Prognostic relevance of previous exposure to ionizing radiation in well-differentiated thyroid cancer

Although exposure to ionizing radiation is a well-known risk factor for well-differentiated thyroid cancer, its effects on tumor behavior remain unclear. This study was undertaken to address this question. We randomly selected 426 patients who underwent thyroidectomy for cancer between 1964 and 2000 and divided them into two groups: previously exposed (ExR); not exposed to radiation (nExR). Data were retrospectively collected. There were 340 female patients and 86 male, age 9-89 years with mean follow-up of 56 months. The ExR group (n=68) was smaller than the nExR group (n=358). Most patients in the ExR group (64.5%) had previously received therapeutic radiation, whereas 25% were occupationally exposed and 10.5% were at risk from environmental radiation. Both groups were similar with regard to extent of thyroidectomy and adjuvant treatment, but neck dissections were more frequent in the nExR group. There were no significant differences between both groups for age, gender, tumor multicentricity, frequency of microcarcinoma, histology, lymph node disease, distant metastasis, and local recurrence. Incidental microcarcinomas were more frequent in the ExR (35.3%) group than in the nExR group (22.1%). Mean tumor size in the ExR group (18.7 mm) was smaller than in the nExR group (22.3 mm). Ionizing radiation increases risk for well-differentiated thyroid cancer (WDTC) but not adverse cancer behavior. Surgeons should be aware of the high incidence of microcancer among patients with previous exposure to radiation.