How accurate is applicator reconstruction in HDR gynecological brachytherapy? Patient-specific results from an electromagnetic tracking system designed to intercept errors before radiation delivery.

PURPOSE: To measure brachytherapy applicator reconstruction accuracy for intracavitary and interstitial gynecological patients before HDR treatment delivery, with an emphasis on intercepting errors and quantifying dose uncertainties METHODS AND MATERIALS: A custom electromagnetic tracking quality assurance (QA) system, EMQA, was employed for pretreatment verification of the treatment planning system (TPS) applicator reconstruction for 25 gynecological HDR brachytherapy implants. TPS versus EMQA differences were quantified for applicator channel tips and shafts, and corresponding dose metric differences were calculated for the target and nearby organs at risk. Accuracy was evaluated as a function of applicator device type and EM sensor position and orientation with respect to the CT table and EM field generator.

RESULTS

Applicator channel reconstruction differences between the clinical plan and EM tracking were (mean ± standard deviation [minimum, maximum]) 0.76 ± 0.43 (0.05, 2.48) mm for channel tips and 0.41 ± 0.26 (0.03, 2.47) mm for shafts. Dose metric differences (% of Rx) for the HRCTV, bladder, rectum, large bowel, and small bowel were <2.3% on average, but differences up to 10.5% were observed. Agreement for needles was improved for needles implanted within a hybrid applicator compared with freehand placement.

CONCLUSIONS

An EM tracking-based QA system enabled patient-specific, pretreatment verification of applicator channel reconstruction before radiation delivery. Although applicator channel reconstruction errors were observed to be small for a variety of gynecological brachytherapy applicator and implant types, the system has the potential to intercept rare and serious errors.