Incidence and prognosis of differentiated thyroid cancer in Sweden

Abstract

Non-medullary differentiated thyroid cancer (DTC) is a rare disorder but, nevertheless, among the most common cancers in individuals below 40 years of age. DTC consists of papillary (PTC) and follicular (FTC) thyroid cancer. The prognosis is excellent with an overall 10-year survival of 90 %. The only established risk factor for DTC is ionizing radiation. Different scoring systems are used for prognostication of DTC. Thyroidectomy is the primary treatment for patients with DTC. How the extent of surgery and postoperative treatment, mainly radioactive iodine treatment, influences the prognosis of DTC remains controversial.

The aim of this study was to analyse the incidence and survival of patients with DTC in Sweden and to identify factors of prognostic importance.

From the population-based Swedish Cancer Registry we identified 5,554 individuals diagnosed with DTC during 1958-1987. The patients were followed until 31 December 1999. The relative survival ratio was used as the measure of patient survival. Within this cohort, a nested case-control study was conducted. One control, matched by age at diagnosis, sex and calendar period, was randomly selected for each case, i.e., patients who died due to DTC. Information of possible prognostic factors was abstracted from the medical records. The effect of prognostic factors on DTC mortality was evaluated using conditional logistic regression.

Incidence of both PTC and FTC was higher among women than men, especially for PTC and particularly during the fertile part of life. Ten year relative survival was 99 % for patients under 40 years of age at diagnosis. Among possible risk factors only smoking had a significantly negative influence on survival for patients with DTC. Previous radiotherapy towards the neck region had no prognostic implication. A family history of DTC influenced prognosis, although not significant due to few numbers. Patients with widely invasive FTC experienced a significantly higher mortality compared with PTC patients. Grade of differentiation significantly influenced mortality. Patients in TNM stage IV had a nine times higher mortality compared with patients in stage II. Patients with lymph node metastases experienced twice as high mortality and patients with distant metastasis a 7-fold higher risk of death compared with those with no metastases. Non-surgically treated patients had a worsened prognosis. Incomplete surgical excision was associated with higher mortality, particularly among patients in stage I. In contrast to completeness of tumour excision, surgical extent did not influence the risk of dying from DTC, neither was lymph node surgery associated with prognosis. For patients with loco-regional recurrence the risk of death was fivefolded. The administration of postoperative treatment (radioactive iodine, external radiotherapy, or chemotherapy) was not associated with better survival.

The data suggest that there may exist a class of thyroid tumours that are diagnosed in women during the fertile part of life and associated with a superior prognosis. Among possible risk factors for DTC, the risk of death due to DTC was increased only for smoking patients. Clinically important prognostic factors for patients with DTC were histopathological subgroup, TNM staging including lymph node metastases and distant metastases, as well as completeness of the surgical excision, although the extent of surgery did not influence the patients' survival. Locoregional recurrence increased the mortality significantly. Postoperative treatments were not associated with better survival.