Behavior of small thyroid cancers found by screening radiation-exposed individuals

Thyroid cancers detected by screening irradiated individuals are often smal l and of uncertain clinical significance. We retrospectively analyzed the e ffect of screening in a cohort of 4296 individuals exposed to radiation as children in the 1940s and 1950s and followed by us from 1974 until the pres ent. We compared the thyroid cancers diagnosed before 1974 (122 cases, rout ine care) with the cancers found in subjects screened by us after 1974 (172 cases, screened), using cancer recurrence as the end point. Screening incl uded a thyroid scan or, more recently, thyroid ultrasound. As expected, man y of the cancers found by screening were very small (52% were < 10 mm), but the range of tumor sizes overlapped those found by routine care. The recur rence rate was significantly lower in the cases found by screening, but whe n the comparison was limited to cancers 10 mm or larger, no difference in t he recurrence rates was seen. This would suggest that the lower recurrence rate observed for small thyroid cancers detected at screening was due to ea rlier diagnosis rather than more effective treatment. By univariate analysi s, four factors were associated with an increased risk of recurrence of sma ll (< 10 mm) thyroid cancers: short latency (i.e. a shorter time interval b etween the radiation exposure and the first thyroid surgery), lymph node me tastases present at diagnosis, multifocal cancers, and higher radiation dos e. In a multivariate analysis combining the four risk factors, only short l atency was significant. As thyroid cancers that escape detection by routine means should be diagnosed at screening, and both large and small thyroid c ancers have the potential to recur, screening may be of value, but only if groups with a sufficiently high prevalence of thyroid cancer can be identif ied to offset the adverse effects of unnecessary treatment due to false pos itive results.