NEW YORK (Reuters Health) – Environmental factors appear to influence the incidence of papillary thyroid microcarcinomas (PTMCs), according to a comparison of epidemiologic data from Sicily and the USA.

Moreover, while such microcarcinomas are usually indolent, “In both registries, more than 35% of PTMCs exhibited 2 or more risk factors, suggesting that they may require surgery and follow-up similar to that of larger carcinomas,” according to the report published online March 12 in the Journal of Clinical Endocrinology and Metabolism.

The authors of the paper note that thyroid microcarcinomas – defined as those smaller than 1 cm in diameter – make up almost half of all thyroid cancer.  Guidelines indicate that most such microcarcinomas do not require aggressive treatment or follow-up.  However, some have the potential to progress and metastasize.

Dr. Pasqualino Malandrino, at the University of Catania in Italy, and colleagues go on to explain that to identify PTMCs that warrant treatment and followup, algorithms have been proposed based on risk factors such as tumor size and multifocality along with patient characteristics such as age and gender.  “To determine whether these risk factors are similarly distributed and associated in different environmental and ethnic settings, we analyzed 2 thyroid cancer registries from geographic areas that are different in terms of ethnicity, environment, and lifestyle.”

The researchers used data on all incident cases of PTMCs recorded over a 5-year period in the Sicilian Regional Registry for Thyroid Cancer and in the US Surveillance Epidemiology and End Results (SEER) registry.

They found that the incidence rate in Sicily was twice that seen in the US: 5.8 versus 2.9 cases per 100,000 population, respectively.  Furthermore, in Sicily, the incidence was 10.4/100,000 in the volcanic area compared 4.6/100,000 in the rest of the island.

There were other differences between the two geographical locations.  The female-male ratio was 5.4-to-1 in Sicily compared to 4.4-to-1 in the US (p=0.003), for example.  While the mean age in the two regions was 49.4 versus 50.6 years (p<0.001), respectively, the higher age in the US was mainly due to a lower percentage of males younger than 45 (37.3% in Sicily vs 26.2% in the US; p<0.0001).

Average tumor size was similar in both registries (5.5 and 5.4 mm), and mean tumor diameter was greater in patients younger than 45 years in both data sets. Furthermore, younger patients were more likely to have nodal metastases than patients older than 45 years (odds ratio 1.26; p<0.0001).

Multifocality was present in of 26.0% of cases in Sicily compared to 33.5% in the US. (p<0.0001), and in both regions multifocality was associated with increased likelihood of lymph node involvement.

The percentage of patients with two or more risk factors for progression was lower in Sicily than in the US (37.8% vs 42.7%), the team found.

Dr. Malandrino and colleagues expect that in future, molecular characterization of PTMCs will define the risk of progression with more certainty.  For now they advise, “Based on the clinical characteristics at presentation (concordant in 2 large registries), PTMC risk stratification should suggest total thyroidectomy in at least 1 of 3 PTMCs, specifically in the case of larger microcarcinoma size and in young, male patients.”

“Moreover,” they add, “a careful assessment of cervical lymph node involvement is required in these patients when the cancer exhibits extrathyroid extension and/or is multifocal.”

SOURCE: Papillary Thyroid Microcarcinomas: A Comparative Study of the Characteristics and Risk Factors at Presentation in Two Cancer Registries

J Clin Endocrinol Metab 2013;98.