Application of a radiophotoluminescent glass dosimeter to nonreference condition dosimetry in the postal dose audit system.
PURPOSE
The purpose of this study was to obtain a set of correction factors of the radiophotoluminescent glass dosimeter (RGD) output for field size changes and wedge insertions.


METHODS
Several linear accelerators were used for irradiation of the RGDs. The field sizes were changed from 5 × 5 cm to 25 × 25 cm for 4, 6, 10, and 15 MV x-ray beams. The wedge angles were 15°, 30°, 45°, and 60°. In addition to physical wedge irradiation, nonphysical (dynamic/virtual) wedge irradiations were performed.


RESULTS
The obtained data were fitted with a single line for each energy, and correction factors were determined. Compared with ionization chamber outputs, the RGD outputs gradually increased with increasing field size, because of the higher RGD response to scattered low-energy photons. The output increase was about 1% per 10 cm increase in field size, with a slight difference dependent on the beam energy. For both physical and nonphysical wedged beam irradiation, there were no systematic trends in the RGD outputs, such as monotonic increase or decrease depending on the wedge angle change if the authors consider the uncertainty, which is approximately 0.6% for each set of measured points. Therefore, no correction factor was needed for all inserted wedges. Based on this work, postal dose audits using RGDs for the nonreference condition were initiated in 2010. The postal dose audit results between 2010 and 2012 were analyzed. The mean difference between the measured and stated doses was within 0.5% for all fields with field sizes between 5 × 5 cm and 25 × 25 cm and with wedge angles from 15° to 60°. The standard deviations (SDs) of the difference distribution were within the estimated uncertainty (1SD) except for the 25 × 25 cm field size data, which were not reliable because of poor statistics (n = 16).


CONCLUSIONS
A set of RGD output correction factors was determined for field size changes and wedge insertions. The results obtained from recent postal dose audits were analyzed, and the mean differences between the measured and stated doses were within 0.5% for every field size and wedge angle. The SDs of the distribution were within the estimated uncertainty, except for one condition that was not reliable because of poor statistics.