Imaging system for real-time, full-field pulse-by-pulse surface dosimetry of UHDR electron beams.

Background: The interest in ultra-high dose rate (UHDR) radiation therapy (RT) has grown due to its potential to spare normal tissue. However, clinical application is hindered by dosimetry challenges, as current irradiators and dosimeters are not designed for UHDR's high fluence. To ensure safe treatment and accurate dose delivery, real-time dose and dose rate quantification methods are essential.

Purpose: We propose a novel scintillation imaging system for in vivo, pulse-by-pulse surface dose monitoring during delivery with a UHDR-capable Mobetron (IntraOp LLC Sunnyvale, CA, USA) system. This setup aims to measure entrance beam dose with high 2D spatial and temporal resolution.

Methods: A modified collimating cone was 3D printed to house the imaging lens. The system featured a 90° sinuscope endoscope attached to a CMOS camera, was gated by the Mobetron's magnetron output signal, and captured light from a scintillator placed on the treatment surface. Three scintillator types were tested for their emission intensity and decay time. Dose and dose rate linearity studies were performed using various pulse lengths and repetition frequencies, respectively, and the imaging data were compared to an EDGE diode detector (SunNuclear Melbourne, FL, USA) and the Mobetron beam-current transformer (BCT) measurements.

Results: Dose (R² = 0.993) and dose rate (within 2%) were linear, and the temporal beam structure agreed with the diode and BCT data, as evident by the fact that it was successfully gated such that it captured each pulse during testing. Dose per pulse measurements agreed with diode and BCT data within 2.0 ± 1.2 cGy (0.6% ± 0.3%) and 2.5 ± 1.0 cGy (1.1% ± 0.4%), respectively.

Conclusions: The developed imaging system met the criteria for measuring entrance beam dose with high spatial and temporal resolution, offering a promising in vivo dosimetry method for UHDR RT in preclinical and clinical trials.