Trastuzumab With or Without Tamoxifen in Treating Women With Progressive Stage IV Breast Cancer

DISEASE CHARACTERISTICS: * Histologically confirmed invasive stage IV breast cancer * Hormone receptor status: * HER2/neu positive (3+ by immunohistochemical \[IHC\] assay or fluorescent in situ hybridization \[FISH\]) * Estrogen receptor or progesterone receptor positive * Measurable or evaluable disease * Must have disease progression within 6 months of initiation of tamoxifen (administered in the adjuvant or metastatic setting) or during aromatase inhibitor therapy PATIENT CHARACTERISTICS: Age * 18 and over Sex * Female Performance status * ECOG 0-2 Hematopoietic * Absolute neutrophil count at least 1,000/mm\^3 * Platelet count at least 75,000/mm\^3 Hepatic * Bilirubin no greater than 1.5 times upper limit of normal (ULN) * SGOT and SGPT no greater than 2.5 times ULN Cardiovascular * LVEF normal by MUGA Other * Not pregnant or nursing * Fertile patients must use effective nonhormonal contraception during and for at least 2 months after study completion * No other concurrent active malignancy except nonmelanoma skin cancer * Patients who have completed prior therapy and are at less than 30% risk of relapse are not considered to have an active malignancy PRIOR CONCURRENT THERAPY: Biologic therapy * No prior trastuzumab (Herceptin) in the adjuvant or metastatic setting Chemotherapy * No more than 1 prior chemotherapy regimen in the metastatic setting * No concurrent chemotherapy Endocrine therapy * See Disease Characteristics * No more than 1 prior hormonal therapy regimen for metastatic disease * Prior aromatase inhibitor therapy administered in the first-line metastatic or adjuvant setting is allowed provided there is disease progression on tamoxifen * No other concurrent hormonal therapy except the following: * Steroids for adrenal failure * Hormones for nondisease-related conditions (e.g., insulin for diabetes) * Intermittent use of dexamethasone as an antiemetic * Vaginal estrogen (or Estring®) for vaginal dryness Radiotherapy * No concurrent palliative radiotherapy except whole brain irradiation for CNS disease Other * Concurrent bisphosphonates allowed * No concurrent cardioprotective drugs