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Comparison of the Depth Dose in the Build-Up Region and Surface Dose for 6MV Flattened and 7MV Unflattened Photon Beams with Different Detectors

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A sigamani

Ashokkumar sigamani1*, Arunai Nambi Raj2, S N Sinha1, Girigesh Yadav1, Kothanda Raman1, Rajesh Thiyagarajan1, M Bhushan1, (1) Rajiv Gandhi Cancer Institute & Research Center, New Delhi, Delhi,India (2) Vellore Institute of Technology, Vellore, Tamilnadu,India

SU-E-T-92 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:

Comparison of the depth dose in the build-up region and Surface dose for 6MV flattened and 7MV unflattened photon beams with different detectors

Methods:

The percentage depth dose in the build-up region and the surface dose for the 6MV-FB and 7MV-UFB photon beams from a Siemens Artiste medical linear accelerator were measured for square field sizes of 5Χ5, 10Χ10 and 15Χ15 cm² using Scanditronix NACP ion chamber, CC 13, and Stereotactic field Detectors (SFD) along the central axis of the beam at 100 cm source to surface distance with IBA blue phantom 2 and solid phantom.

Results:

The consistency between the measured percentage depth dose data from all four detector types were observed for depths beyond the depth of the maximum dose, but were all clearly different from each other data in the build-up region this is observed for both 6MV-FB and 7MV-UFB beams. The measured surface dose (16.2 Percent) applied with improved Velkley correction factors used for 10Χ10 cm² of 6MV-FB using NACP parallel plate chamber values were all in good agreement with previously published data. The measured percentage surface doses obtained using the NACP parallel plate chamber, CC 13 and SFD without any correction factor values are 0.3885,0.4330 and 0.4751, 0.4485,0.4922 and 0.5323,0.3652,0.4076 and 0.4580 respectively for field sizes of 5Χ5,10Χ10 and 15Χ15 cm² for the 6MV-FB photon beams and 0.3800,0.4117 and 0.4398 , 0.4364 , 0.4655 and 0.4965, 0.3611,0.4100 and 0.4349 for the 7MV-UFB photon beams.

Conclusion:

The measured surface dose clearly increases with increasing field size, regardless of the detector used in the measurement for both 6MV-FB and 7MV-UFB photon beams. Compare to NACP parallel plate chamber CC 13 chamber is showing over response and SFD is showing lesser surface dose.


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