Author: Jeremy Eric Hallett, Aubrey Parks, Brian W Pogue 👨🔬
Affiliation: University of Wisconsin - Madison, University of Wisconsin-Madison 🌍
Purpose: Cherenkov imaging visualizes radiotherapy beams, assuming emission is proportional to deposited dose. However, attempts for Cherenkov as a quantitative dose estimation have been unsuccessful because multi-beamlet treatment plans standard in IMRT and VMAT show reduced Cherenkov emission. This work examines the causes of reduced dose-emission proportionality in small fields.
Methods: Cherenkov emission and dose was examined first in water, avoiding complex photon transport in tissue. Cherenkov images of broad beams with a fixed dose in 1 dimension were taken with a time-gated ICMOS camera in an aqueous quinine solution. The beams had longitudinal sizes between 0.5cm and 15.0cm on the imaging axis, a constant lateral width, and prescribed doses of 1Gy at 13mm depth. This was repeated for both 6MV flattened and flattening filter free (FFF) beams. Spatial correlation between Cherenkov and dose was evaluated using a 3%/3mm gamma index. Tissue optical effects were assessed by imaging similar doses in the build-up region from a projection view from a 1% blood 1% intralipid phantom.
Results: The 3%/3mm gamma index revealed strong spatial correlation between Cherenkov and dose across all field sizes in the build-up region. However, Cherenkov emission dropped to 61% of the reference field for flattened beams and 78% for FFF fields, deviating from dose proportionality. Tissue scatter and absorption further exacerbated the divergence between Cherenkov and dose, with the smallest field emitting 38% of the reference field’s Cherenkov emission.
Conclusion: Cherenkov imaging remains reliable for dose localization in the clinic, but its intensity is affected by small beam size, likely due to beam hardening effects and loss of LCPE, as evidenced by reduced divergence in FFF beams. Tissue optical properties further distort Cherenkov emission with field sizes, challenging clinical implementation. Monte Carlo analysis is ongoing to directly attribute causes of signal loss.