Simulation and Characterization of a Monolithic Active Pixel Sensor Designed for Brachytherapy

In order to be effective and to reduce potential issues related to late toxicities in the healthy tissues, Low Dose Rate (LDR) and High Dose Rate (HDR) brachytherapy treatments require both an excellent control on the actual position of the radioactive seeds implanted within the patient body to deliver the intended dose to the target cancer cells. To this end, we developed a small area pad detector (O (1 mm2)) with embedded CMOS electronics that can be mounted on the tip of a thin and flexible prostate catheter. This results in a compact system that can be exploited to monitor the clinical treatment performing in-vivo dosimetry and source tracking, thus improving the therapy outcome through the prompt identification of potential errors related to the misplacement of the implanted radioactive seeds. To assess the detector performance, we electrically characterized the sensor monitoring the chip power consumption. Then, we tested its dynamic response using an optical setup equipped with a fast pulsed IR laser with 1060 nm wavelength coupled to a focusing system able to provide a minimum laser spot size in the order of 10 μm FWHM and we compared the obtained waveforms with the ones predicted by simulations. Finally, we evaluated the detector response to an X-ray tube and different radiation sources, e.g. 90Sr and 133Ba, to calibrate the gain of the embedded amplifier and we exploited an external charge injection circuit to verify the linear dynamic range of the amplifier in its two operating modes. The tests proved that the embedded amplifier can work both in Charge Sensitive Mode with an estimated gain equal to 108±2 mV/fC and a linear response up to ≃ 25 fC and in Transimpedance mode with a conversion factor of 2.10±0.01mVmGy/s.