Inclusion Criteria

DISEASE CHARACTERISTICS:

- Histologically confirmed primary invasive adenocarcinoma of the breast fulfilling the
following criteria:

- Nonmetastatic disease

- Operable and adequately excised

- Patients with nonresectable deep margin invasion are eligible provided they
have had or will receive radiotherapy to the region

- Patients with histologically documented infiltration of the skin (pT4) are
eligible provided they have undergone or will receive radiotherapy
encompassing the tumor bed

- Node-positive OR -negative and determined eligible to receive adjuvant
trastuzumab (Herceptin®)

- No positive or suspicious internal mammary nodes by SNS that have not been or will
not be irradiated

- No supraclavicular lymph node involvement (confirmed by fine needle aspiration or
biopsy)

- Over expression and/or amplification of HER2 in the invasive component of the primary
tumor, according to one of the following:

- 3+ over-expression by IHC (> 30% of invasive tumor cells)

- 2+ or 3+ (in 30% or less neoplastic cells) over-expression by IHC AND in situ
hybridization (FISH/CISH) test demonstrating HER2 gene amplification

- HER2 gene amplification by FISH/CISH (> 6 HER2 gene copies per nucleus, or a
FISH ratio [HER2 gene copies to chromosome 17 signals] of > 2.2.)

- Negative or equivocal overall result (FISH test ratio of < 2.2, < 6.0
HER2-gene copies per nucleus) and staining scores of 0, 1+, 2+, or 3+ (in
30% or less neoplastic cells) by IHC not allowed

- Hormone receptor status known (estrogen receptor with or without progesterone
receptor)

PATIENT CHARACTERISTICS:

- Menopausal status not specified

- ECOG performance status 0-1

- Hemoglobin ≥ 10.0 g/dL

- ANC ≥ 1,500/mm^3

- Platelet count ≥ 100,000/mm^3

- Serum creatinine ≤ 2.0 times upper limit of normal (ULN)

- AST and ALT ≤ 2.5 times ULN

- Alkaline phosphatase ≤ 2.5 times ULN

- Bilirubin ≤ 1.5 times ULN (≤ 2.0 times ULN if known Gilbert syndrome)

- Baseline LVEF ≥ 50% measured by ECHO or MUGA scan

- Not pregnant or nursing

- Negative pregnancy test

- Fertile patients must use effective contraception

- No serious cardiac illness or medical condition including, but not limited to, any of
the following:

- History of documented congestive heart failure (any NYHA class) or systolic
dysfunction (LVEF < 50%)

- High-risk uncontrolled arrhythmias (e.g., ventricular tachycardia, high-grade
atrioventricular-block [second degree or higher], or supraventricular
arrhythmias that are not adequately rate-controlled)

- Angina pectoris requiring antianginal medication

- Clinically significant valvular heart disease

- Evidence of transmural infarction on ECG

- Poorly controlled hypertension (any reading of systolic BP > 180 mm Hg or
diastolic BP > 100 mm Hg)

- No other concurrent serious diseases that may interfere with planned treatment
including severe pulmonary conditions or illness

- None of the following:

- Ulcerative colitis

- Malabsorption syndrome

- Any disease significantly affecting gastrointestinal function

- Inability to swallow oral medication

PRIOR CONCURRENT THERAPY:

- See Disease Characteristics

- No prior mediastinal irradiation except internal mammary-node irradiation for the
present breast cancer

- No prior anti-HER2 therapy for any reason

- No prior biologic or immunotherapy for breast cancer

- No prior resection of the stomach or small bowel

- No other concurrent anticancer therapy including chemotherapeutic agents, biologic
agents, or radiotherapy

- No concurrent anticancer treatment in another investigational trial with hormone
therapy or immunotherapy unless approved by the study chair

- No concurrent CYP3A4 inhibitors or inducers

- No concurrent epoetin alfa, including darbepoetin alfa

- No concurrent oprelvekin