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UBC Theses and Dissertations

Surgical cavity contour and skin marking variability in accelerated partial breast irradiation Kosztyla, Robert

Abstract

Accelerated partial breast irradiation (APBI) has emerged as an attractive alternative to whole-breast radiotherapy (WBRT) in adjuvant radiotherapy of early stage breast cancers. Localization of the target, the surgical cavity (SC), is of the utmost important since a large dose per fraction is delivered during treatment. Differences in the shape and position of the SC between planning and treatment can result in a geographic miss of the target. Skin markings (SM) placed around the surgical scar potentially can be used to improve localization of the SC during treatment. However, analysis of such movements is complicated by interobserver and intraobserver variations in defining the SC. Thus, this thesis aims to (1) develop a method to construct a representative surgical cavity (RSC), a contour that combines contributions from multiple SC contours; (2) investigate contouring uncertainties and dosimetric coverage of the SC as contoured by multiple oncologists; and (3) use the RSC to quantify differences between the positions of the SMs and SC. Twelve patients underwent four CT scans: one at the time of planning and three during treatment. Three radiation oncologists contoured the SC on each CT scan. Oncologists delineated two additional repeat contours for three patients. Interobserver and intraobserver contour variability was assessed by comparing the SC contours with a RSC. Volume-based and distance-based measurements were performed to assess contour variability. Clinical consequences of variability were assessed using the equivalent uniform dose (EUD) formalism. Correlations between the change in position of the SM centre-of-mass (COM) and RSC COM were quantified. Interobserver volume, COM, and spatial variations of the SC contours were observed to be larger than intraobserver variations. However, dosimetric coverage of the SC was adequate despite these variations. The SM COM was found to be strongly correlated with the RSC COM in the lateral, anterior-posterior, and superior-inferior directions. The average change in the distance between the SM COM and RSC COM during treatment was more than 4 mm in less than 5% of cases, suggesting that planning margins for the SC can potentially be decreased if the SMs are used for patient setup.

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