Dosimetric validation of new semiconductor diode dosimetry system for intensity modulated radiotherapy

The new diode Isorad was validated for intensity modulated radiotherapy (IMRT) and the observations during the validation are reported.

The validation includes intrinsic precision, post-irradiation stability, dose linearity, dose-rate effect, angular response, source to surface (SSD) dependence, field size dependence, and dose calibration.

The intrinsic precision of the diode was more than 1% (1 σ). The linearity found in the whole range of dose analyzed was 1.93% (R² = 1). The minimum and maximum variation in the measured and calculated dose were found to be 0.78% (with 25 MU at ioscentre) and 4.8% (with 1000 MU at isocentre), respectively. The maximal variation in angular response with respect to arbitrary angle 0° found was 1.31%. The diode exhibited a 51.7% and 35% decrease in the response in the 35 cm and 20 cm SSD range, respectively. The minimum and the maximum variation in the measured dose from the diode and calculated dose were 0.82% (5 cm × 5 cm) and 3.75% (30 cm × 30 cm), respectively. At couch 270°, the response of the diode was found to vary maximum by 1.4% with ± 60 gantry angle. Mean variation between measured dose with diode and planned dose by TPS was found to be 1.3% (SD 0.75) for IMRT patient-specific quality assurance.

For the evaluation of IMRT, use of cylindrical diode is strongly recommended.