Rodrigo A. Rubo

The contribution from transit dose for Ir-192 HDR brachytherapy treatments

Brachytherapy treatment planning systems that use model-based dose calculation algorithms employ a more accurate approach that replaces the TG43-U1 water dose formalism and adopt the TG-186 recommendations regarding composition and geometry of patients and other relevant effects. However, no recommendations were provided on the transit dose due to the source traveling inside the patient. This study describes a methodology to calculate the transit dose using information from the treatment planning system (TPS) and considering the sources instantaneous and average speed for two prostate and two gynecological cases. The trajectory of the (192)Ir HDR source was defined by importing applicator contour points and dwell positions from the TPS. The transit dose distribution was calculated using the maximum speed, the average speed and uniform accelerations obtained from the literature to obtain an approximate continuous source distribution simulated with a Monte Carlo code. The transit component can be negligible or significant depending on the speed profile adopted, which is not clearly reported in the literature. The significance of the transit dose can also be due to the treatment modality; in our study interstitial treatments exhibited the largest effects. Considering the worst case scenario the transit dose can reach 3% of the prescribed dose in a gynecological case with four catheters and up to 11.1% when comparing the average prostate dose for a case with 16 catheters. The transit dose component increases by increasing the number of catheters used for HDR brachytherapy, reducing the total dwell time per catheter or increasing the number of dwell positions with low dwell times. This contribution may become significant (>5%) if it is not corrected appropriately. The transit dose cannot be completely compensated using simple dwell time corrections since it may have a non-uniform distribution. An accurate measurement of the source acceleration and maximum speed should be incorporated in clinical practice or provided by the manufacturer to determine the transit dose component with high accuracy.

HDR Ir-192 source speed measurements using a high speed video camera

PURPOSE The dose delivered with a HDR (192)Ir afterloader can be separated into a dwell component, and a transit component resulting from the source movement. The transit component is directly dependent on the source speed profile and it is the goal of this study to measure accurate source speed profiles. METHODS A high speed video camera was used to record the movement of a (192)Ir source (Nucletron, an Elekta company, Stockholm, Sweden) for interdwell distances of 0.25-5 cm with dwell times of 0.1, 1, and 2 s. Transit dose distributions were calculated using a Monte Carlo code simulating the source movement. RESULTS The source stops at each dwell position oscillating around the desired position for a duration up to (0.026 ± 0.005) s. The source speed profile shows variations between 0 and 81 cm/s with average speed of ∼ 33 cm/s for most of the interdwell distances. The source stops for up to (0.005 ± 0.001) s at nonprogrammed positions in between two programmed dwell positions. The dwell time correction applied by the manufacturer compensates the transit dose between the dwell positions leading to a maximum overdose of 41 mGy for the considered cases and assuming an air-kerma strength of 48 000 U. The transit dose component is not uniformly distributed leading to over and underdoses, which is within 1.4% for commonly prescribed doses (3-10 Gy). CONCLUSIONS The source maintains its speed even for the short interdwell distances. Dose variations due to the transit dose component are much lower than the prescribed treatment doses for brachytherapy, although transit dose component should be evaluated individually for clinical cases.

Determination of transit dose profile for a 192Ir HDR source

PURPOSE Several studies have reported methodologies to calculate and correct the transit dose component of the moving radiation source for high dose rate (HDR) brachytherapy planning systems. However, most of these works employ the average source speed, which varies significantly with the measurement technique used, and does not represent a realistic speed profile, therefore, providing an inaccurate dose determination. In this work, the authors quantified the transit dose component of a HDR unit based on the measurement of the instantaneous source speed to produce more accurate dose values. METHODS The Nucletron microSelectron-HDR Ir-192 source was characterized considering the Task Group 43 (TG-43U1) specifications. The transit dose component was considered through the calculation of the dose distribution using a Monte Carlo particle transport code, MCNP5, for each source position and correcting it by the source speed. The instantaneous source speed measurements were performed in a previous work using two optical fibers connected to a photomultiplier and an oscilloscope. Calculated doses were validated by comparing relative dose profiles with those obtained experimentally using radiochromic films. RESULTS TG-43U1 source parameters were calculated to validate the Monte Carlo simulations. These agreed with the literature, with differences below 1% for the majority of the points. Calculated dose profiles without transit dose were also validated by comparison with ONCENTRA(®) Brachy v. 3.3 dose values, yielding differences within 1.5%. Dose profiles obtained with MCNP5 corrected using the instantaneous source speed profile showed differences near dwell positions of up to 800% in comparison to values corrected using the average source speed, but they are in good agreement with the experimental data, showing a maximum discrepancy of approximately 3% of the maximum dose. Near a dwell position the transit dose is about 22% of the dwell dose delivered by the source dwelling 1 s and reached 104.0 cGy per irradiation in a hypothetical clinical case studied in this work. CONCLUSIONS The present work demonstrated that the transit dose correction based on average source speed fails to accurately correct the dose, indicating that the correct speed profile should be considered. The impact on total dose due to the transit dose correction near the dwell positions is significant and should be considered more carefully in treatments with high dose rate, several catheters, multiple dwell positions, small dwell times, and several fractions.

