Improving Cherenkov-based radiotherapy dose estimates in diverse patient populations via skin luminance imaging

Abstract Objective . Cherenkov imaging is an emerging technology that detects light naturally emitted from patient tissue during radiation treatment. The initial intensity of Cherenkov light is proportional to radiation dose, but its absorption is highly dependent on patient skin pigmentation, with increasing melanin attenuating more Cherenkov photons per dose. This effect must be calibrated per patient before Cherenkov emission can serve as an accurate surrogate for dose. In this study, we present the first attempt at in vivo Cherenkov imaging of a diverse patient cohort and calibration for the effect of skin pigmentation towards quantitative Cherenkov dosimetry. Approach . A multi-institutional collaboration was designed to increase the diversity of our patient imaging cohort. Cherenkov imaging was completed with a time-gated, iCMOS camera, and color background images were taken with an RGB camera module under standardized lighting. Under an institutional review board-approved retrospective protocol, skin pigmentation was assessed per patient by calculating the relative luminance of their treated area from the color images. Additionally, 2D dose maps were generated by projecting the exponentially-weighted dose from the surface to 5 mm into the body, representative of Cherenkov emission. Main results. Of N 6MV = 23 and N 15MV = 20 breast patients imaged, encompassing a variety of skin pigmentations, Cherenkov intensity varied nearly 5X for the same dose delivered across the examined cohort. Plotting Cherenkov intensity per unit dose revealed a direct correlation with relative luminance, providing a linear calibration factor based on skin pigmentation. Including this calibration factor significantly improved Cherenkov-to-dose linearity, from R 2 = 0.79–0.96 for 6 MV and R 2 = 0.19–0.91 for 15 MV ( p < 0.05). Significance . This study marks the first assessment of Cherenkov imaging in a diverse, representative patient population. It addresses an integral factor towards achieving quantitative in vivo Cherenkov dosimetry and demonstrates significant mitigation of the effect of skin pigmentation, while preserving the non-contact, real-time benefits of Cherenkov imaging.