Overview

The goal of this study is to investigate the differences in toxicity and efficacy between proton and photon hypofractionated radiotherapy following lumpectomy or mastectomy for breast cancer. The main questions it aims to answer are:

1. What is the difference in the incidence of radiation-related grade 2 or higher adverse events within two years following proton versus photon therapy after breast-conserving surgery?
2. What is the difference in the incidence of radiation-related grade 2 or higher adverse events within two years following proton versus photon therapy after mastectomy?

Eligibility

Inclusion Criteria:

1. Histologically confirmed invasive breast cancer.
2. Indications for adjuvant radiotherapy following lumpectomy or mastectomy for left breast cancer:
   1. Post lumpectomy: Patients who have undergone lumpectomy with postoperative pathology staged as pT1-3 N1-3, excluding N3c.
   2. Post mastectomy: Patients who have undergone mastectomy, with postoperative pathology staged as pT1-3 N1-3, excluding N3c, and who have had an implant (prosthesis or tissue expander) placed.
3. Neoadjuvant therapy patients: Patients who have received neoadjuvant chemotherapy or

   endocrine/targeted therapy and have a postoperative stage of ypT1-3 N0-3, excluding N3c, requiring radiotherapy to the chest wall/breast and regional lymphatic drainage areas.

4. No distant metastasis.
5. Age range: 18-80 years.
6. Performance status: Eastern Cooperative Oncology Group (ECOG) score of 0-2.
7. Non-pregnant and non-lactating women.
8. Informed consent: Patients must provide written informed consent prior to receiving radiotherapy.

Exclusion Criteria:

1. Presence of Ductal Carcinoma In Situ (DCIS).
2. Tumor staging: Patients presenting with T4, N0, or N3c disease.
3. History of prior radiotherapy to the ipsilateral chest or breast.
4. Surgical margin status: Margins are either positive or close, defined as:

   Invasive carcinoma within 1 mm of the surgical margin.

5. Bilateral breast cancer: Patients with synchronous or previous contralateral breast cancer.