Evaluation of UVA Emission from MV-Irradiated Tissues and Phantoms

Introduction: RECA (Radiotherapy Enhanced with Cherenkov photo-Activation) is a novel treatment that induces a synergistic therapeutic effect by combining conventional radiation therapy with phototherapy using the anti-cancer and potentially immunogenic drug, psoralen. Psoralen is photo-activated in-situ by UVA (UltravioletA, 320-400nm) Cherenkov Light (CL), produced in tissue directly by the treatment beam. In this study, we develop methods to image and quantify relative CL production (primarily in the UVA range) from a range of tissue and phantom materials upon photon irradiation. These methods are further applied to identify a tissue-equivalent optical phantom, mimicking CL production in the UVA range, in order to facilitate further RECA experiments.

Methods: The imaging system included a deep-cooled, high-sensitivity CCD camera, equipped with either a visible range lens (sensitive to 400-700nm photons) or a UVA-compatible lens assembly and a band-pass filter (sensitive to 320-400nm photons). CL emission was quantified in bulk tissue samples, solid waters (SW brown and white), and agarose gels in a series of experiments. The samples and imaging equipment were placed in a dark, light-blocking chamber to avoid contamination from other light sources. In addition, the camera was carefully positioned with respect to the LINAC head and was also shielded using lead bricks to minimize radiation noise.The samples were then irradiated with clinical photon beams, while simultaneously being imaged by the camera.

Results: In the visible range, solid water had similar CL emission to that from bulk tissue samples (34% less than the maximum and 44% higher than the minimum UVA emitting tissue). A 25% reduction in radiation noise in the UVA spectrum was achieved using lead block shielding of the camera. In the UVA range at 15MV, white SW emitted 66±5%, 64±5% and 76±3% less UVA than chicken, pork loin and pork belly respectively. Similar under-response was observed at 6MV. Brown SW had 21±8% less UVA emission than white SW at 15MV, and no significant emission at 6MV. Agarose samples (1% by weight) doped with 250ppm India Ink exhibited equivalent UVA CL emission to chicken breast (within 8%).

Conclusion: The results confirm that for the same absorbed dose, SW emits lessUVA light than the tissue samples, indicating that prior in-vitro studies utilizing SW as the CL-generating source may have underestimated the RECA therapeutic effect. Agarose gel doped with 250ppm India Ink is a convenient tissue equivalent phantom for further work.