Room: Karl Dean Ballroom B1
Purpose: The emerging clinical implementation of MRI integrated radiotherapy systems requires rigorous understanding of magnetic field effects on dose deposition, particularly at tissue-air interfaces from the electron return effect (ERE). The purpose of this study was to present a new method of visualizing and quantifying ERE in phantoms to validate simulations of an MR-linac treatment planning system (TPS).
Methods: A ViewRay MR-linac (0.34T primary magnetic field, 6MV FFF beams) was studied. To simulate air cavities in tissue, three phantoms were constructed 1) block of acrylic milled with holes of 2cm and 3cm diameter, 2) split block of acrylic with three holes of increasing diameter, into which a piece of film was inserted, and 3) a water tank inset with air-tight tubes (0.95cm, 1.6cm diameters). All cavities were painted black to inhibit optical reflections. Phantoms were individually irradiated while concurrently imaged with an intensified CMOS camera (DoseOptics LLC., Hanover, NH) synchronized to the radiation pulses to detect the generated scintillation and Cherenkov emission. The generated 2D images were compared to the planar sum along the optical axis of the TPS dose simulations in the acrylic or water.
Results: Optical images qualitatively matched the TPS dose predictions in terms of the areas with elevated or lowered dose. With 100% dose defined at Dmax, the optical images of phantom 1 were within 3% of the maximum escalation reported by the TPS. The size of the >100% isodose region was much larger in the optical images than the TPS (23.2% and 809% increase in physical area for 3cm and 2cm holes, respectively).
Conclusion: The optical measurements of dose variation due to the ERE agree with TPS simulations in terms of maximum observed dose, however the size of the area indicated that the TPS may be under-reporting the areas impacted by dose escalation and reduction.
Funding Support, Disclosures, and Conflict of Interest: This work has been funded by NIH grants F31CA192473 and R01EB023909. Potential conflict of interest arises from contributing author B. Pogue, founder and president of DoseOptics LLC developing Cherenkov imaging systems. However, this work was not financially supported by DoseOptics in any way.