Superficial dosimetry imaging of Cerenkov emission in electron beam radiotherapy of phantoms

Cerenkov emission is generated from ionizing radiation in tissue above 264 keV energy. This study presents the first examination of this optical emission as a surrogate for the absorbed superficial dose.. Cerenkov emission was imaged from the surface of flat tissue phantoms irradiated with electrons, using a range of field sizes from 6 cm x 6 cm to 20 cm x 20 cm, incident angles from 0 degrees to 50 degrees, and energies from 6 to 18 MeV. The Cerenkov images were compared with the estimated superficial dose in phantoms from direct diode measurements, as well as calculations by Monte Carlo and the treatment planning system. Intensity images showed outstanding linear agreement (R-2 = 0.97) with reference data of the known dose for energies from 6 to 18 MeV. When orthogonal delivery was carried out, the in-plane and cross-plane dose distribution comparisons indicated very little difference (+/- 2-4% differences) between the different methods of estimation as compared to Cerenkov light imaging. For an incident angle 50 degrees, the Cerenkov images and Monte Carlo simulation show excellent agreement with the diode data, but the treatment planning system had a larger error (OPT = +/- 1 similar to 2%, diode = +/- 2 similar to 3%, TPS = +/- 6-8% differences) as would be expected. The sampling depth of superficial dosimetry based on Cerenkov radiation has been simulated in a layered skin model, showing the potential of sampling depth tuning by spectral filtering. Taken together, these measurements and simulations indicate that Cerenkov emission imaging might provide a valuable method of superficial dosimetry imaging from incident radiotherapy beams of electrons.