SU‐E‐T‐221: An In‐House Developed Resettable MOSFET Dosimeter for Radiotherapy

Purpose: To report the feasibility and clinical validation of an in‐house developed MOSFETdosimetry system and describe an integrated nondestructive reset procedure. Methods: Off‐the‐shelf MOSFETs are connected to a PC using an 18 bit/analogue‐input and 16 bit/output data acquisition card. A reading algorithm was developed defining the zero‐temperature‐coefficient point (ZTC) to determine the threshold voltage. 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 (0—9 V) to the gate. Dosimetric properties have been evaluated for 6 MV and 15 MV clinical photon beams and in vivo benchmarking was performed against TLD for conventional treatments and total body irradiation (TBI). Results: Sensitivity of 0.08 mV cGy−1 can be obtained for 200 cGy irradiations at 5 V bias voltage. Ten consecutive measurements at 200 cGy yield a SD of 2.08 cGy (1.05%). Increasing the dose in steps from 5 cGy to 1000 cGy yields a 1.00 Pearson correlation coefficient and agreement within 2.0%. Dose rate dependence (160–800 cGy min−1) was within 2.5%, temperature dependence within 2.0% (25–37° C). Dose response is stable up to 50 Gy (saturation occurs at approximately 90 Gy), which is used as threshold dose before resetting the MOSFET. An average measured‐over‐calculateddose ratio within 1.05 (SD: 0.04) has been obtained in vivo. TBI midplane‐dose assessed by entrance and exit dosemeasurements agreed within 1.9% with ionization 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 dosemeasurement in clinical routine for open field treatment configurations.