SU-F-T-309: Radiochromic Film Calibration Rescaling for Patient-Specific CyberKnife MLC QA.

PURPOSE: This work investigated the development of EBT3 film as a function of dose rate, and also the accuracy of a commercial calibrationrescaling analysis method, to determine potential application to patientspecific QA of CyberKnife MLC treatments.

METHODS: EBT3 films were exposed to 300cGy at dose rates ranging from 5-300cGy/min, and scanned repeatedly from 2-5760min after exposure using a flatbed scanner. All other films were scanned 1 and 24hr after exposure. Ten calibration films were exposed to doses between 0 and 800cGy. Four verification films were exposed to doses between 80 and 290cGy, and 0 to 30min later, reference films were exposed to 450cGy. The verification and reference films were scanned together and analyzed both with and without calibration rescaling. The dose distribution of a 36min treatment plan was measured. Immediately prior to the measurement, a reference film was exposed. Both films were scanned together and analyzed both with and without calibration rescaling.

RESULTS: The development of EBT3 film is linear with the logarithm of time after exposure. If the midpoint of the exposure duration is taken as time zero, then film development is independent of dose rate. The doses measured with the verification films agreed with the expected dose within 1- 2% with calibration rescaling for all reference films. The delay between the exposure of the reference and QA films was 24min relative to the midpoint of exposure. Analysis of the scan at 60min post-exposure using rescaling of the calibration at 24hr had a gamma passing rate of 92% with 2%/1mm criteria, consistent with analysis of the scan at 24hr post-exposure without calibration rescaling.

CONCLUSION: Using calibration rescaling, measured doses were accurate within 1-2% 60min after exposure. If the midpoint of exposure is taken as time zero, then calibration rescaling can be applied to patient-specific QA for CyberKnife MLC treatments.