3366 - In-Vivo Time-Resolved Optical Dosimetry with Line Measurement and Source Tracking for Prostate High-Dose-Rate Brachytherapy

Purpose/Objective(s):

Brachytherapy enables highly conformal radiotherapy delivery; however, its accuracy relies on surgical precision and few, if any, in-vivo dosimetry systems are available to verify dose. Previous attempts at in-vivo dosimetry lacked the spatial and temporal resolution necessary to accurately measure transit and dwell times, source position, or deposited dose. To address this, we developed a CMOS camera with endoscopic lens capable of imaging a line scintillator inside a transrectal probe. We hypothesized that a scintillator-based optical dosimetry system within a transrectal probe could accurately measure source position, source speed, dwell times, and absolute dose-rate (DR) in real time during HDR brachytherapy, providing a clinically viable method for in-vivo treatment verification.

Materials/Methods:

A transrectal ultrasound probe was designed and manufactured to house an optical dosimetry system consisting of a thin 67mm long scintillator strip, an endoscope, and a CMOS imaging sensor. A water-filled phantom was designed simulating a rectal cavity to house the probe with interstitial brachytherapy needles positioned at 15, 10, and 7mm from source center to the visible surface of the scintillator. Dose was delivered using these 3 channels, with 6 dwell-positions spaced 1cm apart and a 5-second dwell time per position, utilizing a 10.305 Curie Ir-192 source in a clinical afterloader. Recorded video from the treatment was analyzed to measure optical intensity at each dwell position, dwell time, and source speed and then compared to the treatment report from the system. The time at each dwell point was measured by counting frames at each stationary point of intensity and converted to time knowing the camera captured at 30 frames per second. A function between optical intensity and dose rate was determined and used to calculate dose at all points of source transit.

Results:

Using this imaging system, we measured the dwell time to be 5.03s (97% CI:5.01,5.05), agreeing with the machine measured time within SE=0.8% vs 0.5%. The dwell position agreed within a standard error (SE) of 14.3%. Source speed and DR were measured to be 6.13 cm/s (97% CI:5.54,6.73) and 27.18 cGy/s (99% CI:26.49,28.15), respectively, compared to an expected DR of 26.73 cGy/s (SE=4.47%).

Conclusion:

This prototype system shows promise of providing measurements of time and spatially resolved in-vivo dosimetry in HDR prostate brachytherapy treatments. It is packaged ideally to provide treatment verification with minimal change to the current standard-of care workflow. Implementation of this real-time verification enables clinicians to see treatment characteristics otherwise only determined computationally, reducing uncertainty margins and allowing for more optimization of the therapeutic ratio improving tumor control and patient outcomes.