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Penumbral Width Calculation for Flattening Filter Free Beam 7MV and Flattened Beam 6MV Using Inflection Point

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S Sinha

Sujit Nath Sinha*, Girigesh Yadav, S Ashokkumar, Kothanda Raman, Rajesh Thiyagarajan, Manindra Mishra, Rajiv Gandhi Cancer Institute & Research Centre, New Delhi, India N Arunai Nambiraj, SAS , VIT University, Vellore, India

SU-E-T-42 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: The purpose of the study was to investigate penumbra for Flattening filter free beam 7MV(7MV UFB) and 6MV(6MV FB) flattened beam by inflection point method and verify with conventional penumbra width for 6MV FB.

Methods: The conventional definitions of the penumbra based 80 - 20 % values are not applicable for unflattened beam. In house built in program in Matlab were developed to find inflection point in the penumbral region for penumbra. Further to find the penumbra, a factor of 0.4 and 1.4 was applied to the inflection point dose value to obtain the penumbral width. We measured profiles for various field size ranging from 5 x 5 cm² to 30 x 30 cm² with CC 13 chamber at the depth of 5.0 and 10 g/cm² using IBA blue water phantom for 6MV FB and 7MV UFB x-ray beams from a Siemens - ARTISTE linear accelerator.

Results: The calculated penumbra width for 6MV FB using inflection method are compared with conventional calculation and the values differ from 0.1 to 1.61 mm for jaw shaped fields and 0.1 to 1.75 mm for MLC shaped fields at 5.0 cm and 10.0 cm depth for various field size. The calculated penumbra widths in inflection point method values for 7MV UFB and 6MV FB varies from 5.13 to 10.7 mm and 5.55 to 11.85 mm respectively at 10.0 cm depth for 5 x 5 cm² to 30 x 30 cm² field sizes. We observed the calculated penumbra widths with inflection point method values for 7MV UFB are lesser than the 6MV FB for various field sizes and depth.

Conclusions: The penumbra values of 7MV UFB are lesser than the 6MV FB. This is due to softer photon beam spectrum and missing scatter from flattening filter and higher beam quality.

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