Beginning on day 1, patients receive topotecan IV continuously for 3-6 weeks and thoracic radiotherapy 5 days a week for 2, 3, or 6 weeks. Cohorts of 4-6 patients receive escalating doses of topotecan and thoracic radiotherapy until the maximum tolerated dose (MTD) of each therapy is determined. The MTD is defined as the dose preceding that at which 2 of 4 or 6 patients experience dose-limiting toxicity. Six additional patients are treated at the MTD. Patients who fail to achieve complete remission (CR) and continue to have measurable disease at 4-6 weeks after completion of radiotherapy receive topotecan IV continuously on days 1-21 at 1 dose level preceding the MTD as determined by the ongoing Protocol NYU-9123. Treatment continues every 4 weeks in the absence of unacceptable toxicity.