| Summary: | Objective:To determine if subsets of patients may benefit from smaller or larger margins when using laser setup and bony anatomy verification of breast tumour bed (TB) boost radiotherapy (RT). Methods: Verification imaging data acquired using cone-beam CT, megavoltage CT or two-dimensional kilovoltage imaging on 218 patients were used (1574 images). TB setup errors for laser-only setup (dlaser) and for bony anatomy verification (dbone) were determined using clips implanted into the TB as a gold standard for the TB position. Cases were grouped by centre-, patient- and treatment-related factors, including breast volume, TB position, seroma visibility and surgical technique. Systematic (?) and random (?) TB setup errors were compared between groups, and TB planning target volume margins (MTB) were calculated. Results: For the study population, ?laser was between 2.8 and 3.4?mm, and ?bone was between 2.2 and 2.6?mm, respectively. Females with larger breasts (p?=?0.03), easily visible seroma (p???0.02) and open surgical technique (p???0.04) had larger ?laser. ?bone was larger for females with larger breasts (p?=?0.02) and lateral tumours (p?=?0.04). Females with medial tumours (p?<?0.01) had smaller ?bone. Conclusion:If clips are not used, margins should be 8 and 10?mm for bony anatomy verification and laser setup, respectively. Individualization of TB margins may be considered based on breast volume, TB and seroma visibility. Advances in knowledge:Setup accuracy using lasers and bony anatomy is influenced by patient and treatment factors. Some patients may benefit from clip-based image guidance more than others.
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