The purpose of this work is to develop a prototype system for continuous, three-dimensional (3D) monitoring of patient cranial motion during stereotactic radiosurgery. Using novel capacitive detector plates, the goal was to provide detection of cranial position inside a thermoplastic immobilizing mask, without relying on skin monitoring or use of ionizing radiation. A novel capacitive detector array was used to detect cranial translations with sub-millimeter accuracy. The array was comprised of four conductive plates arranged around the cranium. One superior plate was positioned at the cranial vertex, two lateral plates were positioned in sagittal planes at the lateral aspects of the cranium and one plate was located in a coronal plane anterior to the face. The system was calibrated by parameterizing a capacitive signal for each dimension as a function of spatial translation. The detector array performance was evaluated with the help of a volunteer in the absence of radiation. Separately, possible effects of electromagnetic interference and irradiation in the linac suite were assessed. Detector plates mounted at 1 cm original distance to the thermoplastic mask can detect sub-millimeter lateral and superior cranial motion. Detection of sub-millimeter anterior motion is possible when the plate is mounted closer to the patient (5-10 mm). No signal interference was observed when the capacitive array was irradiated. Our prototype detector array provides continuous, 3D translation detection with sub-millimeter precision. The signal provides sufficient signal to noise ratio and is stable in linac room environment and in direct radiation beam. The detector plate is sensitive to the position of the cranium inside a mask and offers the advantage of being insensitive to the mask itself. Future work will involve modifying the array to detect patient rotation.

This work investigated the application of plastic scintillation detectors (PSDs) in dosimetry measurements on the surface of a phantom in megavoltage photon and electron beams. Emission spectra associated with fluorescence in the PMMA optical fibre, Cerenkov in the optical fibre, and the signal from the scintillator were characterized with a spectrophotometer. The angular dependence of the PSD response was studied with the PSD both in the plane and perpendicular to the plane of gantry rotation. Two methods of stem correction were investigated: stem subtraction and full spectral correction. It was found that with the PSD in the plane of gantry rotation, the stem subtraction method overestimates the dose particularly in electrons. The magnitude of overestimation is dependent on the beam energy, scintillator type and the angle of incidence. In this study a maximum error of 12% was observed for 16 MeV with a gantry angle of 45°. It was shown that the cause of this error was a non-negligible emission of Cerenkov radiation from the scintillator itself. This error was not observed when the PSD was oriented perpendicular to the plane of rotation. Similarly, the full spectral method was effective in isolating the scintillation-only signal regardless of PSD orientation.

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