SU-E-T-179: MVCB Pelvic Surface Dosimetry

Medical Physics
J Mallah, D Mihailidis

Abstract

To compare commonly clinically available methods of estimating skin dose for a Megavoltage Conebeam CT of the pelvis. The clinical 6 MV conebeam uses 15 MU on a Siemens Oncor linac. Film tests using XV-2 film were done on a solid water phantom, as were tests using a parallel plate ion chamber at 2 mm depth. A Rando pelvic phantom was set up for a MVCB CT, and a cylindrically symmetric Isorad 6-12 MV PDM diode was placed at various angles around the phantom and irradiated using a 150 MU 6 MV arc beam with the same geometry as the clinical conebeam, giving higher and more accurate readings than would be obtained with the 15 MU conebeam. A comparison was made with the Pinnacle 8.0m treatment planning system. A strip of Optically Stimulated Luminescence dosimeters provides an accurate check of the surface dose distribution. All methods revealed an expected lateral asymmetry in the dose due to the starting and stopping angles of 270 (-90) and 110 for the conebeam. On the sides of the phantom, the diode dose was comparable to the Pinnacle-calculated dose at a depth of 3-5 mm. Near the anterior portion the diode dose was about 5% higher than the maximum Pinnacle-calculated dose at that angle. This difference is partly due to the incre...Continue Reading

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