Comparison of on-site and off-site evaluations of dosimetry data

Traditionally the Radiological Physics Center (RPC) quality assurance audits of institutions have been performed by a qualified RPC physicist on-site, and by a mailed TLD program that monitors machine output calibrations. In 1997, the RPC implemented a comprehensive off-site evaluation of institution dosimetry data and patient dose calculations. A preliminary analysis and comparison of the off-site evaluation for seventeen institutions with the results of their respective RPC on-site review is presented. The comparison shows that a detailed off-site evaluation of the calibration protocol parameters is not effective in identifying calibration dosimetry problems. In general, the off-site evaluation failed to identify percent-depth-dose (PDD) and output factor discrepancies found at the institution during on-site reviews. However, PDD discrepancies found at one institution by the off-site evaluation correctly predicted the on-site findings. The most frequently identified dosimetry problems came from the evaluation of wedge factors and off-axis factors. The off-site evaluation of patient dose calculations shows discrepancies that are significant, but not confirmed by the on-site visit. Our findings indicate that for the off-site dosimetry review to be more successful in predicting potential problems, the RPC needs to continue to refine its "standard" dosimetry data. The comparison of the results of the two reviews has allowed the RPC to evaluate the effectiveness of the remote review, to identify areas where it is more effective and how this remote evaluation program can be revised and improved.