‘Increased incidence of secondary tumours in thyroid cancer patients’: a fact or a sophism?

Abstract

In the paper by Robbert B T Verkooijen et al. (1), they described an increased incidence of second primary tumours (SPTs) in thyroid cancer patients. They found no association with iodine-131 (I-131) treatment of differentiated thyroid cancer (DTC), but surmised a common aetiology and/or genetic mechanism causing the observed increased prevalence of SPTs, especially breast cancer, in thyroid cancer patients. In concluding, they recommended close clinical follow-up for early diagnosis of SPTs in patients with DTC. The astonishing results and conclusions of Verkooijen et al. prompted us to search our own database; it contained 1087 patients (73% females, 27% males) with the diagnosis of thyroid cancer. The follow-up period ranged between a minimum of 1 year and a maximum of 30 years (mean 7G5 years); the mean age of the patients was 51G16 years and 103 patients had passed away. Therapeutic doses of I-131 had been given to 933 patients, and 154 had very small primary lesions of DTC with a very low risk of tumour recurrence, and therefore received no I-131 therapy. We included patients with medullary carcinoma in our analysis, in contrast to Verkooijen et al., but otherwise age, gender distribution, histopathology, ablation doses and time points of treatment were comparable with the group reported by Verkooijen et al. In our group, we found 125 additional SPTs in 112 patients (11 patients had three tumours, 1 patient had four tumours), yielding a percentage of 11.5% vs 14.2% (40 out of 282 patients). Fifty-five tumours had occurred several years prior to the diagnosis of DTC (mean 8.4 years). Thus, our data analysis ultimately revealed 5% of patients who had developed an SPT (following the diagnosis of DTC and treatment) when compared with 7% of those reported by Verkooijen et al. These data are in accordance with others, such as Bhattacharyya et al. and Canchola et al. (2, 3). Verkooijen et al. reported a significantly increased standardized incidence ratio (SIR 3.95) for SPTs only when patients with breast cancer before thyroid cancer were included; their SIR for SPTs following DTC overall was 1.13, which is nearly the same as our value of 1.06. Sadetzki et al. (4) found an elevated risk in women with breast cancers for DTCs. Cengiz et al. (5) also observed