SU-E-T-63: Dose Profile Study On a KV Superficial Radiation Therapy System - Sensus SRT100TM

Abstract

Purpose: Surface dose profiles of a kV skin treatment machine — SRT100™ were measured in order to investigate the output deviations due to the uncertainty of ion chamber positioning. Methods: GafChromic film (EBT3) and a parallel plate ion chamber (PTW Model 23342) were used to measure the dose profiles. The dose response of EBT3 film was calibrated by using 3‐channel technique. The maximum cone size of 5 cm was used in this study. Because of the ion chamber size, the readings were limited to measure at 1 and 2 cm away from the central axis in 4 directions (+/− parallel and perpendicular to cathode‐anode axis). The ion chamber was positioned by using Varian Exact Couch within 1‐mm accuracy. Results: For 5‐cm cone, the dose profile was not entirely uniform especially in cathode‐anode direction and it showed on both ion chamber and film measurements. For film measurements, a maximum of 15% lower dose (normalized to the center) was observed at field edge on the anode side. On the cathode side, a maximum of 6% lower dose was observed. The cross line profile was symmetry and there was a maximum of 10% lower dose on the both side of field edges. Overall, the uniformity of the area of 2‐cm diameter was within 4% in all 4 directions. The ion chamber readings agreed with film measurements within 1–3%. Conclusion: The non‐uniform dose profile may introduce uncertainty to measurements up to 15% for the field of 5‐cm cone. Since there is no inherent tool (crosshair or field light) for aligning with beam axis, the users should be cautious with positions of ion chamber during output measurement.