Radiation Therapy in Treating Patients With Extensive Stage Small Cell Lung Cancer

DISEASE CHARACTERISTICS: * Histologically or cytologically confirmed extensive stage small cell lung cancer (SCLC) diagnosed within the past 6 months\* * Has 1-4 extracranial metastatic lesions NOTE: \*Diagnosis made before treatment with chemotherapy * Has completed 4-6 courses of platinum-based chemotherapy within the past 8 weeks AND meets the following criteria: * Radiographic partial or complete response to chemotherapy in ≥ 1 site of disease according to Response Evaluation Criteria in Solid Tumors (RECIST) criteria (if radiotherapy has been delivered to primary disease with chemotherapy, there must be complete or partial response in ≥ 1 of the sites that has not been treated with radiotherapy) * No progression in any site * No limited stage SCLC, even if disease progressed * No brain or central nervous system (CNS) metastases PATIENT CHARACTERISTICS: * Zubrod performance status 0-2 * Absolute neutrophil count (ANC) ≥ 1,000/mm\^3 * Platelets ≥ 75,000/mm\^3 * Hemoglobin ≥ 8.0 g/dL (the use of transfusion or other intervention is allowed) * Serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤ 2.5 times upper limit of normal (ULN) (for patients who will receive radiotherapy to the liver) * Serum bilirubin \< 1.5 times ULN (for patients who will receive radiotherapy to the liver) * Serum creatinine \< 1.5 times ULN (for patients who will receive radiotherapy to the kidneys) * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective contraception * No severe, active co-morbidity, defined as any of the following: * Acute bacterial or fungal infection requiring IV antibiotics at the time of study registration * Chronic obstructive pulmonary disease exacerbation or other respiratory illness requiring hospitalization or precluding study therapy at the time of study registration PRIOR CONCURRENT THERAPY: * See Disease Characteristics * Recovered from prior chemotherapy (i.e., toxicities ≤ grade 1 \[except for neuropathy and alopecia\]) * Thoracic radiotherapy administered concurrently with or before chemotherapy for the current diagnosis allowed (these patients will not receive mediastinal radiotherapy per protocol) * No prior radiotherapy to the region of this cancer that would result in overlap of radiotherapy fields * No concurrent chemotherapy