Study of Irinotecan on a Weekly Schedule in Children

The medicine being offered in this study is called Irinotecan. Irinotecan has been effective against many animal cancers. It has been given to both adults and children. We are now attempting to determine how much of the drug can be given to children when Irinotecan is given weekly for four weeks in a row every 6 weeks and the toxicities (bad side effects) that occur when irinotecan is administered on this schedule. The purposes of this study are to: 1. To determine the highest dose of Irinotecan that can safely be given to children whose cancer no longer responds to standard treatment. 2. To determine the toxicities of Irinotecan. 3. To determine what studies (laboratory or x-rays) need to be done to evaluate the toxicities of this drug. 4. To determine if irinotecan is beneficial to the patient. 5. To understand how the drug Irinotecan works in the body.