SU‐GG‐T‐369: An In‐House Developed MOSFET Dosimeter with Reset Capabilities

Background and purpose : To report the feasibility and clinical validation of an in‐house developed MOSFET dosimetry system and describing an integrated non‐destructive reset procedure. Material and Methods : Off‐the‐shelf MOSFETs are connected to a common PC using an 18bit/analogue‐input and 16bit/output data acquisition card. A reading algorithm was developed defining the Zero‐Temperature‐Coefficient point to determine the threshold voltage. A wireless interface was established for ease‐of‐use. The reset procedure consists of an internal circuit generating a local heating induced by an electrical current. Sensitivity has been investigated as a function of bias voltage (0V–9V) to the gate. Dosimetric properties have been evaluated for 6MV and 15MV clinical photon beams and in vivo benchmarking was performed against TLD for conventional treatments (2 groups of 10 patients for each energy) and TBI. Results : MOSFETS were pre‐irradiated with 20Gy. Sensitivity of 0.08mV/cGy can be obtained for 200cGy irradiations at 5V bias voltage. Ten consecutive measurements at 200cGy yield a SD of 2.08cGy (1.05%). Increasing dose in steps from 5cGy to 1000cGy yields a 1.00 Pearson correlation coefficient and agreement within 2.0%. Dose rate dependence (160–800cGy/min) was within 2.5%, temperature dependence within 2.0% (25–37°C). A strong angular dependence has been observed for gantry incidences exceeding ±30°. Dose response is stable up to 50Gy (saturation occurs at approximately 90Gy), which is used as threshold dose before resetting the MOSFET. An average measured‐over‐calculated dose ratio within 1.05 (SD: 0.04) has been obtained in vivo. TBI midplane‐dose assessed by entrance and exit dose measurements agreed within 1.9% with ionisation chamber in phantom, and within 1.0% with TLD in vivo. Conclusions : An in‐house developed resettable MOSFET‐based dosimetry system is proposed. The system has been validated and is currently used for in vivo entrance dose measurement in clinical routine for simple (open field) treatment configurations.