Assessment of Positional Accuracy of Ring Applicators Following Source Exchange
Abstract
Purpose
Ring applicators are widely used in high-dose-rate (HDR) brachytherapy, where accurate source positioning is essential for achieving dosimetric precision. Following each radioactive source exchange, positional accuracy is routinely verified; however, clinical experience suggests that the necessity of verification after every exchange may warrant reassessment. This study evaluates the reproducibility of dwell position accuracy in ring applicators following source exchange and examines whether repeated verification is clinically justified.
Methods
Positional verification was performed after source exchange and compared with the original commissioning planning template. Ring applicators were affixed to Gafchromic film, which was initially irradiated using 12 MeV electrons to 700 monitor units to create a reference outline of the applicator geometry. The applicators were then connected to an HDR afterloader. A test plan consisting of nine dwell positions spaced 1 cm apart was delivered to the film. Films were scanned and analyzed to determine measured dwell positions, which were compared to commissioning values to assess positional accuracy and reproducibility. Dosimetric impact was evaluated using previously treated patient plans by applying measured positional shifts and comparing resulting dose metrics to the unshifted clinical plans.
Results
Measured dwell positions demonstrated variability of up to 3 mm relative to initial commissioning. Dosimetric analysis revealed a maximum change of 3.3% in high-risk clinical target volume (HR-CTV) D90 for evaluated patient plans.
Conclusion
Measurable positional variations in ring applicator dwell positions following source exchange were observed. However, clinical use of 5 mm dwell spacing, rather than 1 cm, may mitigate the dosimetric impact of these variations. Ongoing work will further characterize the effect of positional variability on dosimetric quality across a broader patient cohort.