Author: Peter G. Maxim, Michael P. Reilly, Mengying Shi, Maryam Shirmohammad, Darren M. Zuro 👨🔬
Affiliation: University of California Irvine 🌍
Purpose: This study aims to assess the positional accuracy of HDR brachytherapy ring applicators, quantify the extent of positional deviation, and provide clinical recommendations for GYN HDR brachytherapy.
Methods: Ring applicators exhibit a known discrepancy in the planned dwells based on the manufacturers’ provided solid models and the actual dwells delivered. There is a distal shift in the dwells that is corrected by modifying the contour points of the solid model based on 2D film measurements, as recommended by Varian. The commissioning measurements were conducted with 1 cm dwell spacing, followed by film verification measurements. A new ring model was created based on the corrected model and used clinically. The magnitude of the average distal shift was quantified through film measurements before and after the solid model adjustments for 45° and 60° ring applicators. In addition, the robustness of the positional accuracy of the adjusted model templates was evaluated for 5 mm dwell spacing for both rings using film measurements.
Results: Film measurements revealed that, before adjustment, there were distal shifts of 3.1 ±1.0 mm and 5.5 ±1.1 mm, respectively, for the 45° and 60° rings. The magnitude of shift was significantly reduced to 0.5 ±0.3 mm and 0.6 ±0.2 mm after the adjustments. However, the robustness of the adjusted templates for a dwell spacing of 5 mm was observed to be less reliable. The shifts were 1.3 ±0.4 mm and 1.1 ±0.3 mm respectively for the 45° and 60° rings.
Conclusion: Commissioning ring applicators is a complex process and evaluating the difference between delivered and planned dwells is critical. Clinical plans should be delivered with the same dwell spacing (1 cm in our study) used for commissioning measurements. It is not recommended to use smaller dwells of 5 mm.