Implementation of a quality assurance program for computerized treatment planning systems

In the present investigation, the necessary tests for implementing a quality assurance program for a commercial treatment planning system (TPS), recently installed at Sao Paulo University School of Medicine Clinicas Hospital-Brazil, was established and performed in accordance with the new IAEA publication TRS 430, and with AAPM Task Group 53. The tests recommended by those documents are classified mainly into acceptance, commissioning (dosimetric and nondosimetric), periodic quality assurance, and patient specific quality assurance tests. The recommendations of both IAEA and AAPM documents are being implemented at the hospital for photon beams produced by two linear accelerators. A Farmer ionization chamber was used in a 30×30×30cm3 phantom with a dose rate of 320 monitor unit (MU)/min and 50 MU in the case of the dosimetric tests. The acceptance tests verified hardware, network systems integration, data transfer, and software parameters. The results obtained are in good agreement with the specifications of the manufacturer. For the commissioning dosimetric tests, the absolute dose was measured for simple geometries, such as square and rectangular fields, up to more complex geometries such as off-axis hard wedges and for behavior in the build up region. Results were analysed by the use of confidence limit as proposed by Venselaar et al. [Radio Ther. Oncol. 60, 191-201 (2001)]. Criteria of acceptability had been applied also for the comparison between the values of MU calculated manually and MU generated by TPS. The results of the dosimetric tests show that work can be reduced by choosing to perform only those that are more crucial, such as oblique incidence, shaped fields, hard wedges, and buildup region behavior. Staff experience with the implementation of the quality assurance program for a commercial TPS is extremely useful as part of a training program.

ANÁLISES DE PROTOCOLOS TELETERÁPICOS DE CONTROLE DE QUALIDADE DE ALGUNS SERVIÇOS LOCAIS, BASEADOS NO TG40 E ARCAL XXX

In view of the great importance of quality control in radiotherapy services, this paper aimed primarily to evaluate the tests recommended by international protocols TG40 and ARCAL XXX for teletherapic equipments (cobalt, linear accelerator and simulator). A second objective was to evaluate the tests currently used in some radiotherapy services in Brazil and Latin America and to compare these tests with the ones recommended by the international protocols. Our results suggest that ARCAL is more complete than TG40, although the latter includes all the essential basic tests. We concluded that radiotherapy services should implement all basic quality control tests and that all other complementary tests should be implemented according to the need of each service. Finally, suggestions of protocols are presented, elaborated from the official and routine protocols used.

Avaliação dosimétrica de uma combinação de aplicadores para braquiterapia de tumores do colo uterino com acometimento da porção distal da vagina

OBJECTIVE: To evaluate an alternative brachytherapy technique for uterine cervix cancer involving the distal vagina, without increasing the risk of toxicity. MATERIALS AND METHODS: Theoretical study comparing three different high-dose rate intracavitary brachytherapy applicators: intrauterine tandem and vaginal cylinder (TC); tandem/ring applicator combined with vaginal cylinder (TR+C); and a virtual applicator combining both the tandem/ring and vaginal cylinder in a single device (TRC). Prescribed doses were 7 Gy at point A, and 5 Gy on the surface or at a 5 mm depth of the vaginal mucosa. Doses delivered to the rectum, bladder and sigmoid colon were kept below the tolerance limits. Volumes covered by the isodoses, respectively, 50% (V50), 100% (V100), 150% (V150) and 200% (V200) were compared. RESULTS: Both the combined TR+C and TRC presented a better dose distribution as compared with the TC applicator. The TR+C dose distribution was similar to the TRC dose, with V150 and V200 being about 50% higher for TR+C (within the cylinder). CONCLUSION: Combined TR+C in a two-time single application may represent an alternative therapy technique for patients affected by uterine cervix cancer involving the distal vagina.

