Room: Exhibit Hall | Forum 4
Purpose: High-dose-rate interstitial brachytherapy (HDR-ISBT) for gynecologic malignancy can be impacted by variation due to interfractional motion of markers and needles, uncertainties in contouring structures, swelling and organ filling. The purpose of this study is to evaluate the spatial variation of these features as well as their impact on dose distributions and integrated radiobiological metrics and to create tools/solutions to mitigate these factors.
Methods: The spatial positions of 50 implanted markers and 202 needles were tracked in 15 patients treated over 70 fractions of HDR brachytherapy. Marker/needle coordinates were extracted from CT images with contours and dose distributions. Automated analysis determined marker self-consistency and displacements between various elements of the implant. Augmenting DVH parameters, the variations of equivalent uniform biological effective dose (EUBED) and generalized biologically equivalent uniform dose (gBEUD) were evaluated for each patient.
Results: The relative positions of the markers experienced an average mean absolute change in distance of 1.7Â±0.2 mm, the needle tips varied by an average of 1.1Â±2.0 mm over all fractions, both relative to their respective centers of mass (CM) over all fractions, and the distance between CMs of the needles and markers varied by less than 2.5 mm in the majority of patients. The volumes of the prescription region varied from 20 cc to 154 cc. The average gEUD and EUBED were calculated across all patients relative to the prescription dose over the volume encompassed by 100% prescription dose as well as the CTV.
Conclusion: We present a new solution to better self-identify implanted markers based on their relative orientation. Markers and needle tips are clinically reliable across fractions for the majority of cases, and as a result, the dose distributions across patients is largely reproducible. Despite the wide range of prescription volumes, the integrated dose metrics were relatively constant across patients.