Purpose: Craniospinal irradiation (CSI) is a time-consuming treatment, traditionally characterized by multiple fields and isocenters. There exists a practical need for verification of field junctions to assess proper coverage, especially at converging field areas. In this study, Cherenkov imaging was used as a technique to verify beam field overlap between upper and lower spine fields over four fractions delivered to a single patient. Beam intersection areas were quantified from the observed surface intensity profiles, verifying delivery accuracy and intra-fraction repeatability.
Methods: Cherenkov and background images were acquired using an intensified, time-gated CMOS camera (DoseOptics LLC, Lebanon, NH) with custom acquisition software designed for radiotherapy. Cumulative Cherenkov images were stitched together using transformations computed by fiducial point matching from the background images. The summed images were analyzed with focus on the junction between upper and lower spine fields. This field area was summed into a 1D profile of relative Cherenkov emission, from which the day to day variation of the FWHM was assessed and compared to the treatment plan.
Results: Field junction analysis of lower spine and upper spine fields showed excellent agreement, with a percent standard error of 1.5%. FWHM distance to conformity as compared to the treatment plan ranged from 4.2 mm -10 mm. Smaller incidences of overlapping field width can be associated with deeper intersection points of the converging beams and hot spot formation above the spinal cord.
Conclusion: Cherenkov imaging was used as a method to verify spatial field overlap of upper and lower spine fields during supine CSI, requiring no additional time during treatment. With knowledge of the beam divergence parameters, a back-projection technique could be applied to predict the beam intersection locations at the spine from these data, which would be promising for future, more quantitative Cherenkov dosimetry.
Funding Support, Disclosures, and Conflict of Interest: Dr. Brian Pogue (PI) is the president and co-founder of DoseOptics LLC, a company manufacturing Cherenkov imaging systems. This work was not financially supported by DoseOptics.