PATIENT DOSIMETRY QUALITY ASSURANCE PROGRAM WITH A COMMERCIAL DIODE SYSTEM

Purpose: To evaluate a commercial silicone diode dosimeter for a patie nt dosimetry quality assurance program. Methods and Materials: The dio de dosimeter was calibrated against an ion chamber and percentage dept h dose, linearity, anisotropy, virtual source position, and held size factor studies were performed. Correction factors for lack of full sca tter medium in the diode entrance and exit dose measurements were acqu ired. Dosimetry equations were proposed for calculation of dose delive red at isocenter. Diode dose accuracy and reproducibility were tested on phantom and on four patients. A patient dosimetry quality assurance program based on diode measured dose was instituted and patient dose data were collected. Results: Diode measured percentage depth dose and field factors agreed to within 3% with those measured with an ion cha mber. The diode exhibited less than 1.7% angular dose anisotropy and l ess than 0.5% nonlinearity up to 4 Gy. Diode dose measurements in phan tom showed that the calculated doses differed from the prescribed dose by less than 1.5%; the diode exhibited a daily dose reproducibility o f better than 0.2%. On four selected patients, the measured dose repro ducibility was 1.5%; the average calculated doses were all within +/- 7% of the prescribed doses. For 33 of 40 patients treated with a 6 MV beam, measured doses were within +/- 7% of the prescribed doses. For 5 8 of 63 patients treated with an 18 MV beam, measured doses were withi n +/- 7% of the prescribed doses. For 11 out of 12 patients, a second repeat measurements yielded doses within +/- 7% of the prescribed dose s. Conclusions: The proposed diode-based patient dosimetry quality ass urance program with dose tolerance at +/- 7% is simple and feasible. I t is capable of detecting certain serious treatment errors such as inc orrect daily dose greater than 7%, incorrect wedge use, incorrect phot on energy and patient setup errors involving some incorrect source-to- surface-distance vs. source-to-axis-distance treatments.