It’s easy to crunch the numbers on thyroid cancer and assume it is a disease fast on the rise. Incidence has more than doubled since the early 1970s, and for women, it is the cancer with the fastest-growing number of new cases.

But not every statistic tells the obvious tale. Despite the increase, thyroid cancer - a very treatable disease that develops in a butterfly-shaped gland in the neck — is still relatively uncommon.

Many people develop benign lumps, known as nodules, in the neck, but only one in 20, or less than 45,000 cases a year, are malignant. Even fewer patients have an aggressive form of the disease, which has a survival rate of nearly 97 percent after five years and results in an estimated 1,690 deaths each year. Over the last few years, however, studies showing there has been a significant increase in incidence, and even mortality, in certain groups have caused much debate in the scientific community.

“I don’t think there is any question that there is an increasing incidence of thyroid cancer,” said Dr. Kenneth Burman, chief of the endocrine section in the department of medicine at Washington Hospital Center in Washington, D.C. “But it is not that simple. The question is whether or not it is related to detection and radiological studies, or if it is related to an authentic rise in thyroid cancer.”

It is a question that still remains largely unanswered. Evidence from the Surveillance, Epidemiology and End Results database, a registry of cancer cases that is kept by the National Cancer Institute, leaves little question that there are more cases of thyroid cancer today than three decades ago. But the more important question, as Dr. Burman points out, is whether these statistics indicate a true rise in the disease or are simply a result of better diagnostic tools.

Over the last three decades, ultrasound and fine-needle biopsies have helped diagnose thousands of cases that would never have been found before. In many cases, nodules are discovered by accident during another medical investigation.

A study published in The Journal of the American Medical Association first brought this issue to light in 2006. Researchers concluded that the reported 140 percent increase in thyroid cancer from 1973 to 2002 was simply a result of “increased diagnostic scrutiny.”

They argued that a true increase in incidence would be reflected in every stage of the cancer. But the study showed that 87 percent of the increase was from small papillary thyroid cancer tumors - the most common and treatable type of thyroid cancer - that were less than two centimeters in size. Many of these cases, the researchers say, would never have caused any problems. In fact, studies have shown that thyroid cancer is found in nearly 4 percent of all fine-needle aspiration biopsy specimens.

“These cases have been there all along,” said Dr. Louise Davies, assistant professor of surgery in the division of otolaryngology, head and neck surgery at Dartmouth Medical School. “We just didn’t see them until now. Understanding this requires that you think about the word ‘cancer’ in a different way than we usually do. You can have increased rates of incidence without changing the number of people who die.”

But the mortality rate is a little more complicated than that. Survival rates, after five years, increased 4.7 percent in women, who are three times as likely to develop the disease as men, from 1974 to 2001. In men, however, the annual percentage change in thyroid cancer mortality increased significantly, by 2.4 percent, from 1992 to 2000 — the highest jump of any cancer. That is one reason many other experts argue that diagnostic tools are not the only factor.

“I think it is an oversimplification to say the increase in diagnosis is from the overuse of technology and only relates to small tumors that are insignificant,” said Dr. Steven Sherman, medical director of the endocrine center at the University of Texas M.D. Anderson Cancer Center in Houston. “There is a component that relates to increased technology, but until we can do a better job at predicting the outcomes for individuals who develop cancer we still need to treat each case.”

Physicians are fairly clueless about what else could account for this mysterious rise in incidence. Exposure to radiation from the Chernobyl nuclear power plant accident in 1986 and radioactive fallout from nuclear weapons testing in the 1950s have long been linked to thyroid cancer, but they would not account for all the new cases.

Regardless of the reported increase in small tumors, the standard of care for thyroid cancer remains the same as it was two decades ago. Patients must undergo a thryoidectomy, a surgical procedure that removes all or half of the thyroid gland. Afterward, many patients also require a radioactive iodine treatment, which kills any remaining cancer cells.

Dr. Bryan McIver, a physician in the division of endocrinology, diabetes, metabolism and nutrition at the Mayo Clinic in Rochester, Minn., said of the surgical default, “Even though the evidence does not support that it is beneficial, there is an increasing trend in the U.S., and probably worldwide, to treat all thyroid cancers in the most aggressive way.”

As a result, surgeons like Dr. Davies think the increase in diagnosis does patients with small tumors a disservice. “I don’t think it is helpful when patients pick it up by accident,” she said. “It distracts them from the problem they came in with and leads to unnecessary treatment. The mortality rate of papillary thyroid cancer is lower than the surgical complication rates.”

Since thyroid cancer has long been thought of as a disease that requires surgery, experts are starting to rethink how they approach the rapidly increasing number of small tumors.

“Sometimes I think we are doing more harm than good with these small tumors,” Dr. McIver said. “But there is also going to be a subset of these small tumors that are caught early and would have caused a problem. It’s hard to ignore a diagnosis of cancer.”

###

By CAROLYN SAYRE