Increasing diagnosis of subclinical thyroid cancers leads to spurious improvements in survival rates

Background: Survival rates are commonly used to measure success in treating cancer, but can be misleading. Modern diagnostic practices can lead to the appearance of improving cancer survival, as tumors are diagnosed earlier (lead-time bias) or as an increasing proportion are slow-growing (length bias), whereas the actual burden of cancer deaths is unchanged. Increasingly, more subclinical thyroid cancers are being diagnosed. The objective of the current study was to determine whether thyroid cancer survival rates have been affected by this phenomenon.

Methods: The authors analyzed survival data from patients with thyroid cancer who were treated at Memorial Sloan Kettering Cancer Center (MSKCC) from 1950 to 2005, and United States population-based incidence, prevalence, and survival data from 1973 to 2009 in the Surveillance, Epidemiology, and End Results data set.

Results: US thyroid cancer incidence has increased 3-fold from 1975 to 2009. Over time, the proportion of thyroid cancers that are subcentimeter in size has increased from 23% (1983) to 36% (2009). At MSKCC, this percentage rose from 20% (1950) to 35% (2005). The incidence rates of large tumors (>6 cm) and distant metastasis have not changed. In the United States, 10-year relative survival improved from 95.4% to 98.6% (1983-1999). At MSKCC, 10-year disease-specific survival improved from 91.1% to 96.1% (1950-2005). However, when stratified by tumor size and stage, no changes in survival outcomes were observed. US thyroid cancer mortality rates have remained stable (1975-2009).

Conclusions: Modern medical practices increasingly uncover small, asymptomatic thyroid cancers. Survival rates appear improved, but this finding is spurious, attributable instead to shifts in the characteristics of disease being diagnosed. Relying on survival rates to measure success in treating thyroid cancer may reinforce inappropriately aggressive management. Treatment decisions in thyroid cancer should be made based on mortality, not survival data.