KEY POINTS
- Investigators irradiated a tissue-equivalent optical phantom with a 6-MV TrueBeam using 20 rectangular fields ranging from 0.5×5 cm² to 15×5 cm², each delivering 1000 MU, while an intensified CMOS camera measured surface Cherenkov emission.
- Detected Cherenkov emission per unit dose remained relatively stable for larger fields but decreased nonlinearly as fields narrowed. Signal stayed within 95% of the 10×5-cm² reference down to 4×5 cm² and within 90% down to 3×5 cm².
- For the smallest 0.5×5-cm² field, detected Cherenkov signal per unit dose was only 30.5% of that measured for the 10×5-cm² field, demonstrating that image brightness cannot be assumed to scale directly with dose in small-field conditions.
- TOPAS Monte Carlo simulations showed that narrowing the field increased mean incident photon energy by 5.0% and contaminant-electron energy by 2.5%. In contrast, mean energy of the particles actually producing Cherenkov light changed by only about 0.3%.
- Cherenkov production remained strongly linked to secondary charged-particle dose deposition: the number of Cherenkov-producing particles increased approximately linearly with field size, while yield remained near 25 Cherenkov photons per parent particle.
- Secondary electrons generated within the tissue dominated Cherenkov production across field sizes. Changes in particle angle were also minimal: the average Cherenkov emission angle changed by only about 0.2%.
- The authors therefore attribute most of the measured field-size effect to optical absorption, scattering, and loss of optical equilibrium, rather than a large intrinsic change in Cherenkov production per delivered dose. The experiments used static rectangular fields and a phantom, so translation to patient-specific dynamic IMRT/VMAT requires further study.
CLINICAL TAKEAWAY
Cherenkov imaging remains attractive for real-time treatment visualization, but signal intensity should not be interpreted as a simple direct surrogate for dose when small or highly modulated fields are used. Field size and tissue optical transport will need to be accounted for if Cherenkov imaging is to evolve from qualitative treatment visualization toward quantitative in-vivo dosimetry.