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Capacitive Sensing for Intra-Fraction Patient Position Monitoring During Frameless SRS Radiosurgery

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P Sadeghi

P Sadeghi*, E Evila Ruiz , J Robar , Dalhousie Uniersity, Halifax, NS


MO-DE-FS1-4 (Monday, July 31, 2017) 1:45 PM - 3:45 PM Room: Four Seasons 1

Purpose: Development of a novel method for intra-fraction patient position monitoring during frameless SRS. The system provides continuous monitoring without the use of ionizing radiation or monitoring surrogates based on the skin of the patient.

Methods: This technology is based on measurement of capacitance between the patient's cranium and an array of conductive pads during SRS treatment. The comparatively high permittivity of the body, makes this approach sensitive to the cranial position but insensitive, e.g., to the position of the surrounding immobilization mask. Multiple capacitive pads are placed in an array surrounding the cranium. The acquired capacitive signal provides near real-time positional information. The system is designed using Finite Element Analysis (FEA) to determine the capacitance for different plate shapes, plate sizes and array designs. An experimental prototype was evaluated with comparison to ground truth provided by a 6D motion translation stage. In addition, a volunteer in a SRS thermoplastic mask replicated SRS patient conditions, facilitating evaluation of system performance under clinical conditions in the linac suite. Furthermore, FEA was performed to detect lateral motion sensitivity using two plates at either side of the cranium and sensitivity was determined as relative signal difference collected from both lateral plates.

Results: Testing performed with a volunteer indicated that the single copper sensor has the highest sensitivity per unit distance (average of 7.2% change in normalized signal per mm motion for the first 11 mm) which corresponds to the sensitivity of 7.8% per mm calculated using FEA under similar conditions. Lateral motion sensitivity was determined using FEA for sensors originally placed at 2 cm distance from the head. The results show a sensitivity of 1.85% per 0.5 mm motion (0.5-1.5 mm motion range in 0.5 mm steps) .

Conclusion: This novel technique exhibits great promise in detecting cranium motion during SRS treatment.

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