Breast cancer is the most common cancer in women and also has the highest death rate. In 2018, 58,500 new cases were diagnosed in metropolitan France. It affects, in 8 out of 10 cases, women aged 50 and over. Nevertheless, thanks to early detection and improved therapeutic techniques, its mortality rate has continued to decrease each year (a decrease of 1.6% per year was recorded between 2010 and 2018), which allows it to maintain its status as a cancer with a good prognosis. Furthermore, net survival is 87% at 5 years and 76% at 10 years. The median age of death is 74 years. Organized screening was initiated in France in 1990 and generalized in 2004, while maintaining a specific management according to the personal risk of each patient. It consists of a mammogram, plus or minus an ultrasound, every two years from the age of 50. The mammogram is reviewed by two different radiologists. In case of abnormality, a biopsy is performed.

Breast lesions are separated into two nosological entities: palpable tumors, often diagnosed on clinical examination (or autopalpation) and confirmed on mammography, and non-palpable tumors detected during mammographic and/or ultrasound screening.

Thanks to the generalization of screening, the detection of subclinical lesions requiring surgery represents more than half of the breast surgeries and the challenge of their surgical management lies in a precise preoperative location to allow a complete removal, while limiting the sacrifice of healthy glandular tissue. The metallic location technique is currently the reference technique for the preoperative location of these lesions.

For all these reasons, new techniques of preoperative tracking have been developed.

Magnetic tracking (MR) appears to be a simple, non-radioactive and non-aggressive technique to use and organize. MR takes the form of a 1x5mm paramagnetic clip made of iron oxide visible on ultrasound and mammography and is detected by a SentiMag probe. The MR began to be used in France in 2017. It is placed in contact with the lesion under ultrasound guidance. It has the advantage that it can be placed up to 30 days before surgery and has less risk of complication. This MR contains iron particles and is 5mm long. It is pre-loaded in a sterile 18 G needle closed with a wax tip. It is deployed under mammographic and/or ultrasound guidance. The Sentimag probe is used to detect MR in a multidirectional way at 360°, with an audio and visual estimation of the distance. It generates an alternating magnetic field that magnetizes the iron in the MR.

All of these surgical procedures can be performed as outpatient surgery, which corresponds to a hospitalization of less than 12 hours without overnight accommodation, meaning that each patient is admitted and discharged on the same day as her procedure. Outpatient surgery represented 36% of all surgeries in France in 2018. The objective of the HAS is to achieve a majority ambulatory practice of 70% by 2022. This type of surgery has advantages for the patient, by making her journey safer: reduction in the risk of nosocomial infection, thromboembolic risk and anxiety linked to hospitalization for the patient and her family. It also has an important socio-economic advantage by reducing hospitalization costs and the number of hospital beds required. It improves the working conditions of the staff by reducing the workload and night shifts. It has also enabled a clear improvement in the optimization and efficiency of the organization of resources in the surgical technical platforms. The role of the MR is particularly well suited to this new organization.

Inclusion Criteria:

• Patient whose age ≥ 18 years
• Patient with non-palpable benign, atypical or cancerous lesions
• Preoperative biopsy
• Requiring surgical excision
• French speaking patient

Exclusion Criteria:

• Palpable breast lesion
• Contraindication to the installation of a magnetic tracer
• Patient under guardianship or curatorship
• Patient deprived of liberty
• Patient under court protection
• Patient opposing the use of his data for this research

Patients were or are referred for surgical management of non-palpable breast lesions between May 15, 2018 and December 21, 2018.