Influence of Field Size Changes on Cherenkov Radiation Production
Abstract
Purpose
Cherenkov imaging offers real-time visualization and verification of external beam delivery during radiation therapy. However, the relationship between beam parameters and Cherenkov photon yield per unit dose remains underexplored, especially for small-fields such as in IMRT treatments, where Cherenkov imaging is critically important. Here, this link is established for accurate clinical interpretation of Cherenkov imaging.
Methods
Cherenkov beam profiles in an aqueous quinine solution were acquired using an intensified CMOS camera, with field sizes ranging from 0.5x5.0cm2 to 15.0x5.0cm2 to evaluate the dependence upon lateral beam size. Modeling of the Cherenkov production per unit dose was done via Monte Carlo simulation with Tool For Particle Simulation (TOPAS) to examine characteristics such as beam hardening, incident particle contamination, and loss of lateral charged particle equilibrium (LCPE).
Results
Experimentally, the smallest 0.5x5.0cm2 field exhibited a ~45% reduction in Cherenkov emission per unit dose at dmax in the absence of tissue-like optics. Additionally, incident photon spectra hardened by 5% between the smallest and largest fields, while contaminant electrons hardened by 2.5%. There is a decrease of Cherenkov produced directly from contaminant electrons and positrons with decreased field size of ~5%. Finally, TOPAS simulations predicted minimal decrease in the Cherenkov per unit dose along the beam center among the changing field sizes.
Conclusion
Despite reduction in experimentally acquired Cherenkov per unit dose, Monte Carlo simulations indicate that the quantity remains constant with field size, implying that there are additional unexplained effects that alter Cherenkov intensity. Beam hardening has a minimal impact on Cherenkov production with field size because absorbed particles are below the Cherenkov threshold. Simulation study of contaminant electrons showed minimal impact on Cherenkov production per unit dose with field size. Understanding Cherenkov production changes with beam diameter allows for better informed use in online treatment verification for highly modulated IMRT delivery.