The management of axillary nodes in breast cancer patients is a highly debated and evolving field. To date, an increasing number of patients with positive lymph nodes receives primary systemic therapy (PST) prior to surgery leading to down-staging axillary nodes in about 40% of women. However, the available diagnostic methods have several limitations in properly evaluating the response after treatment both in the breast and in the nodes and might lead to either under or over-treatment in these patients. Fully integrated scanners capable of simultaneous acquisition of PET and MRI have now been developed, with the potential to combine the specificity obtained by the functional imaging of PET, with the superior sensitivity of MRI, to provide higher diagnostic accuracy.

It is expected that PET/MRI could better determine the response after PST to distinguish patients with negative versus patients with positive axillary nodes after medical treatment. As the excision of axillary nodes has mainly a staging purpose, the reliable identification of node negative patients might eventually spare women from unnecessary surgery. An accurate over-time and final imaging work-up might help choose the appropriate type of surgery according to the extent of nodal involvement: either SNB or complete axillary clearance.

HYPOTHESIS: Hybrid PET/MRI might be a non-invasive, one-stage, operator-independent imaging modality to accurately define nodal status after PST, properly select type of surgical approach and might eventually lead to the omission of axillar surgery in some breast cancer patients showing complete imaging response.

AIMS: The primary endpoint is to compare the staging power between SNB (or lymphoadenectomy) vs PET/MRI in detecting axillary lymph node macro-metastases (>2 mm).Additionally the PET/MRI and A-US results will be confronted and ultimately related to pathology result, calculating the concordance rate in terms of positive vs negative nodes and number of positive nodes detected by each method. The staging power of both preoperative exams will be evaluated by considering how many times the results from PET/MRI or A-US could have indicated the most appropriate axillary treatment according to pathological findings.

EXPERIMENTAL DESIGN: Patients with breast cancer of any size with positive axillary nodes and candidates to PST will undergo PET/MRI prior to PST.

The study population will then be split into two groups:

• women with positive axillary lymph nodes after PST (Group 1, 60% of the total)
• women with negative axillary lymph nodes after PST (Group 2, 40% of the total).

A total of 110 women will guarantee two well-powered independent analysis. one for Group 1 (n=66; power>90%; Type I error rate of 0.05) and one for Group 2 (n=44; power>80%; Type I error rate of 0.05).

Inclusion Criteria:

• Signed informed consent;
• Age > 18 years;
• Proven diagnosis of early BC of any size;
• Patients candidate to primary systemic therapy (PST);
• Positive axillary nodes at diagnosis, confirmed by either citology or histology confirmation. Patients with clear overt clinical and radiological nodal involvement might be enrolled as well without FNA or microhystology

Exclusion Criteria:

• inflammatory BC;
• pregnancy;
• contraindication to PET;
• distant metastases;
• no surgery after PST;
• contraindication to MRI;
• claustrophobia;
• allergy to the MR contrast agent;
• severe renal insufficiency.