Development of in vivo MOSFET dosemeter for applications in radiotherapy

In radiotherapy (teletherapy), the maximum difference between the prescribed dose and the dose delivered to the patient must be ±5%. For this reason, the use of dosimeters is essential to meet this specification and, consequently, to ensure the success of this type of cancer treatment. In this work we present a new in vivo MOSFET dosimeter suitable for in vivo radiotherapy applications which combines a simple and accurate readout procedure with a small size, low-cost, and cable/battery-free sensor. The absence of cable and battery during irradiation is interesting because metals that are present in these components can deflect radiation and, consequently, change the dose received by the patient. The dosimeter that we developed uses the integrated circuit (IC) CD4007UBM (Texas Instruments) as the radiation sensor. This IC was chosen because its MOS transistors have a gate oxide thickness of 120 nm, which is indicated for the desired application since it has adequate radiation sensitivity (7 mV/Gy) and is able to operate at relatively low voltage (|V_T | ∼= 1,6 V). Moreover, this IC has a very low cost, US$0.5/IC, and small dimensions (35 mm² and thickness lower than 2 mm). MOSFET dosimeters sense the total dose by the variation of the threshold voltage (VT ) due to ionizing radiation; for this reason, the VT -extractor circuit is a fundamental block in this type of dosimeter. In our dosimeter we used an accurate and low power constant-current (CC) V_T-extractor circuit that directly determines the V_T. It is worth to mention that this CC extractor operates in moderate inversion (conventional CC extractors are biased in strong inversion) and extracts a value of V_T that has physical meaning. Experiments with ionizing radiation were carried out at the Centro de Pesquisas Oncol´ogicas (CEPON) in Florian´opolis/Santa Catarina using linear accelerators to generate X-rays of 6 MV and 15 MV. Among the main results of the CD4007 MOSFET dosimeter we can highlight: radiation sensitivity 98.1 mV/Gy, thermal dependence 0.5 cGy/◦C, angular dependence of 13%, energy dependence of.1.3%, linearity of 97.5%, and attenuation to the radiation beam of 0.14%. (author)