The Effects of Backscattered Radiation into Beam Monitor Chamber: Study of 6 and 18 MV Conventional and Removed Flattening Filter Clinical Accelerator

Mansour Zabihzadeh^1,2*, Seyyed Rabi Mahdavi³, Mohammad Reza Ay⁴ and Zahra Shakarami¹

¹Department of Medical Physics, School of Medicine, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran.

²Department of Radiotherapy and Radiation Oncology, Golestan Hospital, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran.

³Department of Medical Physics, School of Medicine, Iran University of Medical Sciences, Tehran, Iran.

⁴Department of Medical Physics and Biomedical Engineering, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran.

DOI : http://dx.doi.org/10.13005/bbra/1601

ABSTRACT: In some linear accelerators (Linac), the collected charges in beam monitor chamber (BMC) is partly caused by the backscattered particles from the accelerator components downstream the BMC that influence the Linac output factors. In the intensity modulated radiation therapy technique, the desired dose distribution can be achieved through an unflattened beam. Although removing the flattening filter provides some advantages, the amount of backscatter radiation into BMC can be changed. In this study, contribution of backscattered particles into the BMC response of a Varian 2300 C/D Linac with and without a flattening filter was determined for 6, 18 MV photon beams. The experimental procedure included telescopic method and calculation procedure consisted of Monte Carlo simulation (MCNPX, version 2.4.0), were used to investigate the contribution of backscattered particles into the BMC performance. Our results showed a 2.3 % and 3 % increase in backscatter for a 0.5 × 0.5 cm2 field compared to a 40 × 40 cm2 field for 6 MV and 18 MV, respectively. The energy deposition from backscattered radiation is mainly caused by backscattered electrons. Removing the flattening filter did not change the BMC performance for a conventional Linac with a flattening filter. However, this result was not valid for small fields (e.g. 0.5 × 0.5 cm2, 18 MV). The corrected backscatter factors is necessary to taking into account the contribution of backscattered radiation in the monitor chamber response for small fields in the case of the free flattening filter Linacs (18 MV).

KEYWORDS: Backscatter radiation; Flattening filter; Linac; MC simulation; Telescopic analysis

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