PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: See Disease Characteristics No more than 1 prior chemotherapy regimen Endocrine therapy: Not specified Radiotherapy: See Disease Characteristics At least 4 weeks since prior radiotherapy and recovered No prior mantle irradiation, hemibody irradiation, or radiation to the pelvis or lumbar spine Surgery: See Disease Characteristics Other: No concurrent phenytoin, phenobarbital, or other antiepileptic medication No concurrent prochlorperazine on day of irinotecan administration No concurrent prophylactic loperamide