First Demonstration of Medical Radiation Treatment Beam Monitoring via Air Scintillation Imaging

Abstract

Error rates in radiation therapy administration, although reportedly low, can have devastating consequences. Recent fatal incidents could potentially have been mitigated if real-time methods of monitoring delivery of radiation during treatment had been utilized. However, few existing methods are practical enough to be used routinely without disrupting clinical operations. The purpose of this study is to investigate a novel non-perturbing method of monitoring radiation therapy. The proposed method for monitoring radiation delivery exploits the fact that when ionizing radiation propagates through the atmosphere, it excites nitrogen gas on its path. This excitation induces weak luminescence (primarily in the 300-400 nm range) via the process of air scintillation. While not visible to the naked eye, air scintillation can be imaged with a sensitive camera to probe the physical characteristics of a radiation beam. An electron-multiplication charge-coupled device camera (f/0.95 lens) was set-up in a clinical treatment vault and was used to capture air scintillation images of kilovoltage and megavoltage beams. The vaults were prepared to block background light and a short-pass filter was utilized to block light above 440 nm. Megavoltage electron beams from a linear accelerator, as well as an orthovoltage unit (50 kVp, 30 mA) were visualized with a relatively short exposure time (10 s), showing an inverse intensity falloff (r2 = 0.89) in image intensity along the central axis. For a fixed exposure time (100 s), air scintillation was shown to be proportional to dose rate (r2 = 0.9998). As beam energy increased from 6 MeV to 20 MeV, air scintillation images exhibited a decrease in the divergence of the electron beam and improved penumbra. The irradiation of a transparent phantom also showed that Cherenkov luminescence does not interfere with the detection of air scintillation. In a final illustration of the capabilities of this new technique, air scintillation was captured during a total skin irradiation electron treatment. Air scintillation can be measured to monitor a radiation beam in an inexpensive and non-perturbing manner. This new method is particularly advantageous for monitoring a wide area in a single acquisition, which may be useful for online verification of total body/skin/lymphoid irradiation treatments. Future work on air scintillation aims to further validate this method using accurate Monte-Carlo simulations and translate this technology to the clinic.