Purpose: Cherenkov emission (CE)-based dosimetry is an emerging technique that is especially promising for small-field measurements. Here, we present Monte Carlo (MC) simulations and relative experimental validation of photon-beam CE, optimize detection angle geometry, and simulate optical sectioning in a small field with added noise.
Methods: Additional photon beam experimental validation is provided for a previously-published MC code. Both large and small fields are measured at 90° to beam in water with lenses focusing into a fiber+CCD. Small field measurements are 1D with field size varied only transverse to the optical axis (10x10 to 0.5x10 cm2), pending detector development. Optimization of polar angle (relative to beam) and aperture is performed for a 10 MV photon beam MC model, on percent-depth dose-CE (PDD-PDC) root-mean-squared error and Gamma index, with consideration of convenience. In addition, we simulate 1D (along optical axis) optical sectioning of a 5x5-cm2 small-field Matlab model, assuming diffraction-limited optics and using the theoretical PSF with added Poisson noise. Based on these findings and the literature, the achievable detection system specifications and potential costs are laid out.
Results: Measurements and simulations were in agreement within uncertainty (<1% for 10x10 cm2, <7% for 0.5x10 cm2). Optimal detection included multiple angles, but a sufficiently accurate single-angle compromise was 57.5°±5.0° [uncertainty (k=1): 1.2%/0.3mm in build-up and 0.2% in drop-off, with <1mm upstream shift in build-up and <0.5% correction in drop-off]. The optical sectioning test case and accompanying analysis revealed that shot-noise limited detection should be possible and reasonably priced, with expected system quantum efficiency >90%, ±2.5° polar angle resolution, short acquisition times (~seconds), sensitive volume of 0.0013 mm3 (5 μm along optical axis), and traceable calibration uncertainty of 0.5%.
Conclusion: These findings indicate that, especially for small fields, optical sectioning may be an attractive (viz. accurate and affordable) dosimetry alternative, unique to CE.