Gefitinib in Treating Patients With Previously Untreated Stage IIIB or Stage IV Non-Small Cell Lung Cancer

DISEASE CHARACTERISTICS: * Pathologically confirmed non-small cell lung cancer (NSCLC) * No squamous cell histology * Stage IIIB (with pleural effusion) or stage IV disease * Must meet ≥ 1 of the following criteria: * Female * Adenocarcinoma tumor histology * No history of smoking, defined as smoking \< 100 cigarettes (5 standard packs of cigarettes) in a lifetime, \< 20 oz of pipe tobacco in a lifetime, OR \< 100 cigars in a lifetime * Asian/Pacific Rim ethnicity, defined as Japanese, Chinese, Korean, or other Asian/Pacific Rim ethnicity * Must have activating mutations in the TK region of the epidermal growth factor receptor (EGFR) gene * Measurable disease * No symptomatic or newly diagnosed CNS metastases that have not been definitively treated with radiotherapy and/or surgery * History of CNS metastases or cord compression allowed if definitively treated and clinically stable PATIENT CHARACTERISTICS: * See Disease Characteristics * ECOG performance status 0-2 * Life expectancy ≥ 12 weeks * Absolute neutrophil count ≥ 2,000/mm³ * Platelet count ≥ 100,000/mm³ * Bilirubin ≤ 1.25 times upper limit of normal (ULN) * ALT and AST ≤ 2.5 times ULN (5 times ULN if liver has tumor involvement) * Creatinine ≤ 1.5 times ULN OR creatinine clearance ≥ 60 mL/min * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective contraception * No known severe hypersensitivity to gefitinib or any other component of gefitinib tablets * No evidence of clinically active interstitial lung disease * Patients with chronic, stable radiographic changes who are asymptomatic are eligible * No other concurrent malignancy or malignancy diagnosed within the past 5 years except for basal cell carcinoma of the skin or cervical cancer in situ * No concurrent severe or uncontrolled systemic disorder * No evidence of any other significant clinical disorder or laboratory finding that, in the opinion of the investigator, would preclude study participation * Able to tolerate protocol treatment, in the opinion of the investigator PRIOR CONCURRENT THERAPY: * See Disease Characteristics * No prior systemic chemotherapy, biological therapy, immunotherapy, or hormonal therapy for NSCLC, including adjuvant and neoadjuvant treatment * No prior radiotherapy to the target lesion * Prior radiotherapy to bony disease or CNS disease allowed * At least 2 weeks since prior radiotherapy and recovered * More than 30 days since prior non-FDA approved or investigational agents * No prior EGFR antagonists * At least 2 weeks since prior and no concurrent phenytoin, carbamazepine, rifampin, barbiturates, or Hypericum perforatum (St. John's wort) * No concurrent chemotherapy, immunotherapy, hormonal therapy, nonpalliative radiotherapy, surgery for cancer, or other experimental medications * No other concurrent specific antitumor therapy