1107 - Monte Carlo-Validated In Vivo Optical Dosimetry and Source Tracking for HDR Brachytherapy

05:25pm - 05:30pm ET

Room 156

Presenter(s)

Kevin Willy, BS - Dartmouth College, Hanover, NH

K. Willy¹, M. Clark², P. Bruza¹, S. M. McVorran³, and D. J. Gladstone⁴; ¹Thayer School of Engineering, Dartmouth College, Hanover, NH, ²Stanford University, Stanford, CA, ³H. Lee Moffitt Cancer Center and Research Institute, Department of Radiation Oncology, Tampa, FL, ⁴Geisel School of Medicine at Dartmouth & Norris Cotton Cancer Center, Dartmouth Hitchcock Medical Center, Lebanon, NH

Purpose/Objective(s):

Prostate HDR brachytherapy delivery accuracy depends on correct source positioning, dwell times, and catheter geometry, yet routine in vivo verification is uncommon and typically limited to surrogate measures of cumulative dose. We hypothesize that a transrectal scintillator/CMOS dosimetry system can measure the delivered HDR brachytherapy dose rate/total dose, source position, dwell times, and transit times/speed, providing independent verification of treatment delivery.

Materials/Methods:

A custom transrectal probe houses a line scintillator and an endoscopic camera. The probe was placed in a water-filled phantom simulating a rectal cavity, with HDR needles positioned at varying distances from the scintillator. A 3-channel Ir-192 plan with six dwell positions (1-cm spacing; nominal 5-s dwell) was delivered on a clinical afterloader. The recorded video was processed to extract time-resolved scintillation intensity (converted to dose via calibration), source position, dwell times, transit times, and source speed. The absolute dose was validated using Monte Carlo modeling of the experimental geometry; source motion/speed and dwell times were validated using independent optical measurements and afterloader log files.

Results:

Video-integrated dose measured 14.08 Gy compared with an expected 14.16 Gy (absolute difference 0.08 Gy, 0.6%). The system measured an average intradwell source speed of 6.1 ± 0.92 cm/s, agreeing with independent validation within 3%. Average dwell time was 5.22 ± 0.2 s for nominal 5-s dwells, demonstrating the ability to resolve clinically relevant deviations in delivered timing and quantify transit-dose contributions.

Conclusion:

A transrectal scintillator/CMOS optical probe can provide time- and position-resolved in vivo verification of HDR brachytherapy delivery, including absolute dose, dwell times, and source motion (position/speed/transit). These parameters are otherwise inferred only from the plan and afterloader logs, rather than directly observed in vivo. This approach may enable detection of delivery deviations (e.g., dwell-time errors, unexpected transit behavior, positional inconsistencies) and support future clinical workflows for independent verification during prostate HDR treatments.