Sensitivity Response Analysis of Optical Surface Monitoring Systems Using the Fitzpatrick Scale: A Phantom Study

Abstract

PurposeOptical Surface Monitoring systems (OSMS) have gained substantial attention in modern radiotherapy, specifically in the context of Surface Guided Radiation Therapy (SGRT) which offers real-time patient surface monitoring, ensuring accurate and effective radiotherapy treatments. The aim of this paper is to evaluate the OSMS camera sensitivity towards different skin tones, categorized according to the Fitzpatrick scale, a universal classification of human skin tones, using a phantom. Methods and MaterialThe study utilized Catalyst and SentinelTM OSMS systems (C-Rad, Uppsala, Sweden). The Alderson RANDO Female Pelvis phantom, located at the isocenter in CT simulation and treatment rooms, served as an experimental subject. Eighteen skin tone matching cotton cloths, selected based on Von Luschan's chromatic and Fitzpatrick scales, were wrapped around the phantom for sensitivity evaluation. Camera sensitivity was optimized by adjusting threshold/gain (100-600%) and integration time during individual scans in both rooms. Temporal response analysis spanned two months, with sixteen measurements for each OSMS taken in varying light conditions. ResultsThe OSMS systems successfully detected the surface of cloth-covered phantoms with varying mean (standard deviation) integration times: 550 (34) to 950 (43) μs for the Sentinel system and 2300 (71) to12000 (400) μs for the Catalyst system. The sensitivity parameters differed for each skin tone, with lighter skin requiring shorter integration times and gain/threshold values. Darker skin tones necessitated higher parameters for optimal surface images. The reliability of the systems declined with excessive parameters, leading to noise and compromised accuracy in patient positioning. ConclusionsOptimized sensitivity parameters tailored to individual skin tones are crucial for effective real-time patient surface monitoring in radiation therapy, as variations in skin colour can affect the accuracy of measurements. The precision of skin colour measurements in OSMS relies on carefully adjusting camera sensitivity parameters. However, careful consideration is essential, as larger values are required for darker skin tones, compromising reliability. This suggests the need for exploring alternative image guidance methods for patients with darker skin tones.