LAUSR

PURPOSE OSLDs are often used to make in vivo dose measurements. Most users calibrate the OSLDs using the software provided by the vendor which typically is intended for doses up to about 300 cGy with an uncertainty of ±5.5%. OSLDs that reach that dose are then discarded, and new ones purchased and calibrated. A method has been developed for reusing OSLDs and predicting dose sensitivity up to at least 7000 cGy. MATERIALS AND METHODS The nanoDot OSLDs used in this study were routinely used to do in vivo measurements for TBI patients. Instead of using the calibration program provided by the vendor, each nanoDot was bleached to about 100 counts (~0.1 cGy), then calibrated with 50 cGy to produce a sensitivity specific to each nanoDot prior to the patient measurement. NanoDots were read in the hardware mode and the sensitivity factor was applied manually to subsequent patient in-vivo TBI measurements. This was followed by bleaching prior to the next use. The changes of nanoDot sensitivity relative to accumulated dose was analyzed among 9 nanoDots. In addition, a method to predict a nanoDot's sensitivity was investigated which aims to reduce the number of sensitivity calibrations while retaining dosimetric accuracy. RESULTS Individual per-use nanodot calibrations were performed up to 7000 cGy for 37 clinical TBI patients. Among the 9 nanoDots analyzed in this paper, the sensitivity versus accumulated dose decreased linearly up to about 3000 cGy, with linear fitting curve R2 values above 0.93. After 3000 cGy of accumulated dose, the sensitivity started to plateau and tended to increase by 6000 cGy, with 2nd order polynomial curve R2 values above 0.94. With this finding, an efficient and accurate method to predict nanoDots' sensitivities was developed. With the method applied to the 9 OSLDs, a total of 127 sensitivities were predicted and retrospectively compared with measured sensitivities. The predicted sensitivities agreed with measured sensitivities within ±4.0% with an average of -0.8%. CONCLUSIONS This study is the first to demonstrate the reuse of nanoDot OSLDs on numerous patients with accumulated dose up to 7000 cGy. Our nanoDot re-usage methodology is accurate, cost-saving and feasible. A time saving method is also provided that allows a user to reuse a nanoDot with sensitivities predicted with better accuracy than the 5.5% value provided by the conventional batch calibration method.