Pilot, prospective, monocentric study aimed at evaluating the rate of patients with circulating "cancer cell/macrophage" hybrid cells in the peripheral blood and the evolution of this rate over time.

The study will be conducted on a population of patients with leiomyosarcoma and treated in the context of routine care. 20 patients will be included:

• 10 patients with localized disease.
• 10 patients with metastatic disease.

For each included patient, blood samples will be collected during baseline visit and up to 24 months after inclusion.

Inclusion Criteria:

1. Patient with leiomyosarcoma.
2. Diagnosis of sarcoma histologically confirmed by the RRePS network (Réseau de Référence en Pathologie des Sarcomes et des Viscères)
3. Localized or metastatic disease
4. Newly diagnosed patient who has not yet initiated specific treatment for sarcoma
5. Age ≥ 18 years
6. Patient affiliated to a Social Security system in France.
7. Patient having signed informed consent prior to inclusion in the study and prior to any specific study procedures

Exclusion Criteria:

1. Diagnosis of any other histological subtype of soft tissue sarcoma
2. Associated pathology(ies) that may interfere with the study procedure
3. Pregnant or breastfeeding woman
4. Any psychological, family, geographical or sociological condition that does not allow for compliance with the medical follow-up and/or procedures provided for in the study protocol.
5. Patient who has forfeited his/her freedom by administrative or legal award or who is under legal protection (curatorship and guardianship, protection of justice).