANT4 (através da interface TOPAS). Os objetos simuladores cilíndricos representados foram compostos inteiramente por diversos materiais tecido-equivalentes. Três grupos de estudo guiaram as simulações, o primeiro manteve a composição e densidade originais dos materiais, ao seguinte foi atribuída a todos os materiais heterogêneos a mesma densidade da água, mas mantiveram-se suas composições químicas originais; e por fim, foram realizadas simulações com as densidades originais dos materiais heterogêneos e composição química da água para todos os casos. Através da análise da distribuição de dose longitudinal variando com a profundidade, foi possível observar o comportamento da influência dos parâmetros de composição e densidade no alcance do feixe (d90) para os diferentes materiais e energias analisados. O estudo mostrou que, o efeito que a densidade dos materiais tecido equivalentes exerce sobre a deposição de dose é mais expressivo que o efeito de sua composição. A maior exatidão no range de tratamento permite evitar uma sub ou sobre dosagem da área irradiada. Esta é uma das diversas linhas de pesquisa que contribuem para a diminuição das incertezas em protonterapia.

One of he most difficult steps when preparing a Monte Carlo calculation is the design of their geometries. Such process is an error-prone, time-consuming, and complex step for any simulation in the field of medical physics. The software VoxelMages has been developed to help the user in this complex task. It allows to design arbitrary geometries and to process DICOM image files for simulations with the general-purpose Monte Carlo code PENELOPE. Its main characteristics are described in the following.