Validation of In vivo Diode Characteristics at Extended Source to Surface Distance for Total Body Irradiation

The application of ISORAD diode detectors in treatments like total body irradiation (TBI), where small fractions of potentially lethal doses are delivered, lacks comprehensive clarity and requires further investigation. The study evaluates the feasibility of using ISORAD diode detectors as in vivo dosimeter for extended source to surface distance (SSD) treatments like TBI, despite a 250 cm limit in the manufacturer's specifications. Four identical ISORAD-1163 n-type cylindrical diode detectors manufactured by Sun Nuclear Corporation were studied for their operational characteristics and compared with 0.6cc Farmer chamber. The performance evaluation involved assessing reproducibility, linearity, field size dependence, and dose rate dependence at an SSD of 100 cm, extending up to 400 cm. Postirradiation signal drift, short-term reproducibility, and Signal Stability were monitored at extended SSDs. Directional dependence was evaluated in both the axial plane and transverse plane at 100 cm SSD. A diode was then used as an in vivo dosimeter for the bilateral TBI technique at 380 cm SSD. Acceptable dose rate effect (<0.91%), linearity (<0.99%), and reproducibility (<0.19%) with minimum deviations in postirradiation signal drift (<0.09%) and signal consistency (<0.09%) were observed up to 400 cm SSD for all diodes. Significant directional dependence of more than 1.1% was observed beyond ± 30° in the transverse plane of the detector, while it was insignificant in the axial plane. In vivo measurements of TBI patients with diodes at head, neck, umbilicus, chest, ankle, and shoulder showed that the maximum dose difference in the calculated and actual diode readings did not exceed 8.5%. Operational characteristics of in vivo diodes revealed to be consistent up to 400 cm SSD, supporting their applicability for in vivo measurements in TBI patients.