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Characterization of a 3D Interstitial Ring Applicator Using Fluroscopic Images and Gafchromic Films to Accurately Model High-Dose-Rate (HDR) Brachytherapy Delivery

R Patel

R Patel*, A Plypoo , M Mysz , M Surucu , J Roeske , W Small , M Harkenrider , H Kang , Loyola Univ Medical Center, Maywood, IL


SU-E-205-4 (Sunday, July 30, 2017) 1:00 PM - 1:55 PM Room: 205

Purpose: Our institution is the first to clinically use the Varian 3D interstitial ring applicator. For the GammaMedPlus iX afterloader, there are uncertainties in the source positions along the full circular path of the ring. This study is designed to characterize the exact dwell locations to propose an empirical treatment planning solution specific to our clinic.

Methods: Fluoroscopic images of the applicator with the source within the ring were acquired using a C-Arm unit. The distal end of the source wire was located for each dwell position (5.0mm step size), and results were substantiated with film measurements. Both tests were repeated to determine the uncertainty in dwell positions. Nine cervix cases using the ring applicator were retrospectively planned with and without offset correction using known dwell positions, and the resulting doses were evaluated.

Results: Fluoroscopic imaging and film measurements of the source wire revealed a 3.0mm dead space at the distal end of the ring lumen. Additionally, the step size for dwell positions between 130-128.5cm was non-uniform and varied between 1.5–7.0mm. After 128.5cm, the measured step-size was uniformly 5.0mm for the remainder of the ring dwell positions. The positional uncertainty based on fluoroscopy was 0.6mm and for film was 1.5mm. Based on this study, we propose a planning solution to extend the applicator 2.0mm beyond the distal end of the ring channel and not load the 1st (130cm) and 3rd(129cm) dwell positions. This technique ensures all loaded dwell positions coincide with measured positions (<1.0mm). Retrospectively planned data suggests that not accounting for this offset minimally effects target coverage while having up-to 4% effect on rectal dose.

Conclusion: A new interstitial ring applicator was characterized with direct imaging of the HDR source for our clinic. Our planning proposal accounts for accurate dwell positions and generates plans consistent with treatment delivery.

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