Eligibility Criteria:
DISEASE CHARACTERISTICS:
* Histologically confirmed diagnosis of early-stage breast cancer
* HER2 positive by immunohistochemistry (IHC) (3+) or fluorescent in situ hybridization (FISH)
* Ductal carcinoma in situ (DCIS) components should not be counted in the determination of degree of IHC staining or FISH amplification
* No locally advanced tumors (i.e., T4) at diagnosis, including the following:
* Tumors fixed to chest wall
* Peau d'orange
* Skin ulcerations or nodules
* Clinical inflammatory changes (e.g., diffuse brawny cutaneous induration with an erysipeloid edge)
* Has undergone mastectomy or lumpectomy with axillary node or sentinel node dissection within the past 84 days
* Patients who have undergone a mastectomy must meet the following criteria:
* No evidence of gross or microscopic tumor (i.e., invasive DCIS) at the surgical resection margins noted in final surgery or pathology reports
* Patients with close margins are eligible
* Radiation therapy is required for 4 or more positive lymph nodes and must be started after completion of chemotherapy
* Patients who have undergone a lumpectomy with axillary node or sentinel node dissection must meet the following criteria:
* No evidence of invasive cancer or DCIS at the surgical resection margins
* No gross residual adenopathy
* Planning to undergo radiation therapy to the breast with or without regional lymphatics after completion of chemotherapy
* No active hepatic or biliary disease
* Patients with liver metastases, stable chronic liver disease, Gilbert's syndrome, or asymptomatic gallstones are eligible
* Hormone receptor status:
* Estrogen receptor and progesterone receptor status known
PATIENT CHARACTERISTICS:
* Male or female
* Menopausal status not specified
* ECOG performance status 0-2
* Absolute neutrophil count ≥ 1,500/mm³
* Platelet count ≥ 100,000/mm³
* Hemoglobin ≥ 10.0 g/dL
* Bilirubin ≤ 1.5 times upper limit of normal (ULN)
* AST and ALT ≤ 2.5 times ULN
* Alkaline phosphatase ≤ 2.5 times ULN
* Creatinine normal OR creatinine clearance ≥ 60 mL/min
* LVEF ≥ 50% by MUGA scan or echocardiogram
* Able to complete questionnaire(s) by themselves or with assistance
* Able and willing to provide blood and tissue samples
* No known sensitivity to benzyl alcohol
* No sensory neuropathy ≥ grade 2
* No active cardiac disease, including any of the following:
* Myocardial infarction within the past 6 months
* Prior or concurrent congestive heart failure
* Prior or concurrent arrhythmia or cardiac valvular disease requiring medications or that is clinically significant
* Uncontrolled hypertension, defined as diastolic blood pressure (BP) \>100 mm Hg or systolic BP \> 200 mm Hg on 2 separate occasions ≥ 14 days apart
* Clinically significant pericardial effusion
* Prior or concurrent uncontrolled or symptomatic angina
* Other cardiac condition that, in the opinion of the treating physician, would put the patient at hazardous risk
* No history of allergic reactions attributed to compounds of similar chemical or biologic composition as lapatinib ditosylate
* No uncontrolled intercurrent illness including, but not limited to, the following:
* Ongoing or active infection
* Psychiatric illness or social situations that would preclude study compliance
* Able to swallow and retain oral medication
* No history of gastrointestinal (GI) disease resulting in an inability to take oral medication, including any of the following:
* Malabsorption syndrome
* Requirement for IV alimentation
* Prior surgical procedures affecting absorption
* Uncontrolled inflammatory GI disease (e.g., Crohn's disease or ulcerative colitis)
* Not pregnant or nursing
* Negative pregnancy test
* Fertile patients must use effective contraception during and for 6 months after completion of study treatment
PRIOR CONCURRENT THERAPY:
* See Disease Characteristics
* No prior chemotherapy, radiation therapy, immunotherapy, or biotherapy for breast cancer
* No primary breast radiation therapy as part of breast-conserving treatment
* No prior anthracycline or taxane therapy for any malignancy
* No prior epidermal growth factor receptor-targeting therapies (e.g., gefitinib, cetuximab, erlotinib hydrochloride, rituximab, trastuzumab \[Herceptin®\], lapatinib ditosylate, panitumumab, or nimotuzumab)
* At least 14 days since prior and no concurrent CYP3A4 inducers, including the following:
* Rifamycin-class antibiotics (e.g., rifampin, rifabutin, or rifapentine)
* Anticonvulsants (e.g., phenytoin, carbamazepine, or barbiturates \[e.g., phenobarbital\])
* Antiretrovirals (e.g., efavirenz or nevirapine)
* Glucocorticoids (e.g., oral cortisone, hydrocortisone, prednisone, methylprednisolone, or dexamethasone)
* Daily oral dexamethasone ≤ 1.5 mg (or equivalent) allowed
* Modafinil
* Hypericum perforatum (St. John's wort)
* At least 7 days since prior and no concurrent CYP3A4 inhibitors, including the following:
* Antibiotics (e.g., clarithromycin, erythromycin, or troleandomycin)
* Antifungals (e.g., itraconazole, ketoconazole, fluconazole \[\> 150 mg daily\], or voriconazole)
* Antiretrovirals and protease inhibitors (e.g., delaviridine, nelfinavir, amprenavir, ritonavir, indinavir, saquinavir, or lopinavir)
* Calcium channel blockers (e.g., verapamil or diltiazem)
* Antidepressants (e.g., nefazodone or fluvoxamine)
* Gastrointestinal agents (e.g., cimetidine or aprepitant)
* Grapefruit and grapefruit juice
* At least 6 months since prior and no concurrent amiodarone
* No herbal or alternative medicines or supplements ≥ 14 days before, during, and for 30 days after completion of study treatment
* No concurrent hormonal agents (e.g., birth control pills, ovarian hormonal replacement therapy, or raloxifene)
* Adjuvant hormonal agents (e.g., tamoxifen, aromatase inhibitors) allowed after completion of chemotherapy as part of treatment for breast cancer
* No concurrent antiretroviral therapy for HIV-positive patients
* No concurrent digitalis or beta-blockers for congestive heart failure
* No concurrent arrhythmia or angina pectoris medication
* No other concurrent investigational agents or anticancer therapies, including cytotoxic agents or immunotherapy