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A Comparison of Out-Of-Field Dose and Its Constituent Components for 6MV Flattened and 7MV Unflattened Photon Beam

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A sigamani

Ashokkumar Sigamani1*, Arunai Nambi Raj2, S N Sinha1, Girigesh Yadav1, Kothanda Raman1, Rajesh Thiyagarajan1, Manindra Bhushan1, (1) Rajiv Gandhi Cancer Institute & Research Center, New Delhi, Delhi,India.(2) Vellore Institute of Technology, Vellore, Tamilnadu,India.

SU-E-T-93 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:

A Comparison of out-of-field dose and its constituent components for 6MV Flattened and 7MV unflattened Photon Beams

Methods:

The flattening filter and primary collimator are the major sources of producing the scattered radiation. The out-of-field photon dose, Sc,p , Sc and Sp were measured with CC 13 chamber for square open fields from 5 x 5 cm² to 30 x 30 cm² using IBA blue phantom and solid phantom as a function of distance from central axis and field size at depth of maximum for respective photon energies. Scatter radiation, which predominantly contributes to peripheral dose at larger distance from the field edge, was measured using a CC 13 ionization chamber.

Results:

All the values of Sc,p and Sc are normalized to 10 x 10 cm² field size. The measured values of Sc,p for 6MV-FB and 7MV-UFB varies from 0.9437 to 1.0651 and 0.9690 to 1.0283 respectively. The Sc values for 6MV-FB maximum (max.) 2.1 percent lesser than 7MV-UFB for up to 10 x10 cm² field size and 6MV-FB max.1.7 percent higher than the 7MV-UFB for above 10 x 10cm² field size. Out of field dose is max.42.0 percent higher for 6MV-FB compare to 7MV-UFB at 25 cm from the field edge for up to 10 x 10 cm2 field size and max.50.39 percent reduced dose for 7MV-UFB for above 10 x 10 cm². Out of field dose is max.70.0 percent reduced for 7MV-UFB compare to 6MV-FB above 50 cm away from the field edge for up to 10 x 10 cm² field size and max.68 percent reduced dose for 7MV-UFB for above 10 x 10 cm².

Conclusion:

7MV-UFB may be advantageous by reduced out-of-field dose compare to 6MV-FB, but differences are small in absolute terms.

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