Cherenkov-excited fluorescence imaging for small-field dosimetry and SRS plan verification: a feasibility study

To develop a high-spatial-resolution Cherenkov-excited fluorescence dosimetry system (CEFD) with rapid optical readout using quinine sulfate solution and evaluate its feasibility for small-field dosimetry and verification of patient-derived stereotactic radiosurgery (SRS) dose distributions. Approach. A 3D-printed optical phantom containing 1 g/L quinine sulfate solution was imaged with a CMOS camera through a 45° mirror. Sensitive-volume thicknesses of 2.5, 5, 10 and 15 mm were compared using calibration response, signal-to-noise ratio, output factors, dose-rate dependence, and small-field beam profiles benchmarked against a PTW 60023 microSilicon detector. The selected CEFD configuration was further evaluated for thermal response, repeatability, signal drift, low-MU response, and small-field gamma performance, and then used for fixed-gantry verification of 31 patient-derived SRS dose distributions. GafChromic EBT3 film was used as the high-spatial-resolution comparator. Main results. The CEFD achieved 0.065 mm/pixel spatial sampling. CEFD-10 showed the most balanced overall performance across sensitivity, dose-rate response, output-factor agreement, and small-field profile measurements. Dose-rate deviations remained within 1.7% at 300-1400 MU/min. Passive heat sinking reduced the coefficient of variation from 0.26% to 0.13% and limited signal drift from 4.3% to 0.97% over 30 consecutive measurements. For the 0.5 × 0.5 cm² field, the CEFD-10 central-axis dose ratio relative to the TPS-projected dose remained within ±2% from 10 MU onward. For 0.5 × 0.5-3 × 3 cm² static fields, CEFD-10 achieved 99.56%-100.0% global 2%/1 mm gamma passing rates with a 10% low-dose threshold. For patient-derived SRS verification, CEFD-10 achieved mean global gamma passing rates of 99.47 ± 1.58% and 97.53 ± 3.58% for 2%/2 mm and 2%/1 mm criteria, respectively, with mean differences from EBT3 film of -0.32% and -0.21%, respectively. Significance. This study demonstrates compact, high-spatial-resolution, rapid-readout optical dosimetry for small-field and patient-derived SRS dose patterns. Agreement with EBT3 film supports CEFD-10 as a controlled-geometry SRS dose verification.