The goal of this phase II clinical trial is to study the effectiveness of a treatment with Serratus Plane Block (SPB) or Botox-A in comparison with capsaicin for the control of chronic neuropathic pain of post-mastectomy syndrom, that is not adequately managed by systemic treatment alone. The primary outcome will be the pain evaluation at 8 weeks.

123 patients with chronic neuropathic pain of post-mastectomy syndrom insufficiently relieved by systemic treatment alone will be recruited over 24 months at the Centre Oscar Lambret.

Patients will be randomly assigned to one of three treatment groups (41 patients per group):

Capsaicin Botulinum toxin A SPB

Patients will be followed for 24 weeks after the study treatment. The follow-up will include remote evaluation and 2 medical visits during which pain and quality of life will be assessed.

SerCaBot is a randomized, open-label, phase II clinical trial designed to evaluate the efficacy of a treatment with Serratus Plane Block (SPB) or Botox-A compared to capsaicin for the control of chronic neuropathic pain of post-mastectomy syndrom, that is not adequately managed by systemic treatment alone.

123 patients with will be recruited over 24 months at the Centre Oscar Lambret.

This clinical trial will be proposed to patients with chronic neuropathic pain of post-mastectomy syndrome that is not adequately managed by systemic treatment alone.

After consent, an inclusion assessment will be carried out including clinical examination, anamnesis, pain assessment (questionnaires PCS, NPS, DN4, NPSI and collect of antalgic treatment) and evaluation of quality of life (questionnaire SF12) and depression (questionnaire HADS).

Patients will then be randomly assigned to one of three treatment groups : Capsaicin, Botox-A, SPB (41 patients per group). The randomization will be balanced 1:1:1, controlled by minimisation (with a random factor set at 0.8) for the distribution of the following factors:

• Pain level at enrolment (continuous NPS)
• Axillary dissection (yes vs. no)
• Loco-regional adjuvant radiotherapy (yes vs. no)
• Prior change of systemic treatment (yes vs. no)

The study treatment will be administered 1 to 2 weeks after randomization. Treatment will be stopped prematurely in the event of unacceptable toxicity or complication. In each group, a repeat of the treatment may be considered after 12 weeks if further pain control is required. In the SPB group, a repeat is also possible every 2 weeks.

Patients will be followed for 24 weeks after the study treatment. Follow-up will include:

• 2 medical visits at 8 weeks and at 24 weeks including pain assessment (questionnaires NPS, NPSI, record of antalgic treatment), record of adverse events related to study treatment and assessment of quality of life (SF12 questionnaire) and depression (questionnaire HADS).
• remote assessment at week-1, week-2, week-4 and week-6 including assessment of pain (questionnaires NPS, NPSI, record of antalgic treatment) and record of adverse events. This remote assessment will be repeated in case of repetition of treatment at week 12.

Patients will withdraw from the study after the 24 week medical visit. Early withdrawal will be possible in case of patient's decision of withdrawal, breast surgery during follow-up, or death.

Inclusion Criteria :

1. Women aged ≥ 18 years;

2. Unilateral breast cancer treated by total or partial mastectomy:

   • with sentinel lymph node technique (SLN) or axillary dissection;
   • with or without immediate reconstruction using a prosthesis;
   • associated or not with radiotherapy and/or chemotherapy;

3. Presenting moderate to severe chronic neuropathic pain, defined by:

   • Numerical Pain Scale (NPS) ≥ 3 and DN4 ≥ 4,
   • on a localised aera ((≤ 240 cm²), at the surgical site, in the axillary hollow, or on the inner side of the ipsilateral arm,
   • between 3 and 9 months after breast surgery,
   • with indication of additional locoregional treatment in complement to a systemic treatment of chronic neuropathic pain, as recommended by the SFETD - French Society for the Study and Treatment of Pain (tricyclic antidepressants or IRSNA or gabapentinoid PLUS lidocaine patch, at the appropriate dosage) and implemented for at least 4 weeks;

4. Patient affiliated with a health insurance plan;

5. Patient informed and having consented to participate in the trial.

Exclusion Criteria :

1. Ipsilateral breast cancer recurrence, regardless of the first treatment;

2. History of breast or thoracic surgery prior to mastectomy with residual pain;

3. Painful polyneuropathy related to chemotherapy requiring treatment;

4. Ongoing or planned loco-regional adjuvant radiotherapy within the next 8 weeks;

5. Treatment area not suitable for potential botulinum toxin type A treatment;

6. Breast reconstruction using flap or lipomodelling;

7. Indication for breast reconstruction within the next 8 weeks;

8. Chronic pain of another etiology such as:

   • Neuropathic pain secondary to a neuroma (localized pain),
   • Radiodermatitis,
   • Phantom breast pain,
   • Lymphedema,
   • Complex regional pain syndrome,
   • Adhesive capsulitis,
   • Fibromyalgia;

9. Hypersensitivity or allergy to anesthetics, capsaicin, naropein, clonidine hydrochloride, an amide-type local anesthetic, botulinum toxin type A, or any excipient contained in the preparations;

10. Infection or inflammation at the injection site;

11. Therapeutic/effective anticoagulation;

12. Clinical signs or medical history leading to the diagnosis of:

    • Hemostasis disorder,
    • Local infection,
    • Severe renal insufficiency (creatinine clearance < 30 mL/min),
    • Thrombocytopenia < 50,000 platelets/mm3;

13. Generalized muscle activity disorders (e.g., myasthenia, Lambert-Eaton syndrome);

14. Heart rate lower than 60/minute;

15. Severe bradyarrhythmia due to sick sinus syndrome or second or third-degree atrioventricular block;

16. State of depression (HADS score ≥ 11);

17. Other contraindication to any of the study treatments;

18. Inability for the patient to follow the study schedule;

19. Inability for the patient or the healthcare team to perform the treatment within 2 weeks;

20. Pregnant or breastfeeding women, women who are able to conceive and who do not use a highly effective method of contraception during the trial and for at least 1 month after the end of treatment ;

21. Patient under guardianship or curatorship.

N/A (since the study is open label)