Evaluation of different magnetic resonance imaging contrast materials to be used as dummy markers in image-guided brachytherapy for gynecologic malignancies

Objective To identify a contrast material that could be used as a dummy marker for magnetic resonance imaging. Materials and Methods Magnetic resonance images were acquired with six different catheter-filling materials-water, glucose 50%, saline, olive oil, glycerin, and copper sulfate (CuSO4) water solution (2.08 g/L)-inserted into compatible computed tomography/magnetic resonance imaging ring applicators placed in a phantom made of gelatin and CuSO4. The best contrast media were tested in four patients with the applicators in place. Results In T2-weighted sequences, the best contrast was achieved with the CuSO4-filled catheters, followed by saline- and glycerin-filled catheters, which presented poor visualization. In addition (also in T2-weighted sequences), CuSO4 presented better contrast when tested in the phantom than when tested in the patients, in which it provided some contrast but with poor identification of the first dwell position, mainly in the ring. Conclusion We found CuSO4 to be the best solution for visualization of the applicator channels, mainly in T2-weighted images in vitro, although the materials tested presented low signal intensity in the images obtained in vivo, as well as poor precision in determining the first dwell position.

Testes dosimétricos para comissionamento de sistemas de planejamento em radioterapia 3DCRT

A evolucao da Radioterapia 2D para a Radioterapia 3D conformacional deve-se ao advento dos diversos sistemas de planejamento comercialmente disponiveis e das tecnicas de imagem tridimensionais, como a tomografia computadorizada. Os sistemas de planejamento possuem ferramentas que permitem delinear tridimensionalmente os volumes das estruturas envolvidas em um tratamento a partir de imagens tomograficas, alem de contar com ferramentas de calculo de dose, que permitem avaliar a dose recebida em cada uma das estruturas delineadas. A aquisicao de um desses sistemas ou a atualizacao de versao deve ser acompanhada pela realizacao de diversos testes dosimetricos e nao dosimetricos com a finalidade de determinar as limitacoes e verificar o correto funcionamento do sistema, alem de verificar que a insercao dos dados de comissionamento no Sistema de Planejamento foi feita corretamente. Este trabalho foi baseado nos protocolos da Agencia Internacional de Energia Atomica (IAEA), documentos Task Group (TG) da Associacao Americana de Fisica em Medicina (AAPM) e artigos da literatura. Realizou-se uma serie de testes dosimetricos com a finalidade de comissionar o novo sistema de planejamento Eclipse 10.0.28 (Varian Medical Systems).Tal versao possui dois algoritmos de calculo para fotons (Pencil Beam Convolution (PBC) e Analytical Anisotropic Algorithm - AAA) e algoritmo Gaussian Pencil Beam para eletrons. No entanto, nao foram realizados testes para o AAA. Os resultados mostraram que os dados dosimetricos foram inseridos corretamente no Sistema de Planejamento. Alguns resultados permitiram entender as limitacoes de calculo de dose do sistema de planejamento Eclipse 10.0.28, em algumas situacoes, as quais foram consideradas clinicamente irrelevantes na rotina do servico.

Residence program on Medical Physics at Hospital das Clínicas of São Paulo University

The main goal of the Residence Program on Medical Physics at Clinicas Hospital is to provide a specialization course of 24 months. The candidate selection is made in 2 steps: a general examination with 50 questions of multiple choices and a specific examination which does not presuppose the candidates must have certain knowledge in the area. After this second written exam, an interview is promoted in order to evaluate other aspects not covered by the previous exams, including some ethical issues, the ability to deal with patients and the multidisciplinary aspect among other professionals in the hospital. The Board Committee also evaluates the candidates’ curriculum according to rules established by the Residence Coordination (participation in scientific meetings, trainings performed during graduation in Radiotherapy and scientific publications). The residence is thus directed to students which concluded bachelor’s degree or a degree in Physics, and aims to train professionals skilled in Radiotherapy area, including the obtaining of further qualified professional title offered by the Brazilian Association of Medical Physics. The students attend lectures, seminars and the routine of the hospital. They also perform experimental work and develop a monograph to be presented at the end of the residence’s period. In the last 5 years, almost 50 candidates have been attending to the selective process for only 2 available positions per year; the candidates come from different parts of the country, including students from the North, Northeast, Central-West, South and Southeast regions. It should be noticed in the last 5 years that most of the approved candidates are students coming from Graduation on Medical Physics, especially in the Sao Paulo state. For the last 10 residents, 20% were hired by companies in the Radiotherapy area; the remaining residents were hired by hospitals, 20% have found jobs in the Northeast region and 60% in Sao Paulo state.

Quality Assurance Program for Radiosurgery at Clinicas Hospital: Results of Implementation

Due to the complexity of the Radiosurgery and fractioned stereotactic radiotherapy processes, it is mandatory to establish a proper quality assurance program for each patient to be treated with this treatment modality [1]. In this work, such program will be described. The QA tests require a proper phantom as well as appropriate dosimetric equipments such as ion chambers and stereotactic diodes, solid water phantom, radiographic films in order to perform all periodic tests.