Medical Physics Seminar – Monday, February 21, 2022
Quantifying radiotherapy dose & FLASH effects with optical signals
Dr. Brian Pogue
Professor & Chair of the Dept. of Medical Physics, UW - Madison
Measurement of dose delivery has always been a challenge in radiotherapy, and the invention of advanced treatment planning methods combined with extensive linac QA has made this less critical. However, it is still important to be able to quantify dose delivery in complex situations, or when unexpected errors occur. Cherenkov imaging was a technology developed to allow visualization of the beam delivery and has been commercialized into a tool that provides real-time images of the beam on the patient. However, this approach does not exactly quantify dose on the patient, and so additional methods such as scintillators can be combined with this to quantify dose to a precision that is clinically useful. The steps towards what could be called non-contact radiation dosimetry will be discussed, and future goals outlined.
These tools for optical sensing of radiation can be applied to the recently developed area of Ultra-High Dose Rate (UHDR) radiotherapy. In this approach, it has been observed that UHDR beams can be designed to deliver less radiobiological damage to normal tissues while still preserving the dose-response of tumor tissue, termed the FLASH effect. In order to verify these observations are valid, it is critical to compare doses, dose rates, and what is called the beam temporal structures. Beam monitoring for FLASH-RT is challenging due to UHDR conditions, where many detector systems fail to provide accurate doses or are not sufficiently fast to measure the dos rates per pulse. In early and ongoing studies, dose-rate independent radioluminescent-based techniques have been used for resolution of the dose from individual pulses from an electron linac (pulse time=4 microsec and >1Gy/pulse). Control of the linac has been a challenge and methods to advance reliable dosimetry in FLASH are an active area of research and development. Additionally sensing oxygen within tissue is achieved optically, and is especially useful for mechanistic studies in FLASH radiotherapy.