Article — From the April 2008 issue

Contagious Cancer

The evolution of a killer

( 4 of 10 )

The phenomenon of transmissible tumors isn’t confined to canines, Tasmanian devils, and Syrian hamsters. There have been human cases, too. Forty years ago a team of physicians led by Edward F. Scanlon reported, in the journal Cancer, that they had “decided to transplant small pieces of tumor from a cancer patient into a healthy donor, on a well informed volunteer basis, in the hope of gaining a little better understanding of cancer immunity,” which they thought might help in treating the patient. The patient was a fifty-year-old woman with advanced melanoma; the “donor” was her healthy eighty-year-old mother, who had agreed to receive a bit of the tumor by surgical transplant. One day after the transplant procedure, the daughter died suddenly from a perforated bowel. Scanlon’s report neglects to explain why the experiment wasn’t promptly terminated—why they didn’t dive back in surgically to undo what had been done to the mother. Instead, three weeks were allowed to pass, at which point the mother had developed a tumor indistinguishable from her daughter’s. Now it was too late for surgery. This cancer moved fast. It metastasized, and the mother died about fifteen months later, with tumors in her lungs, ribs, lymph nodes, and diaphragm.

The case of the daughter–mother transplant and the case of the Syrian hamsters have one common element: the original sources of the tumor and the recipients were genetically very similar. If the genome of one individual closely resembles the genome of another (as children resemble their parents, and as inbred animals resemble one another), the immune system of a recipient may not detect the foreignness of transplanted cells. The hamsters were highly inbred (intentionally, for experimental control) and therefore not very individuated from one another as far as their immune systems could discern. The mother and daughter were also genetically similar—as similar as two people can be without being identical twins. Lack of normal immune response, because of such closeness, goes some way toward explaining why those tumors survived transference between individuals.

Low immune response also figures in two other situations in which tumor transmission is known to occur: pregnancy and organ transplant. A mother sometimes passes cancer cells to her fetus in the womb. And a transplanted organ sometimes carries tiny tumors into the recipient, vitiating the benefits of receiving a life-saving liver or kidney from someone else. Cases of both kinds are very rare, and they involve some inherent or arranged compatibility between the original victim of the tumor and the secondary victim, plus an immune system that is either compromised (by immuno-suppressive drugs, in the organ recipient) or immature (in the fetus).

Other cases are less easily explained. In 1986, two researchers from the National Institutes of Health reported that a laboratory worker, a healthy nineteen-year-old woman, had accidentally jabbed herself with a syringe carrying colon-cancer cells; a colonic tumor grew in her hand, but she was rescued by surgery. More recently, a fifty-three-year-old surgeon cut his left palm while removing a malignancy from a patient’s abdomen, and five months later he found himself with a palm tumor, one that genetically matched the patient’s tumor. His immune system responded, creating an inflammation around the tumor, but the response was insufficient and the tumor kept growing. Why? How? It wasn’t supposed to be able to do that. Again, though, surgery delivered a full cure. And then there’s Henri Vadon. He was a medical student in the 1920s who poked his left hand with a syringe after drawing liquid from the mastectomy wound of a woman being treated for breast cancer. Vadon, too, developed a hand tumor. Three years later, he died of metastasized cancer because neither the surgical techniques of his era nor his own immune system could save him.

's previous article for Harper's Magazine, "Darwin's Conundrum," appeared in the December 2006 issue.

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