In vivo semiconductor dosimetry as part of routine quality assurance

This paper describes the initial physics testing necessary before diodes can be used for in vivo dosimetry as well as the development of a protocol for clinical use in head and neck treatment and the preliminary results acquired. 50 patients were entered into the pilot study. A total of 300 treatment set-ups were measured (184 entrance doses and 116 exit doses). Wedged and unwedged components of each field were measured separately, making the total number of entrance doses 284 and total number of exit doses 207. There was no significant systematic deviation in the measured entrance dose compared with the expected (mean +0.4%, SD 2.7%). Discrepancies between the observed and expected entrance doses of greater than 5% were recorded in 6% (16/284) of measurements. The mean of the measured exit doses was 2.4% lower than expected (SD 4.8%). Discrepancies between the observed and expected exist doses of greater than 5% were recorded in 32% (67/207) of measurements. Possible causes for these discrepancies are discussed. Overall analysis of the data for individual patients suggest that in one patient out of the 50 there may have been a delivered target volume dose discrepancy of greater than 5% (+6.5%). The significance of the results and the implications for routine use are discussed.