Enhancing Radiation Therapy Through Cherenkov Light-Activated Phototherapy.

PURPOSE: This work investigates a new approach to enhance radiotherapy through a photo therapeutic agent activated by Cherenkov light produced from the megavoltage photon beam. The process is termed Radiotherapy Enhanced with Cherenkov photo-Activation (RECA). RECA is compatible with various photo-therapeutics, but here we focus on use with psoralen, an ultraviolet activated therapeutic with extensive history of application in superficial and extracorporeal settings. RECA has potential to extend the scope of psoralen treatments beyond superficial to deep seated lesions. METHODS AND MATERIALS: In vitro studies in B16 melanoma and 4T1 murine breast cancer cells were performed to investigate the potential of RT plus RECA versus RT alone for increasing cytotoxicity (local control) and increasing surface expression of major histocompatibility complex I (MHC I). The latter represents potential for immune response amplification (increased antigen presentation), which has been observed in other psoralen therapies. Cytotoxicity assays included luminescence and clonogenics. The MHC I assays were performed using flow cytometry. In addition, Cherenkov light intensity measurements were performed to investigate the possibility of increasing the Cherenkov light intensity per unit dose from clinical megavoltage beams, to maximize psoralen activation.
RESULTS: Luminescence assays showed that RECA treatment (2 Gy at 6 MV) increased cytotoxicity by up to 20% and 9.5% for 4T1 and B16 cells, respectively, compared with radiation and psoralen alone (ie, Cherenkov light was blocked). Similarly, flow cytometry revealed median MHC I expression was significantly higher in RECA-treated cells, compared with those receiving radiation and psoralen alone (approximately 450% and 250% at 3 Gy and 6 Gy, respectively, P << .0001). Clonogenic assays of B16 cells at doses of 6 Gy and 12 Gy showed decreases in tumor cell viability of 7% (P = .017) and 36% (P = .006), respectively, when Cherenkov was present.
CONCLUSION: This work demonstrates for the first time the potential for photo-activation of psoralen directly in situ, from Cherenkov light generated by a clinical megavoltage treatment beam.