Room: AAPM ePoster Library
Purpose: The objective was to report dose distribution of shielded cylinders in HDR Brachytherapy in a commercial treatment-planning-system-(TPS).
Methods: Patient treatment-plans with partial shielded cylinders were generated in BrachyVision-BV planning with Eclipse-version15.6. TG-43 dose calculation formalism was used for patient treatment delivery. A TG-186 MBCDAs based algorithm Acuros-BV was employed with the same source locations and dwell-times to compare dose calculations for shielded attenuation corrections. Solid applicators with 180° tungsten alloy shielding segment in TPS library were employed for Acuros calculations. The difference between these algorithms were evaluated in target, critical structures, and dose distribution.
Results: Patient planning with 180° shielded cylinders in anterior/posterior were created for focused vaginal treatment and minimization of local rectum/bladder on the patient CT images. The dwell time in the source positions based upon 3D TG-43 algorithm were calculated with ignored shielding effect of the applicator resulting in symmetric dose distribution along the cylinder. With Acuros-BV dose calculation, 3D non-symmetric dose distribution was displayed as expected in planar views within BV. The low dose bath in the shielded areas remained about 12-14% of unshielded dose. About 10% higher dwell time in Acuros-BV was required to reach the same dose level as TG-43 calculation. The 180° shielded cylinder was effectively shielded up to 215° wide regime for either posterior or anterior portion in dose distribution.
Conclusions: The Acuros-BV calculation for the shielded cylinder suggests about 10% longer time needed to reach the same dose level in TG-43 calculation in TPS. The shielded area in dose distribution seems wider than the physical shielding segment. The shielded area remains in the low dose bath up to 12-14% of the prescribed dose. The metal block existence appears to reduce the scattered dose deposition to non-shielded area and to effectively widen the block physical angle.