Cherenkov emission-based external radiotherapy dosimetry: II. Electron beam quality specification and uncertainties.

Purpose: Cherenkov emission (CE) is ubiquitous in external radiotherapy. It is also unique in that it carries the promise of 3D, micrometer-resolution, perturbation-free, in-water dosimetry with a beam quality-independent detector response calibration. Our aim is to bring CE-based dosimetry into the clinic and we motivate this here with electron beams. We Monte Carlo (MC) calculate and characterize broad-beam CE-to-dose conversion factors in water for a clinically representative library of electron beam qualities, address beam quality specification and reference depth selection, and develop a preliminary uncertainty budget based on our MC results and relative experimental work of a companion study (Paper I).

Methods: Broad electron beam CE-to-dose conversion factors include CE generated at polar angles θ ± δθ on beam axis in water. With modifications to the EGSnrc code SPRRZnrc, factors are calculated for a total of 20 electron beam qualities from four BEAMnrc models (Varian Clinac 2100C/D, Clinac 21EX, TrueBeam, and Elekta Precise). We examine beam quality, depth, and detection angle dependence for (4π detection), , , and . As discussed in Paper I, 4π detection offers the strongest CE-dose correlation and with small δθ is most practical. The two additional configurations are considered as a compromise between these two extremes. We address beam quality specification and reference depth selection in terms of the electron beam quality specifier , obtained from the depth of 50% CE , and derive a best-case uncertainty budget for the CE-based dosimetry formalism proposed in Paper I at each detection configuration.

Results: The factor was demonstrated to capture variations in the beam spectrum, angle, photon contamination, and electron fluence below the CE threshold (∼260 keV in the visible) in accordance with theory. The root-mean-square deviation and maximum deviation of a second-order polynomial fit of simulated values in terms of were 0.05 and 0.11 mm at 4π and 0.20 and 0.33 mm at detection, respectively. The fit performance on experimental data in Paper I was in agreement with these values within experimental uncertainties (±1.5 mm, 95% CI). A two-term power function fit of in terms of at a reference depth resulted in total -dependent dose uncertainty contribution estimate of 0.8% and 1.1% and preliminary best-case estimate of the combined standard dose uncertainty of 1.1% and 1.3% at 4π and detection, respectively. The results and corresponding uncertainties with the two intermediate apertures were generally of the same order as the 4π case. In addition, a theoretically consistent downstream shift of the percent-depth CE (PDC) by the difference between and improved the depth dependence of the 4π conversion by an order of magnitude (±2.8%). Therefore, a large aperture centered on a θ value between and combined with a downstream PDC shift may be recommended for beam-axis CE-based electron beam dosimetry in water.

Conclusions: By delivering -based CE-to-dose conversion data and demonstrating the potential for dosimetric uncertainty on the order of 1%, we bring CE-based electron beam dosimetry closer to clinical realization.