SU-E-T-733: Effect of IGRT Couch and Alpha-Cradle on Dose Distributions in the Buildup Region: Comparison of EBT2 Film Data and TPS Calculation

Abstract

Purpose: To evaluate current treatment planning system (TPS) dose calculation algorithm for radiation fields with normal and oblique incidence through alpha‐cradle and IGRT couch using radiochromic EBT2 film dosimetry. Methods: A polystyrene phantom (25×25×15cm3) with seven EBT2 films separated by polystyrene slabs, at depths of 0, 1, 2, 5, 10, and 14mm, was positioned above an alpha‐cradle (AC), which was ∼1cm thickness. Such phantom and AC assembly were CT scanned and the CT‐images were transferred to the TPS for calculations in three scenarios: (1) ignoring AC and couch, (2) accounting for AC only, (3) accounting for both AC and IGRT couch. A single PA field and two posterior‐oblique fields (6MV photons, 10×10cm2 from a Varian Trilogy) were planned in the TPS. Each beam was delivered to expose a stack of EBT2 films in the phantom. All films were scanned using an Epson 10000XL flatbed scanner with 72 dpi pixel resolution. The pixel values were converted to doses based on an established calibration curve. Point doses and planar distributions generated from the TPS for the three scenarios were compared with the measurements. Results: The EBT2 film data agree with the doses calculated by our TPS for scenario (3), within the uncertainty of the EBT2 data. The TPS generated doses were lower than the EBT2 doses by 34%, 32%, 31%, 12% for scenario (1) and by 26%, 23%, 22%, 7% for scenario (2) at the depths of 0, 1, 2, and 5mm, respectively. There were no significant differences for the depths of 10 and 14mm. Conclusions: TPS calculation of doses in the buildup region accounting for AC and IGRT couch top was validated by EBT2 film data. Ignoring the presence of AC and/or IGRT couch in TPS calculation would significantly underestimate the doses in the buildup region.