Spatially and Temporally Resolved Optical Calorimetry for Proton Flash Beams
Abstract
Purpose
Ultra-high Dose-Rate (UHDR) radiotherapy challenges conventional dosimetry methods. Optical calorimetry (OC) enables direct, dose-rate-independent measurement of spatially and temporally resolved dose-to-water distributions. This work validates OC as a dosimetry method in UHDR proton beams for the first time.
Methods
The study was conducted on the University of Washington 50 MeV UHDR research proton beam line. Images of the Bragg Peak (BP) and buildup region were continuously recorded with digital holographic interferometry at 60 frames-per-second. The measurement probe was an expanded laser beam with a diameter of 14 mm, and data acquisition occurred continuously during irradiation. Absolute entrance dose was measured by translating the laser beam to probe the water cell surface. Reconstructed two-dimensional absolute dose distributions were obtained for a range of dose-rates between 80–102 Gy/s and compared with Ion Chamber (IC) measurements and Monte Carlo (MC) simulation.
Results
Radiation dose was measured simultaneously at all depths within the laser beam diameter without introducing a physical probe into the water. Minute radiation-induced temperature changes were holographically reconstructed in under one minute and converted to dose-to-water using fundamental calorimetric equations. OC successfully resolved the BP with a resolution of 50 µm. OC‑derived depth‑dose curves were consistent with MC modelling and IC results. At 102 Gy/s the IC determined entrance dose (8.8 Gy) agreed with OC (9.3 Gy) to within the current OC measurement uncertainty (9.9%).
Conclusion
OC accurately measured spatially and temporally varying dose distributions in a UHDR proton beam with high resolution. Absolute dose determination was achieved without dose-rate-dependent correction factors and showed strong agreement with IC dosimetry. Further work is required to reduce type-A measurement uncertainty and improve the practical implementation of OC dosimetry.