SU-F-T-570: Comparison of Synthetic Diamond, Microionization Chamber, and Radiochromic Film for Absolute Dosimetry of VMAT Radiosurgery.

PURPOSE: Patient specific quality assurance of stereotactic radiosurgery (SRS) plans is challenging because of small target sizes and high dose gradients. We compared three detectors for dosimetry of VMAT SRS plans.

METHODS: The dose at the center of seventeen targets was measured using a synthetic diamond detector (2.2 mm diameter, 1 µm thickness), a 0.007 cm(3) ionization chamber, and radiochromic film. Measurements were made in a PMMA phantom in the clinical geometry - all gantry and table angles were delivered as planned. The diamond and chamber positions were offset by 1 cm from the film plane, so the isocenter was shifted accordingly to place the center of the target at the detector of interest. To ensure accurate detector placement, the phantom was positioned using kV images. To account for the shift-induced difference in geometry and differing prescription doses between plans, the measurements were normalized to the expected dose calculated by the treatment planning system.

RESULTS: The target sizes ranged from 2.8 mm to 34.8 mm (median 14.8 mm). The mean measurement-to-plan ratios were 1.054, 1.076, and 1.023 for RCF, diamond, and chamber, respectively. The mean difference between the chamber and film was -3.2% and between diamond and film was 2.2%. For targets larger than 15 mm, the mean difference relative to film was -0.8% and 0.1% for chamber and diamond, respectively, whereas for targets smaller than 15 mm, the difference was -5.3% and 4.2% for chamber and diamond, respectively. The difference was significant (p=0.005) using the two-sample Kolmogorov-Smirnov test.

CONCLUSION: The detectors agree for target sizes larger than 15 mm. Relative to film, for smaller targets the diamond detector over-responds, whereas the ionization chamber under-responds. Further work is needed to characterize detector response in modulated SRS fields.