Cherenkov emission in realistic optical body phantoms to study effects of skin tone on imaging delivery technique

Objective.In radiotherapy, Cherenkov light emitted from tissue originates predominantly from depths of 0-5 mm within the body, and subsequently darker skin can reduce the observed signal passing through the epithelium. The goal of this study was to assess the spatial correlation between patient near-surface dose and Cherenkov emission with varying patient skin tones and radiotherapy delivery techniques. The work focused on tissue phantoms that recapitulated the layered structure of human melanin pigment in skin allowing for a systematic study difficult to perform in humans.Approach.Six tissue-like, layered silicone phantoms following the progressive Fitzpatrick skin scale were created using a thermoformed mold of a torso phantom. Pigments were applied in thin silicone coatings to simulate the epidermis. Each skin was successively placed on the phantom and computed tomography-scanned for whole breast irradiation treatment planning. Plans were delivered to each phantom for: two tangent fields, dynamic field-in-field, and volumetric modulated arc therapy (VMAT), with online Cherenkov imaging.Main Results.The measured Cherenkov intensity showed a linear trend with luminosity (r²≈0.97) and a melanin index (MI) range consistent with literature (MI range 37-120). Cherenkov image shape comparison with the expected surface dose maps showed a dice score range of 0.93-0.95 for all treatment plans on lighter skin phantoms. For the darkest skin type (MI = 120), the dice score decreased to ∼0.88 for the tangent and field-in-field plans and to ∼0.80 for VMAT. When disagreement was found it tended to be in the lowest dose edges of the field.Significance.Large MI values (type VI) result in lower Cherenkov-to-plan agreement as compared to measurements made with lighter skin tone phantoms, especially when imaging VMAT plans which show reduced optical flux. These results highlight the need for strategies that increase captured optical signal for situations where the Cherenkov light is attenuated by high melanin.