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

Optical surface monitoring systems (OSMSs) have gained substantial attention in modern radiation therapy, specifically in the context of surface guided radiation therapy, which offers real-time patient surface monitoring, ensuring accurate and effective radiation therapy treatments. The aim of this article is to evaluate the OSMS camera sensitivity toward different skin tones, categorized according to the Fitzpatrick scale, a universal classification of human skin tones, using a phantom.

This study used Catalyst and Sentinel OSMSs (C-RAD). The Alderson RANDO female pelvis phantom, located at the isocenter in computed tomography simulation and treatment rooms, served as an experimental subject. Eighteen skin tone-matching cotton cloths, selected on the basis of Von Luschan 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 2 months, with 16 measurements for each OSMS taken in varying light conditions.

The OSMSs successfully detected the surface of cloth-covered phantoms with varying mean (SD) integration times: 550 (34) to 950 (43) μs for the Sentinel system and 2300 (71) to 12,000 (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.

Optimized sensitivity parameters tailored to individual skin tones are crucial for effective real-time patient surface monitoring in radiation therapy, as variations in skin color can affect the accuracy of measurements. The precision of skin color measurements in OSMSs 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.