“The really interesting question,” Dr. Tlsty said, “is not so much why do we get cancer as why don’t we get cancer?”

There is growing evidence that cancers can go backward or stop, and researchers are being forced to reassess their notions of what cancer is and how it develops.

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clipped from www.nytimes.com Call it the arrow of cancer. Like the arrow of time, it was supposed to point in one direction. Cancers grew and worsened. “The old view is that cancer is a linear process,” said Dr. Barnett Kramer, associate director for disease prevention at the National Institutes of Health. “A cell acquired a mutation, and little by little it acquired more and more mutations. Mutations are not supposed to revert spontaneously.” So, Dr. Kramer said, the image was “an arrow that moved in one direction.” But now, he added, it is becoming increasingly clear that cancers require more than mutations to progress. They need the cooperation of surrounding cells and even, he said, “the whole organism, the person,” whose immune system or hormone levels, for example, can squelch or fuel a tumor. Cancer, Dr. Kramer said, is a dynamic process. But some of the skeptics have changed their minds and decided that, contrary as it seems to everything they had thought, cancers can disappear on their own.