SU-F-T-328: Real-Time in Vivo Dosimetry of Prostate SBRT Boost Treatments Using MOSkin Detectors.

PURPOSE: To provide in vivo measurements of dose to the anterior rectal wall during prostate SBRT boost treatments using MOSFET detectors.

METHODS: Dual MOSkin detectors were attached to a Rectafix rectal sparing device and inserted into patients during SBRT boost treatments. Patients received two boost fractions, each of 9.5-10 Gy and delivered using 2 VMAT arcs. Measurements were acquired for 12 patients. MOSFET voltages were read out at 1 Hz during delivery and converted to dose. MV images were acquired at known frequency during treatment so that the position of the gantry at each point in time was known. The cumulative dose at the MOSFET location was extracted from the treatment planning system at in 5.2° increments (FF beams) or at 5 points during each delivered arc (FFF beams). The MOSFET dose and planning system dose throughout the entirety of each arc were then compared using root mean square error normalised to the final planned dose for each arc.

RESULTS: The average difference between MOSFET measured and planning system doses determined over the entire course of treatment was 9.7% with a standard deviation of 3.6%. MOSFETs measured below the planned dose in 66% of arcs measured. Uncertainty in the position of the MOSFET detector and verification point are major sources of discrepancy, as the detector is placed in a high dose gradient region during treatment.

CONCLUSION: MOSkin detectors were able to provide real time in vivo measurements of anterior rectal wall dose during prostate SBRT boost treatments. This method could be used to verify Rectafix positioning and treatment delivery. Further developments could enable this method to be used during high dose treatments to monitor dose to the rectal wall to ensure it remains at safe levels. Funding has been provided by the University of Newcastle. Kimberley Legge is the recipient of an Australian Postgraduate Award.