Abstract
Studies have shown that source dwells within Varian's HDR CT/MR compatible ring applicators can deviate from intended positions by several millimeters. Quantifying this offset is an important part of commissioning. The aims of this study were to: 1) determine the reproducibility of the offset, 2) study the interobserver variation in the offset's measurement, and 3) quantify the dosimetric impact of the offset. Offsets were measured for four ring applicators: two 30°, one 45°, and one 60°. Dwell positions were measured five times for each ring to determine the reproducibility of source positioning. Experiments were done to compare two separate source wires, as well as different time points within a single source wire's lifecycle. Data were analyzed by three independent observers. To quantify the dosimetric impact of the offset, a treatment plan was generated using BrachyVision. The dose to point A, and the D2cc metric for rectum and bladder were calculated with and without the offset. For the 45° and 60° rings, measured offsets were 3.0 mm and 3.6 mm, respectively. The 30° ring showed substantial variation in distal dwell positions (maximum difference between the five experiments of 2.9 mm). Subsequent testing of a replacement ring showed an offset of 2.4 mm that was more reproducible. Offsets varied less than 1 mm between different source wires, and changed less than 1 mm over the course of a source wire's lifecycle. When comparing observers, the average range in a measurement of a dwell position was 0. 5mm (σ = 0.2 mm, max 1.3 mm). The offset resulted in dose variations to point A, bladder, and rectum of less than 1%, 2%, and 5%, respectively. Results indicate that Varian rings can show systematic and random offsets of more than 3 mm. Some can be considered defective and should be replaced. Each applicator should be individually commissioned and reproducibility should be confirmed with multiple tests.PACS number: 87.56.‐v, 87.53.Jw
Highlights
Low-dose rate (LDR) was once the cornerstone of cervical brachytherapy, but increasing availability of remote high-dose rate (HDR) afterloaders has lead many centers to transition to HDR.[1,2,3,4] With HDR, a classical tandem and ovoid applicator can still be used, but some centers prefer to use a ring and tandem instead
This curvature introduces an uncertainty in the determination of the actual dwell position of the HDR source. This positional error is the subject of this study. Data from both end users and the manufacturer have shown that delivered dwell positions within Varian HDR ring applicators can deviate from the intended positions by several millimeters.[5,6] The discrepancy arises because the source wire is much smaller than the diameter of the ring lumen through which it travels
Once this systematic offset is determined, a correction can be applied to treatment plans such that the delivered dwell positions will match the dwell positions used in the treatment plan
Summary
Low-dose rate (LDR) was once the cornerstone of cervical brachytherapy, but increasing availability of remote high-dose rate (HDR) afterloaders has lead many centers to transition to HDR.[1,2,3,4] With HDR, a classical tandem and ovoid applicator can still be used, but some centers prefer to use a ring and tandem instead Both have similar dosimetric characteristics, but the ring applicator is generally smaller than a pair of ovoids, and the larger range of dwell positions offers more flexibility for treatment planning. 51 McMahon et al Reproducibility and variability of tandem and ring applicators offset This curvature introduces an uncertainty in the determination of the actual dwell position of the HDR source. Once this systematic offset is determined, a correction can be applied to treatment plans such that the delivered dwell positions will match the dwell positions used in the treatment plan
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