Room: AAPM ePoster Library
Purpose: algorithm presented is based on patient specific measurements and assures the accuracy required for in vivo EPID dosimetry to measure actual delivered patient dose using VMAT.
Methods: transmission ratio It/I0 is defined at the center of the field through thickness t where I0 is the in air value and It is the transmitted intensity. Data is acquired for several representative depths and field sizes using calibrated Varian equipment. Slabs of solid water are attached to the head of the linac so that the transmitted intensity It is measured throughout the VMAT arc. For relatively large fields an off axis factor is used to correct It/I0. The VMAT patient specific field F is represented as a series of arc segments f that can be computed by creating a verification plan such that F = SUM(f). The equivalent field size is computed from the measured transmission at one depth. The transmission through thickness t is measured for the range of expected patient depth and field size. At each point within field f that goes through a patient thickness t, the EPID response It = I0 x It/I0. Basically the ratio It/I0 acts as a compensator that attenuates the traditional in air portal dose I0.
Results: test the hypothesis a 60 degree VMAT plan was created for a 26.5 cm water cylinder. Analysis with a commercial system indicated 5% to 10% errors. Use of the algorithm described in this work has significantly better results with a mean error in the central 80% of the field vertical axis of 0.4% and a standard deviation of 0.6%. Fields up to 10x10 cm did not require off axis correction.
Conclusion: process is suitable for general implementation and can serve as a standard for clinical use, accrediting agencies, and acceptance testing of commercial products.