Cancer researchers have known for years that it was possible in rare cases for some cancers to go away on their own. There were occasional instances of melanomas and kidney cancers that just vanished. And neuroblastoma, a very rare childhood tumor, can go away without treatment. But these were mostly seen as oddities - an unusual pediatric cancer that might not bear on common cancers of adults, a smattering of case reports of spontaneous cures. And since almost every cancer that is detected is treated, it seemed impossible even to ask what would happen if cancers were left alone. Now, though, researchers say they have found a situation in Norway that has let them ask that question about breast cancer. And their new study, to be published Tuesday in The Archives of Internal Medicine, suggests that even invasive cancers may sometimes go away without treatment and in larger numbers than anyone ever believed. Robert M. Kaplan, the chairman of the department of health services at the School of Public Health at the University of California, Los Angeles, [is] persuaded by the analysis. The implications are potentially enormous, Dr. Kaplan said. If the results are replicated, he said, it could eventually be possible for some women to opt for so-called watchful waiting, monitoring a tumor in their breast to see whether it grows. People have never thought that way about breast cancer," he added. Dr. Kaplan and his colleague, Dr. Franz Porzsolt, an oncologist at the University of Ulm, said in an editorial that accompanied the study, "If the spontaneous remission hypothesis is credible, it should cause a major re-evaluation in the approach to breast cancer research and treatment."