In Greek mythology, a terrifying swamp monster known as Hydra would regrow two heads for every one lost when one of the serpent's many necks was severed, making the beast nearly impossible to slay. Modern scientists think they may have discovered why cancer often behaves like Hydra, refusing to die despite every seemingly mortal blow that medicine inflicts upon it.
Current cancer therapies may attack only the equivalent of Hydra's head -- the majority of cancer cells removed by surgery or destroyed by radiation and chemotherapy. Spared is a crucial pool of mutant cells that acts as the source of the malignancy, leaving the cancer able to rise again and again.
According to this theory, which has steadily been gaining credence, the only effective strategy for defeating cancer will be found in treatments that stanch cancer's ability to regrow, such as what Hercules did when he finally slew the beast of ancient Greece by cauterizing each of the monster's necks.
In the case of cancer, the solution would lie in stamping out the highly specialized cells, known as cancer stem cells, that appear to give rise to the cancer in the first place. Such cells are largely impervious to current treatments, enabling them to lurk silently until they repeatedly spawn new tumors, either in the same part or in other parts of the body.
"What we've been doing is simply making the tumor shrink -- leaving the equivalent of the source of the head behind. So it just regrows," said Michael Clarke, a professor of medicine at the University of Michigan in Ann Arbor, who has found evidence for the existence of breast cancer stem cells. "We need to figure out how to sever the head so it doesn't grow back."
In addition to breast cancer, scientists have produced evidence for the existence of cancer stem cells in two leukemias and a variety of brain cancers.
In the most recent evidence, published in the Aug. 12 issue of the New England Journal of Medicine, researchers at Stanford University showed that among the millions of cancerous cells found in patients suffering from chronic myelogenous leukemia, only a small, discrete population had the ability to replenish the cancer.
"We showed that only certain cells have the ability to self-renew," said Irving Weissman, who directs Stanford's Institute for Cancer/Stem Cell Biology and Medicine.
These cells appear to have specific characteristics -- they are mutant versions of normal stem cells, which are the immature versions of all cells that have been the focus of attention in recent years because of their potential for treating a host of ailments.
It remains unclear how cancer stem cells originate. But they probably arise as a result of genetic defects or exposure to toxins, researchers said.