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Investigation of Dose Perturbations Around Brachytherapy Seeds in High-Energy Photon Beams

S Ogawa1*, K Yasui2, Y Saito3, K Ouchi1, J Nagata1, N Hayashi2, (1) Graduate school of Health Sciences, Fujita Health University, Toyoake, Aichi, JP, (2) School of Medical Sciences, Fujita Health University,Toyoake, Aichi,JP, (3) Division Of Radiology, Fujita Health University Hospital,Toyoake, Aichi,JP,

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose:
Combination of external-beam radiation therapy and brachytherapy (combo-RT) is useful for the treatment of high-risk prostate cancer. However, the dose calculations on the treatment planning system is inaccurate due to the scattering dose around high atomic number materials like brachytherapy seeds (BTS). The purpose of this study is to investigate dose perturbations around BTS in high-energy photon beams.

Methods:
We calculated the dose distribution around BTS by Monte Carlo (MC) simulation under several geometrical conditions. The calculation of dose distribution for two kinds of seed models TheraAgX100 (TAX: Theragenics Corporation) and Bard BRACHYSOURCE STM125I (BBS: C.R.Bard, Inc.) were carried out with Monte Carlo code (PHITS ver.3.11). Phase space files of the photon beam (6 MV, 10 MV; with flattening filter (WFF), flattening filter-free (FFF)) with TrueBeam (Varian Medical Systems) provided by Varian Medical Systems were used for the source of MC simulation. MC simulations were undergone in two geometric conditions with one BTS at a depth of 5.0 cm (condition (a)) or three BTS at the depths of 4.8, 5.0 and 5.2 cm (condition (b)) on the beam axis in water. These calculation results were compared with dose distribution without BTS

Results:
As a result of condition (a), BTS yielded a maximum dose enhancement of 11.9% (TAX, 10 MV-FFF) upstream and a maximum dose reduction of -11.6% (TAX, 6 MV-FFF) downstream of BTS. As a result of condition (b), BTS yielded a maximum dose enhancement of 13.0% (BBS, 6 MV-WFF) upstream and a maximum dose reduction of -14.9% (TAX, 10 MV-WFF) downstream of the photon beam.

Conclusion:
Dose perturbations around BTS in high-energy photon beams were confirmed in this investigation. The magnitudes of dose perturbations are depended largely on the placement of BTS in combo-RT.

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Keywords

Monte Carlo, Brachytherapy, Backscatter

Taxonomy

TH- External Beam- Photons: Computational dosimetry engines- Monte Carlo

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