Electronic design for a bleeding detector to be used in intraoperative radiotherapy applications

An electronic design for a bleeding detector used with mobile linac during intraoperative radiotherapy (IORT) is presented. It consists on a readout circuit based on a De Sauty Bridge and a wireless power supply using a supercapacitor. The goal is the measurement of the height of fluid present in the applicator in order to avoid a misadministration dose during the irradiation process. A minimum measurement height of 5 mm is required by clinic to be confident with the selected radiation dose. The measurement circuit uses a capacitive sensor which detects variations lower than 0.1 pF. The readout circuit converts dynamic capacitance variation from the bleeding detector into an analog voltage signal. Tests have been performed with a 40 mm diameter polymethyl methacrylate applicator (PMMA) using water. Non-bevelled PMMA 0° and 45° tilted have been considered, being this the worst condition used in the clinic. The actual height of the fluid with respect to that measured by our system shows good linearity and repeatability within acceptable limits. For 0° tilted, a 2 mm height fluid detection is achieved well below the 5 mm height demanded by clinic. For 45° tilted, a 5 mm height fluid detection is achieved.