This trial is a pilot, prospective, single-center study conducted in a population of patients with metastatic breast cancer (whatever the immunohistochemical subtype) or metastatic prostate cancer. The aim of this exploratory study is to compare the sensitivity of three different techniques (CellSearch®, Parsortix® and SmartCatch®) in detecting circulating tumor cells (CTCs). After the patient's agreement, and before starting anti-tumor treatment, a blood sample will be taken using the 3 different CTC detection techniques.

Each patient will participate in the study for one day.

A total of 54 evaluable patients (36 patients with metastatic breast cancer and 18 patients with metastatic prostate cancer) will be included in this interventional study.

Inclusion Criteria:

1. Patient with breast cancer (whatever the immunohistochemical subtype: triple-negative, RH+/HER2-negative or HER2-positive) or prostate cancer, multi-metastatic, eligible for a new line of treatment.
2. Patient not yet initiated on a new specific treatment for breast or prostate cancer at inclusion.
3. Age ≥ 18 years and WHO ≤ 2
4. Patient affiliated to Social Security scheme in France.
5. Patient having signed informed consent prior to inclusion in the study and prior to any study-specific procedures.

Exclusion Criteria:

1. Associated pathology(ies) likely to prevent the study procedure from running smoothly
2. Any psychological, family, geographical or sociological condition that prevents compliance with the medical follow-up and/or procedures stipulated in the study protocol.
3. Patient deprived of liberty or under legal protection (guardianship, legal protection)
4. Patient who has had another solid tumor (excluding carcinoma in situ of the breast or cervix) within 5 years.