chemotherapeutic regimen for treatment of elderly, high- risk surgical patients was reported by Gianfelice et al. in 2003 [46]. Since no surgical resection was performed after MRI-guided FUS treatment, repeat large core-needle biopsy and contrast-enhanced MRI were used to detect foci of residual tumour. The total follow-up time was 6 months. Overall, 19/24 (79%) patients had negative biopsy results after one or two MRI-guided FUS treatment sessions. All patients completed at least one treatment session. Recently, Furusawa et al. reported on the use of MRI-guided FUS for local treatment of breast cancer in 21 patients: 17 patients were treated once and 4 patients twice [47]. Mean follow-up was 14 months. During follow-up patients underwent 3-monthly breast ultrasound and MRI to detect residual tumour. Based on MRI findings, one case (5%) of recurrence, i.e., mucinous carcinoma, was detected. No evidence of recurrence was detected in the other 20 patients (95%). However, results on the outcome of MRI-guided FUS ablation of breast cancer in non- surgical candidates need to be interpreted with caution, since other adjuvant treatment regimens such as radiation therapy and/or chemotherapy have likely influenced the results. In all studies, MRI-guided FUS treatment was performed under local anesthesia. Patients were given analgesic and sedative agents intravenously to reduce pain, unnecessary motion, and claustrophobia. All patients tolerated the MRI-guided FUS procedure well, and only one minor complication, a skin burn, was reported. An overview of studies on MRI-guided FUS ablation of breast tumours is provided in Table 1. US-guided FUS for breast tumour ablation Experience with ultrasound (US)-guided FUS ablation of breast cancer is limited and has only been reported by Wu et al. They reported three studies on the accuracy of US- guided FUS ablation for breast cancer treatment [30, 33, 36]. In the first controlled clinical trial reported in 2003, a total of 48 women were randomized in two treatment groups: (1) the control group in which modified radical mastectomy was performed (n = 25) and (2) the US-guided FUS ablation group, in which modified radical mastectomy was performed within 1–2 weeks following ablation (n = 23) [36]. Of the patients that underwent US-guided FUS ablation prior to surgery short-term follow-up (1–2 weeks), pathologic and immunochemical stains were performed to assess the therapeutic effect of ablation on breast tumour tissue. Pathologic findings revealed that complete necrosis was obtained with US-guided FUS ablation in all patients (100%). The same result on therapeutic effect of US- guided FUS ablation of breast cancer, i.e., 100% necrosis, in the same patients was republished recently [30]. In this most recent report Wu et al. described that a safety margin of 1.5–2 cm around the US visible tumour was chosen to ensure complete tumour ablation [30]. The patient cohort 1435 of 23 US-guided FUS-treated patients served as a basis for several other publications mainly focussing on the immu- nohistochemical results as well [31, 32]. More recently, the same group reported on US-guided FUS as adjunct to chemotherapy, radiotherapy, and tamoxifen for treatment of 22 patients with 23 malignant breast tumours (stage I to IV) [33]. Post-procedural biopsy revealed coagulation necrosis of the entire target tumour in all cases (100%). After a median follow-up of 55 months, one patient died (5%) and two patients (9%) developed local recurrence. The 5-year disease-free survival was 95%. However, since US-guided FUS was used in combination with radiother- apy, chemotherapy, and tamoxifen, it is likely the combination of treatment regimens that resulted in the 5-year survival of 95%. No skin burns or serious complications (bleeding or infection) were observed. It is remarkable that in all US-guided FUS ablation studies a percentage of 100% tumour necrosis was achieved in all patients. In general, this percentage clearly exceeds the success percentage of MRI-guided FUS ablation, although MRI is expected to be the most reliable imaging modality for breast cancer detection and delineation. One potential explanation could be that during the US-guided FUS ablation a larger ablation margin was used (Table 2). Obstacles for clinical implementation of breast FUS ablation Breast-conserving surgery is the gold standard In the past decades, screening programmes and develop- ment in breast imaging techniques have led to an increase in the detection of early stage breast cancer [4]. Breast- conserving surgery with radiotherapy has become the gold standard treatment for localized breast cancer [3]. The technique is easy to perform for qualified surgeons and ensures tumour removal. Although breast-conserving sur- gery is the standard treatment, positive margins are found in between 10% and 53% of the patients [48, 49]. Factors significantly associated with positive margins include large tumour size, younger age, axillary lymph node-positive status, and presence of extensive intraductal component [49]. Furthermore, in breast-conserving surgery, although categorized as a low-morbidity procedure, complications such as bleeding (2%-10%), infection (1%-20%), seroma formation (10%-80%) and chronic incisional pain (20%- 30%) occur [5]. To be equivalent to surgical removal, the effect of FUS ablation should be to achieve total (100%) tumour necrosis. Results of studies on FUS breast cancer ablation to date have been variable, with histopathologic analysis demon- strating complete tumour necrosis in 20% to 100% of patients treated [30, 33, 36, 40–47]. Several factors may be responsible for the variable outcome of FUS ablation. Differences in patient selection, imaging techniques used,