Determination of transit dose profile for a Ir-192 HDR source

Author(s)

Fonseca, G. P. ^{[1, 2]} ; Rubo, R. A. ^[3] ; Minamisawa, R. A. ^[4] ; dos Santos, G. R. ^[3] ; Antunes, P. C. G. ^[2] ; Yoriyaz, H. ^[2]

Affiliation

^[1] Maastricht Univ, Med Ctr, GROW Sch Oncol & Dev Biol, Dept Radiat Oncol MAASTRO, NL-6201 BN Maastricht - Netherlands

^[2] CNEN SP, Inst Pesquisas Energet & Nucl IPEN, BR-05508000 Sao Paulo - Brazil

^[3] Univ Sao Paulo, HC FMUSP, Hosp Clin, BR-05403900 Sao Paulo - Brazil

^[4] Paul Scherrer Inst, Lab Micro & Nanotechnol, CH-5232 Villigen - Switzerland

• Document type: Journal article
• Source: Medical Physics; v. 40, n. 5 MAY 2013.
• Web of Science Citations: 6

Abstract

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 (R) 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. (c) 2013 American Association of Physicists in Medicine. (AU)

• FAPESP's process: 11/22778-8 - 3D dosimetry based on medical images and Monte Carlo codes to use in brachytherapy
• Grantee: Hélio Yoriyaz
• Support Opportunities: Regular Research Grants
• FAPESP's process: 11/23765-7 - Dose-volume relationship in planning systems for 3D brachytherapy using mcnp5 code, brachyvision and oncentra
• Grantee: Gabriel Paiva Fonseca
• Support Opportunities: Scholarships abroad - Research Internship - Doctorate
• FAPESP's process: 11/01913-4 - Monte Carlo modelling of the patient and treatment delivery complexities for high dose rate brachytherapy
• Grantee: Gabriel Paiva Fonseca
• Support Opportunities: Scholarships in Brazil - Doctorate