Optimizing the treatment of breast cancer remains a major topic of interest. In current clinical practice, breast-conserving therapy is the standard of care for patients with localized breast cancer. Technological developments have fueled interest in less invasive breast cancer treatment. Magnetic resonance-guided high-intensity focused ultrasound (MR-HIFU) is a completely noninvasive ablation technique. Focused beams of ultrasound are used for ablation of the target lesion without disrupting the skin and subcutaneous tissues in the beam path. MRI is an excellent imaging method for tumor targeting, treatment monitoring, and evaluation of treatment results. The combination of HIFU and MR imaging offers an opportunity for image-guided ablation of breast cancer. Previous studies of MR-HIFU in breast cancer patients reported a limited efficacy, which hampered the clinical translation of this technique. These prior studies were performed without an MR-HIFU system specifically developed for breast cancer treatment. In this article, a novel and dedicated MR-HIFU breast platform is presented. This system has been designed for safe and effective MR-HIFU ablation of breast cancer. Furthermore, both clinical and technical challenges are discussed, which have to be solved before MR-HIFU ablation of breast cancer can be implemented in routine clinical practice.

ObjectivesTo assess the safety and feasibility of MRI-guided high-intensity focused ultrasound (MR-HIFU) ablation in breast cancer patients using a dedicated breast platform.MethodsPatients with early-stage invasive breast cancer underwent partial tumour ablation prior to surgical resection. MR-HIFU ablation was performed using proton resonance frequency shift MR thermometry and an MR-HIFU system specifically designed for breast tumour ablation. The presence and extent of tumour necrosis was assessed by histopathological analysis of the surgical specimen. Pearson correlation coefficients were calculated to assess the relationship between sonication parameters, temperature increase and size of tumour necrosis at histopathology.ResultsTen female patients underwent MR-HIFU treatment. No skin redness or burns were observed in any of the patients. No correlation was found between the applied energy and the temperature increase. In six patients, tumour necrosis was observed with a maximum diameter of 3–11 mm. In these patients, the number of targeted locations was equal to the number of areas with tumour necrosis. A good correlation was found between the applied energy and the size of tumour necrosis at histopathology (Pearson = 0.76, p = 0.002).ConclusionsOur results show that MR-HIFU ablation with the dedicated breast system is safe and results in histopathologically proven tumour necrosis.Key Points• MR-HIFU ablation with the dedicated breast system is safe and feasible • In none of the patients was skin redness or burns observed • No correlation was found between the applied energy and the temperature increase • The correlation between applied energy and size of tumour necrosis was good

This study has shown the effects of magnetic susceptibility changes induced by heating of breast fatty tissue samples. The resultant errors can be reduced by the use of a model-based correction procedure.

MR-guided HIFU ablation is a promising technique for the non-invasive treatment of breast cancer. A phase I study was performed to assess the safety and treatment accuracy and precision of MR-HIFU ablation in breast cancer patients (n=10) using a newly developed MR-HIFU platform dedicated to applications in the breast. In this paper a technical analysis of the performance of the dedicated breast MR-HIFU system during breast tumors ablation is described. The main points of investigation were the spatial targeting accuracy and precision of the system and the performance of real-time respiration-corrected MR thermometry.The mean targeting accuracy was in the range of 2.4-2.6 mm, whereas the mean targeting precision was in the range of 1.5-1.8 mm. To correct for respiration-induced magnetic field fluctuations during MR temperature mapping a look-up-table (LUT)-based correction method was used. An optimized procedural sedation protocol in combination with the LUT-based correction method allowed for precise MR thermometry during the ablation procedure (temperature standard deviation <3 °C). No unwanted heating in the near field (i.e. skin) nor in the far field (pectoral muscle) was detected.The newly developed dedicated breast MR-HIFU system allows for safe, accurate and precise ablation of breast tumors.

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