Rise in Thyroid Cancer Attributed to Better Detection

KINGSTON, Ontario, Nov. 19 -- Wider use of medical imaging techniques in recent years is turning up significantly more subclinical tumors, researchers here said.Those small tumors -- too tiny to be palpable -- are enough to account for most of the documented increase in thyroid cancer in Canada, the U.S., and Europe, Stephen Hall, M.D., and colleagues at Queen's University, said in the Nov. 20 issue of the Canadian Medical Association Journal.
Action Points
Explain to interested patients that thyroid cancer, although still rare, has been increasing in incidence, for reasons that are not clear.
Note that this study suggests that with better equipment, more small tumors are being detected.
The finding appears to rule out alternate explanations, such as radiation exposure in childhood or hormonal and reproductive factors, the researchers said.
"The detection of small, asymptomatic, so-called subclinicial tumors accounts for the majority of the increase in incidence," Dr Hall said.
Dr. Hall and colleagues used data from the Ontario Cancer Registry, a population-based registry of all new cases of cancer among the 11 million people in Canada's largest province.
For the 12-year period from Jan. 1, 1990 through Dec. 31, 2001, they identified 7,422 cases of differentiated thyroid carcinoma -- 92.2% of them papillary thyroid and 7.8% follicular.
The researchers obtained pathology reports for a random 10% in each year of the study period, and reviewed 1,227 thyroid-related reports. (Some patients had more than one such report.)
The final study group included 605 patients who had surgery in Ontario as initial treatment for differentiated thyroid carcinoma, excluding those under age 18, or who had retrosternal tumors or underwent surgery for another condition.
The researchers compared tumor sizes by year, grouping patients according to whether the tumor was small (no larger than two centimeters), medium (from two through four centimeters), or large (greater than four centimeters).
The size criteria correspond to the American Joint Committee on Cancer's tumor-node-metastasis staging system.
The study found that:
The number of reported cases increased from 403 in 1990 to 990 in 2001, an increase of 1.46 times in 12 years or 13% per year. The increase was significant at P=0.03.
Between-group comparisons showed a statistically significant difference in incidence between the small and medium tumors over the study period, at P=0.007, as well as between the small and large tumors, at P=0.0002, but no difference between the medium and large tumors.
The increase was largely driven by increased detection in women, who made up 77.4% of the study cohort.
Among women, significantly more small tumors were detected, compared with medium and large tumors, at P=0.004 and P=0.0001, but there was no significant difference between medium and large tumors.
Among men, there were no significant differences in incidence among small, medium, or large tumors.
One implication of the finding is that the current treatment paradigm -- total thyroidectomy as the optimal surgical procedure -- may be more aggressive than needed, Dr. Hall and colleagues said.
"For a disease that has an excellent prognosis for many patients," they concluded, "management of small thyroid tumors may be based on an outdated clinical model and result in unnecessary investigations and overly aggressive surgery."
But it's too early to abandon that paradigm, according to Jacques How, M.B.Ch.B., and Roger Tabah, M.D., both of McGill University Health Center in Montréal.
In an accompanying commentary on the report, Drs. How and Tabah noted that thyroid cancer is "highly heterogeneous."
"Despite similar histological appearances," they said, the "biology and clinical behavior [of thyroid cancer] can vary remarkably from one patient to another."
The variability, they said, suggests that "we need to find a compromise between overdiagnosis and beneficial early screening."
The study was supported by funding from the Clinical Teachers of Queen's University. Dr. Hall reported no conflicts. Drs. How and Tabah also reported no conflicts.Additional source: Canadian Medical Association JournalSource reference: Kent WDT, et al "Increased incidence of differentiated thyroid carcinoma and detection of subclinical disease"CMAJ 2007; 177(11): 1357-61. Additional source: Canadian Medical Association JournalSource reference: How J, Tabah R, "Explaining the increasing incidence of differentiated thyroid cancer"CMAJ 2007; 177(11): 1383-84.