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Dose Profile Study On a KV Superficial Radiation Therapy System - Sensus SRT100TM

R Sheu

R Sheu*, Y Lo, Mount Sinai Medical Center, New York, NY

SU-E-T-63 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

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.

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