PHSOR06 Presentation Time: 9:25 AM

Abstract

Purpose

Two high dose rate (HDR) remote afterloaders (RALs) were recently accepted and commissioned for use in our clinic. These RALs are unique in that prior to treatment, they measure each connected applicator + transfer guide tube (TGT) channel length with the dummy wire. If this measurement deviation is within some user-specified tolerance, the RAL automatically adjusts the planned channel length (and subsequent dwell positions) used for treatment by the active wire. This work evaluates the positional accuracy of these units and reports their performance over the first few months of gynecologic patient treatments.

Materials and Methods

Acceptance testing and commissioning were performed for both units: RAL1 in August 2023, and RAL2 in November 2023. Applicator + TGT channel length verification tests were performed as part of this process. Channel lengths were measured with two independent ruler systems provided by the manufacturer. Manually measured channel lengths were within 0.5 mm of the baseline nominal values provided by the manufacturer. In order to quantify RAL positional accuracy, rigid fixed length applicators - including tandems, ovoids, cylinders, and Simon-Heyman capsules - were affixed to radiochromic film, and double exposure irradiations were performed. First, they were irradiated using a mobile C-Arm fluoroscopy unit. An autoradiograph was then acquired using mock treatment plans designed using nominal channel lengths as the planned lengths, and delivered by the RALs using an ideal applicator setup, i.e. minimizing curvature of TGTs. We configured the RAL to automatically adjust dwell positions when the difference between the RAL-measured and planned channel lengths were within 2 mm. Dwell position deviations measured on film were compared with differences between planned and RAL-measured channel lengths. The record and verify system's treatment summary was queried to extract the RALs’ measured channel lengths. The performance of each RAL was assessed over time by comparing the treatment summary channel lengths for patient treatments with the planned values.

Results

Double exposure films indicated that, on average, dwell positions deviated towards the applicator tip by 0.8mm (max=1.5mm). Mean difference between planned and RAL-measured channel lengths was 0.5 mm (max=0.9mm). On both RALs, more curved applicators had larger length deviations during commissioning and patient treatments. This increased curvature resulted in larger overestimates of channel length, up to 1.7mm for ovoids. Ovoid channel length deviations were similar for left and right ovoids on RAL1 at just over 1mm initially, and decreased over a 23 week period to be <1mm. The deviations however remained steady for RAL2 over 10 weeks of treatment, with 90% of all values being >1mm(Fig.1). Left ovoids showed larger deviation than right (1.45 vs 1.18mm respectively) (p<0.01).

Conclusions

Two HDR RALs were commissioned, and the inherent channel length verification measurement and adjustment feature was tested and validated as part of physics commissioning tests of the units. Channel length deviations were as high as 1.7mm for some applicators but gradually decreased overtime. The most curved applicator, ovoids, had the highest deviation.