Session Item

During intracavitary pulsed dose rate (PDR) brachytherapy (BT) for cervical cancer using ring applicators, intermittent friction or obstruction errors are encountered with the check and source cables, requiring operator intervention. This study ascertains patterns and plausible causes of these errors by analyzing the actual source paths in the rings.

Errors from 60 consecutive 58-hourly-pulse BT treatments using Interstitial CT/MR ring applicators on 2 microSelectron PDR afterloaders (Elekta, Sweden) were identified. We evaluated obstruction patterns caused by the check and source cables during both out- and in-drives. To determine the Ir-192 source paths in the rings (diameters of 26 and 30 mm), a portable x-ray machine (GE AMX-4, GE Healthcare, USA) was used to image the source capsule (Fig. 1(a)) at pre-programmed dwell positions: (i) 6 dwells at 15 mm separation (Fig 1(b); illustrates complete path in ring) and (ii) 5-6 dwells at 5 mm separation (Fig 1(c); illustrates capsule position at adjacent dwells). The check cable path was fluoro-imaged when the capsule reached the distal ring end; however, the cable’s high travel speed precluded images suitable for drawing conclusions. Repeated runs were performed with the transfer tube in different arrangements (alignment, curvature) relative to the ring to reproduce the obstructions.

In 45 of 60 ring treatments (26 mm (n=37) and 30mm (n=23), the check cable produced in total 625 obstructions, mostly in the curved portion of the ring (81.9% for 26mm (54.9% from 180°-0°) and 78.8% for 30mm (55.7% from 0°-180°)) during out-drive. Only 2 obstructions arose during in-drive. Image analysis showed that the source capsule tip comes quite close to the outer wall of the lumen (width ~ 3.0mm for all rings) near 90° and 270° (Fig 2). With greater drive power for the source cable (~ 10% more), fewer obstructions were encountered: 24 during out-drive and none during in-drive. 15.1% (26mm) and 16.4% (30mm) of obstructions during check cable out-drive occurred at the neck where the source enters the curved ring from a straight path in the shaft. None of the obstructions were reproducible during repeated runs with the transfer tube aligned with the ring. However, with curving of the transfer tube (as can be seen in clinical PDR setups), occasional obstructions could be generated, but only for the 26mm ring, demonstrating the importance of transfer tube alignment with the rings during treatment. 

Obstruction errors in cervix BT may be mitigated by delivery in lithotomy setup and ideal transfer tube alignment with the ring, more easily achievable in high-dose-rate BT settings. For PDR treatments with multiple hourly in- and out-drives in the ring, and patient movement during/in-between treatment, frequent transfer tube inspections to maintain path of least curvature between the ring and afterloader indexer head should be performed, particularly for smaller ring sizes.