The goal of this clinical trial is to learn whether transcutaneous auricular vagus nerve stimulation, also called taVNS, can help relieve aromatase inhibitor-related joint and muscle pain in adult women with hormone receptor-positive breast cancer.

The main questions this study aims to answer are:

1. Does taVNS reduce pain caused by aromatase inhibitor treatment?
2. Does taVNS improve quality of life, mood symptoms, and the need for pain medicine?
3. What side effects or medical problems occur during treatment?

Researchers will compare active taVNS with sham stimulation. Participants in both groups will receive mild electrical stimulation around the ear, but the sham stimulation will be applied to an area not expected to activate the vagus nerve.

Participants will:

1. Be randomly assigned to active taVNS or sham stimulation
2. Receive one 30-minute treatment session every day for 28 days
3. Complete pain, mood, and quality-of-life questionnaires before treatment, after treatment, and during follow-up
4. Report pain medicine use and any side effects
5. Provide small blood samples to measure inflammatory markers, T-cell profiles, and tumor markers

Participants will be followed for up to 6 months after treatment. For some participants who do not have enough pain relief after taVNS, an optional second-stage study may be offered. In this stage, participants may receive low-dose interleukin-2, also called IL-2, by injection every other day for 2 weeks.

Inclusion Criteria:

1. Female participants aged over 18 years;

2. Patients with pathologically confirmed stage I-III primary breast cancer;

3. Patients who have recovered from systemic symptoms related to surgery, radiotherapy, or chemotherapy;

4. Patients with estrogen receptor- and/or progesterone receptor-positive breast cancer who are receiving aromatase inhibitor therapy and have aromatase inhibitor-related musculoskeletal pain, with a score greater than 3 on the Worst Pain item of the Brief Pain Inventory (BPI-WP);

5. Patients who have received aromatase inhibitor therapy for more than 30 days and are expected to continue treatment for more than 1 year;

6. Patients who voluntarily agree to participate in this study and sign the informed consent form.

   For participants receiving combined IL-2 treatment, the following additional criterion applies:

7. Pain relief rate of less than 50% after taVNS treatment and a lower peripheral blood Treg count than before treatment.

Exclusion Criteria:

1. Patients with advanced breast cancer;

2. Patients with other significant organ dysfunction, such as major cardiovascular disease, diabetes mellitus, or hyperthyroidism;

3. Patients with other cancers;

4. Patients with a history of fracture or surgery around the painful joint area within the past 6 months;

5. Patients currently receiving corticosteroids or opioid therapy;

6. Patients with contraindications to vagus nerve stimulation;

7. Patients who are unable to communicate normally, such as those with cognitive impairment or hearing impairment;

8. Patients who have participated in another clinical trial within the past 1 month.

   For participants receiving combined IL-2 treatment, the following additional exclusion criteria apply:

9. Severe cardiac or renal disease, or hematologic disease;

10. Previous IL-2-related toxic reaction or allergy;

11. Use of other biologic agents or immunosuppressive drugs within the past 1 month;

12. Any other condition that the investigator considers unsuitable for study participation.

To protect patient privacy and ensure the security of clinical data, the individual participant data (IPD) collected in this study will be used only for scientific analysis and publication related to this trial. No public disclosure, sharing, or uploading to public repositories is planned. Therefore, IPD will not be made available.