Table 2 Overview of of studies on ultrasound-guided FUS ablation of breast tumours 1437 Study 1 Wu et al. (2003)* [36] 2 Wu et al. (2005) [33] 23 23 3 Wu et al. (2007)* [30] 23 *Both studies report on the same patient population treated with US-guided FUS and tumour ablation protocols are three factors, but one of the most critical factors may be the size of ablated margins, i.e., healthy breast tissue surrounding the tumour that is to be ablated as part of the procedure. Size of the ablation margins Several studies have examined the importance of tumour- free surgical margins after breast-conserving therapy [50]. Ideally the tumour along with a margin of at least 10 mm of normal-appearing tissue is resected to attempt to remove any microscopic cancerous tissue in the region [49]. The minimum cosmetically acceptable tumour-free margin in relation to the risk of local or distant recurrence has been debated in many studies [48, 50–52]. For image-guided FUS ablation successful treatment of the entire tumour relies on accurate tumour volume delineation with the imaging technique (US or MRI) used for targeting and monitoring the ablation procedure. In this respect MRI is most accurate imaging technique currently available [22]. MRI may also underestimate tumour size, especially when an extensive ductal carcinoma in situ (DCIS) component is present [25]. To address the problem of tumour size underestimation and ensure total tumour ablation, different groups have proposed ablation of a margin of “healthy” breast tissue surrounding the breast tumour during FUS treatment. Gianfelice et al. reported in their study that the residual tumour cells were mainly found at the periphery of the treatment field and proposed that this was caused by Tumours Breast tumour characteristics (n) Outcome of the ablation procedure - Invasive breast cancer, not specified - Surgical resection (n=21) - Non-invasive breast cancer, not - Complete necrosis in 23 lesions (100%) specified (n=2) - All tumours <6 cm in size - Only TTC staining used - Invasive breast cancer, not specified - No surgical resection (n=21) - Non-invasive breast cancer, not - Follow-up range 3–60 months specified (n=1) - Missing (n=1) - Complete necrosis initially (2 weeks) in 23 lesions (100%) - All tumours <5 cm in size - Local recurrence (after 18 and 22 months, respectively) in two lesions (9%) - Invasive breast cancer, not specified - Surgical resection (n=21) - Non-invasive breast cancer, not - Complete necrosis in 23 lesions (100%) specified (n=2) - All tumours <6 cm in size - Only TTC staining used lack of ablating an adequate amount of surrounding healthy breast tissue around the tumour [41]. Wu et al. treated 1.5– 2.0 cm of normal tissue surrounding the tumour and found no residual cancer in all patients [30]. Based on breast- conserving therapy studies, the tumour and a margin of at least 10 mm healthy breast tissue should be ablated to increase the probability of complete tumour necrosis [41]. Methods for margin assessment and residual tumour detection after FUS ablation If FUS is to be implemented in clinical breast cancer treatment as an alternative to conventional surgery in the future, a reliable method for tumour margin assessment and detection of residual tumour after FUS ablation must be available. Since FUS treatment would lack a surgically excised pathological specimen, the tumour margin status could not be assessed by histopathological analysis. To date, most protocols have been treat-and-resect protocols, i.e., FUS ablation followed by surgery, which allows histopathological tissue examination to evaluate the ther- apeutic effect of FUS. In the few FUS ablation studies where treated tumour tissue was left in situ after ablation, different strategies have been used to assess outcome. Furusawa et al. used clinical follow-up by MRI and US every 3 months [47]. If one of the modalities showed suspicious findings suggestive for residual tumour, large core-needle biopsy (LCNB) of the lesion was performed, and if residual tumour or tumour recurrence was found, the