Dose rate correction of a diode array for universal wedge field dosimetric verification.

PURPOSE

To study the performance of MapCHECK 3 (MC3) in measuring universal wedge fields and propose a dose rate correction strategy to improve MC3 measurement accuracy.

MATERIALS AND METHODS

Universal wedge fields with different wedge angles and field sizes were measured at different depths using MC3. Considering the more prominent dose rate dependence of type 4 diodes equipped by MC3, a program was developed to automatically correct the measurements based on the instantaneous dose rate (IDR) correction curve. Central axis (CAX) doses and off-axis doses along the wedge direction, with and without the correction, were compared with those measured by an ion chamber under the same condition. Measurements using MC3 with and without correction were also compared with the planned doses calculated by the treatment planning system (TPS).

RESULTS

If MC3 was used for universal wedge field measurement with the dose calibration factor (DCF) derived from a reference open field, an error of up to -2.4% would be introduced into the CAX dose. Other factors (field size and measurement depth) would also affect the accuracy of measurement when they differed from the absolute dose calibration and the maximum error was up to -2.9%. While greater errors were observed in the off-axis doses at the heel side of the wedge compared to the toe side due to the greater effective thickness of the wedge inserted into the beam. After dose rate correction, the deviations in the CAX dose were reduced to within ± 1.5%. The average gamma pass rate was also improved to over 99.5%.

CONCLUSION

Because of the more prominent dose rate dependence of type 4 diodes, MC3 is not suitable for universal wedge field measurement using the methodology for open field measurement. The correction strategy proposed in this study is convenient and can improve the accuracy of universal wedge field measurement using MC3.