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Objectives: Accurate contouring of anatomical structures allows for high‐precision radiotherapy planning, targeting the dose at treatment volumes and avoiding organs at risk. Manual contouring is time‐consuming with significant user variability, whereas auto‐segmentation (AS) has proven efficiency benefits but requires editing before treatment planning. This study investigated whether atlas‐based AS (ABAS) accuracy improves with template atlas group size and character‐specific atlas and test case selection. Methods and materials: One clinician retrospectively contoured the breast, nodes, lung, heart, and brachial plexus on 100 CT scans, adhering to peer‐reviewed guidelines. Atlases were clustered in group sizes, treatment positions, chest wall separations, and ASs created with Mirada software. The similarity of ASs compared to reference contours was described by the Jaccard similarity coefficient (JSC) and centroid distance variance (CDV). Results: Across group sizes, for all structures combined, the mean JSC was 0.6 (SD 0.3, p =.999). Across atlas‐specific groups, 0.6 (SD 0.3, p = 1.000). The correlation between JSC and structure volume was weak in both scenarios (adjusted R2−0.007 and 0.185). Mean CDV was similar across groups but varied up to 1.2 cm for specific structures. Conclusions: Character‐specific atlas groups and test case selection did not improve accuracy outcomes. High‐quality ASs were obtained from groups containing as few as ten atlases, subsequently simplifying the application of ABAS. CDV measures indicating auto‐segmentation variations on the x, y, and z axes can be utilised to decide on the clinical relevance of variations and reduce AS editing. Advances in knowledge: High‐quality ABASs can be obtained from as few as ten template atlases. Atlas and test case selection do not improve AS accuracy. Unlike well‐known quantitative similarity indices, volume displacement metrics provide information on the location of segmentation variations, helping assessment of the clinical relevance of variations and reducing clinician editing. Volume displacement metrics combined with the qualitative measure of clinician assessment could reduce user variability. [ABSTRACT FROM AUTHOR]

BackgroundDuring left-sided breast radiotherapy, the heart is often exposed to radiation dose. Shielding can be utilised to reduce heart exposure, but compromises the dose delivered to the breast tissue and, in a proportion of patients, to the tumour bed. Deep inspiration breath hold (DIBH) can be used as a technique to move the heart away from the treatment area and thus reduce heart dose. This study examines the efficacy of the Elekta Active Breathing Coordinator (ABC), a DIBH method, in reducing heart dose.Materials and methodsIn total, 12 patients receiving radiotherapy to the left breast were planned for treatment with both a free-breathing (FB) and an ABC scan. The dose volume histogram data for the plans was analysed with respect to heart V13, V5 Gy, mean heart dose and ipsilateral lung V18 Gy. Tumour bed D98%, threshold lung volume in breath hold (BH) and the maximum BH time for each patient was also measured. Patients then received their radiotherapy treatment using the ABC plan and the systematic error in the craniocaudal, lateral and vertical axes was assessed using orthogonal imaging.ResultsThe median heart V13 Gy for FB and DIBH patients was 3% (range, 0·85–11·28) and 0% (range, 0–1·56), respectively, with a mean heart dose of 2·62 Gy (range, 1·21–4·93) in FB and 1·51 Gy (range, 1·17–2·22) in ABC. The median lung V18 Gy was 8·7% (3·08–14·87) in FB plans and 9% (4·88–12·82) in ABC plans. The mean systematic set-up errors in all three planes were within the departmental set-up tolerance of 5 mm for both techniques. Median FB tumour bed D98% was 97·4% (92·8–99·5) and 97·5% (97·3–98·5) for ABC.ConclusionABC represents a good method of reducing radiation dose to the heart while not compromising on dose to the tumour bed, and it has a clear advantage over FB radiotherapy in reducing the risk of cardiac toxicity. It is tolerated well by patients and does not produce any difficulties in patient positioning. [ABSTRACT FROM PUBLISHER]