Commissioning of Varian ring & tandem HDR applicators: reproducibility and interobserver variability of dwell position offsets

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 normalD2cc 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

A multicenter study to quantify systematic variations and associated uncertainties in source positioning with commonly used HDR afterloaders and ring applicators for the treatment of cervical carcinomas

“…and GammaMed afterloaders were largely in agreement with the previous reports for similar rings by Awunor et al, 10 Stern and Liu, 20 and McMahon et al, 21 who reported maximum offsets of up to 6.1, 5.9, and 3.6 mm, respectively, for similar rings.…”

Practical experience commissioning MRI‐compatible tandem and ring applicators for use with the Bravos HDR afterloader

“…Dwell position accuracy, including within the ring applicator, is necessary for robust treatment planning. It has been confirmed in the literature that the trajectory of the source wire along the lumen in ring applicators can undergo noncircular “snaking,” and it is recommended that each ring applicator be individually characterized for an offset correction 8–10 . The required positional offset was measured and verified during this commissioning process.…”

“…It has been confirmed in the literature that the trajectory of the source wire along the lumen in ring applicators can undergo noncircular "snaking," and it is recommended that each ring applicator be individually characterized for an offset correction. [8][9][10] The required positional offset was measured and verified during this commissioning process.Preliminary measurements were completed following Varian's recommendations included in their instructions for use documentation, 11 and then initial values from this process were used to verify the final offset. Radiochromic EBT3 film was used to acquire radiographs and autoradiographs on the rings following the same process as used for the tandems, as described in the previous section.…”

Аnalysis of the technical and dosimetric characteristics of the brachytherapy afterloaders. Ring applicator

“…Соблюдение программы гарантии качества лучевой терапии является одним из важнейших методов повышения качества лучевого лечения пациентов. Ввод аппликаторов в клиническую эксплуатацию, как часть программы гарантии качества в лучевой терапии, включает в себя проверку дозиметрических и геометрических параметров аппликаторов [1,2]. Точность позиционирования источника, которая определяется как соответствие между реальным положением источника ионизирующего излучения (ИИИ) и положением, определяемым реконструкцией аппликатора на системе планирования лечения, является основным параметром программы контроля качества.…”