Prolongation of tumour volume doubling time (midDT) is associated with improvement in disease-specific survival in patients with rapidly progressive radioactive iodine refractory differentiated thyroid cancer selected for molecular targeted therapy.

Purpose: To assess molecular targeted therapy (MTT)'s ability to affect tumour volume doubling time (TVDT) and disease-specific survival (DSS) in patients presenting with lung metastasis from radioactive iodine refractory progressive thyroid cancer.

Methods: In this retrospective study, we examined the clinical characteristics, average tumour volume doubling times of lung metastasis and disease-specific survival of patients with lung metastasis from differentiated thyroid cancer who were treated with MTT.

Results: The 5-year DSS from the distant metastasis (DM) diagnosis was 72% with median survival of 8 years (95% CI: 6.6-9.5). The median survival was 2.9 years after MTT start (95% CI: 2.1-3.6). On MTT, lung average tumour volume doubling time (midDT) was prolonged to midDT ≥3 years in 75% of patients with baseline midDT ≤1 year and 100% of patients with midDT 1-3 years. In patients with rapidly progressive thyroid cancer (midDT ≤1 year at baseline), the median survival was 4.5 years in those with MTT-achieved midDT ≥3 years (95% CI: 2.9-6.2), as opposed to 2.3 years (95% CI: 0.3-4.3) and 0.7 years (95% CI: 0.2-1.3) in those with MTT-achieved midDT of 1-3 years and MTT-achieved midDT ≤1 year, respectively (log rank P < 0.001).

Conclusion: Lung midDT is a useful and important clinical marker of disease-specific survival for patients with progressive radioactive iodine refractory (RAIR) metastatic thyroid cancer. In patients with rapidly progressive metastatic RAIR thyroid cancer, molecular targeted therapy prolongs lung tumour volume doubling time and is associated with improved disease-specific survival.