RATIONALE: Monoclonal antibodies such as trastuzumab can locate tumor cells and either kill them or deliver tumor-killing substances to them without harming normal cells. Radiation therapy uses high-energy x-rays to damage tumor cells. Combining monoclonal antibody therapy with radiation therapy may be an effective treatment for breast cancer.

PURPOSE: Phase II trial to study the effectiveness of combining trastuzumab and radiation therapy in treating women who have stage III or stage IV invasive primary cancer of the breast.

OBJECTIVES: I. Determine the tolerability of trastuzumab (Herceptin) and radiotherapy in women with stage III or IV invasive primary carcinoma of the breast that continues to overexpress HER2 following neoadjuvant systemic chemotherapy. II. Determine the pathologic response rate, locoregional recurrence rate, and time to locoregional recurrence in patients treated with this regimen who did not achieve clinical complete remission following neoadjuvant systemic chemotherapy.

OUTLINE: Patients who received trastuzumab (Herceptin) in combination with prior neoadjuvant chemotherapy receive trastuzumab IV over 30 minutes on day 1, followed by radiotherapy on days 1-5. Patients who did not receive trastuzumab in combination with prior neoadjuvant chemotherapy receive a loading dose of trastuzumab IV over 90 minutes, followed one week later by trastuzumab IV over 30 minutes on day 1, followed by radiotherapy on days 1-5. Radiotherapy begins within 3-4 weeks after completion of neoadjuvant chemotherapy. Treatment continues weekly for 5 courses in the absence of unacceptable toxicity. Patients are followed at 3 weeks for surgical resectability, then every 3 months for 2 years, and then every 6 months for 3 years.

PROJECTED ACCRUAL: A total of 18-35 patients will be accrued for this study.

• Biological: trastuzumab
• Radiation: radiation therapy

DISEASE CHARACTERISTICS: Histologically proven invasive primary carcinoma of the breast by fine needle aspiration cytology, core needle biopsy, or incisional biopsy No prior excisional biopsy Stage III or IV disease, including the following: Any T with N2 or N3 Inflammatory breast cancer or otherwise inoperable T4 Supraclavicular/infraclavicular adenopathy Distant metastasis with measurable disease in the breast or lymph nodes Must have received neoadjuvant systemic chemotherapy with or without trastuzumab (Herceptin) within the past 4 weeks and have residual tumor that continues to overexpress HER2 Measurable or evaluable locoregional disease after completion of neoadjuvant chemotherapy Synchronous bilateral primary carcinoma of the breast allowed if the more serious of the 2 carcinomas meets the above staging criteria Candidate for locoregional therapy consisting of radiotherapy followed by surgery Hormone receptor status: Not specified

PATIENT CHARACTERISTICS: Age: Over 18 Sex: Female Menopausal status: Any status Performance status: Not specified Life expectancy: Not specified Hematopoietic: WBC greater than 3,000/mm3 Platelet count greater than 100,000/mm3 Hemoglobin greater than 12 g/dL Hepatic: Not specified Renal: Not specified Cardiovascular: Left ventricular ejection fraction normal by radionuclide ventriculography Other: Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception

PRIOR CONCURRENT THERAPY: Biologic therapy: See Disease Characteristics Chemotherapy: See Disease Characteristics Endocrine therapy: Not specified Radiotherapy: See Disease Characteristics No prior radiotherapy to breast or regional lymph nodes Surgery: See Disease Characteristics