Ellen McCarthy was scheduled to receive her monthly dose of an ovarian cancer drug at Massachusetts General Hospital last month when she got distressing news: The hospital had run out. There was a nationwide shortage of the drug, Doxil, but her medical team scrambled and after a few days of uncertainty located an extra vial at a clinic in suburban New York. The 60-year-old retiree drove with her husband and dog to Mount Kisco, NY, received the intravenous treatment, then headed back home to Martha's Vineyard -- a 10-hour round trip. This week, McCarthy received Doxil at Mass. General. But with the manufacturer writing letters to physicians warning that the drug will be intermittently available in the months ahead, McCarthy doesn't know what to expect. "And my life depends on it,'' she said. "It never occurred to me in the course of treating my cancer, I was going to deal with a lack of supply.'' Drug shortages have been on the rise in recent years, affecting everything from antibiotics to anesthesia drugs, for reasons that range from manufacturing problems to companies discontinuing a medication. But shortages are particularly harrowing with cancer drugs, when time is of the essence and substitutes aren't always available.