Imagine being a youngster in the fight of your life against cancer--or the parent of such a child--and learning the medicine your doctor says would help is not available. This happens a lot.Holly Kamm Wahl’s teen-aged son David is being treated for leukemia. She describes how they’ve had to cope with inadequate doses of certain drugs, including an essential pain reliever. Pediatric oncologist Dr. Yoram Unguru has been raising the alarm about medication shortages for years. He says it’s getting worse.