Advancing HDR Brachytherapy: Real-Time Optical in vivo Dosimetry with Source Tracking
Abstract
Purpose
Interstitial brachytherapy enables highly conformal radiotherapy, but accuracy depends on surgical skill, and few in vivo dosimetry systems exist. Effective monitoring requires not just dose but tracking source position, dwell time, and transit times. We developed a CMOS camera-based dosimeter with an endoscopic lens imaging a line scintillator in a transrectal probe. We hypothesized that a scintillator-based optical dosimetry system could accurately measure source position, source speed, dwell times, and absolute dose rate in real time during HDR brachytherapy, providing in vivo treatment verification.
Methods
A transrectal probe was designed to accommodate an optical dosimetry system comprising a scintillator, an endoscope, and a CMOS imaging sensor. A water-filled phantom simulates a rectal cavity, housing the probe and brachytherapy needles at varying distances from the scintillator. A 3-channel treatment plan with six dwell positions (1 cm spacing, 5-second dwell time) was delivered using Ir-192 in a clinical afterloader. The video from the treatment was processed and analyzed to find cumulative dose, transit dose, source positions, and speed. Validations used Monte Carlo for dose, optical measurement for source motion, and system logs.
Results
Dose was measured by integrating the pixel intensities of the recorded video and converting the result to dose using a calibration factor, yielding a total dose of (14.08 Gy) with an ideal expected dose of (14.16 Gy). Average intradwell speed was (6.1 +/- 0.92 cm/sec) and matched the validation within 3%, average dwell time was (5.22 +/- 0.2 seconds).
Conclusion
This system shows time and spatially resolved in vivo dosimetry during HDR brachytherapy treatments. This real-time verification reveals characteristics that are otherwise known only computationally. The dosimeter provides independent verification of delivered dose and source positions, as well as the data necessary to assess the clinical significance of often-overlooked parameters, such as source speed and transit dose.