Objective: The primary purpose of this study was to validate the proposed modified 2009 American Thyroid Association Risk Stratification System (M-2009-RSS) in patients with thyroid cancer and to compare the findings with those of the 2009 ATA Risk of Recurrence (2009 ATA-RR) and the Ongoing Risk of recurrence system. The secondary purpose was to assess which risk stratification system had the best predictive value to foresee the probability of structural incomplete response or the no evidence of disease (NED) status at the end of follow-up.

Subjects and methods: This retrospective review included 149 patients with differentiated thyroid cancer who had low and intermediate 2009 ATA-RR and were treated at a single experienced center and followed-up for a median of 6 years (range 3-12 years). Each patient was risk stratified using both the 2009 ATA-RR and the M-2009-RSS. The primary endpoints were 1) the best response to initial therapy defined as either excellent response, biochemical or structural incomplete response, or indeterminate response; 2) clinical status at final follow-up defined as either NED, biochemical incomplete response, structural incomplete response, indeterminate response, or recurrence (biochemical or structural disease identified after a period of NED), and 3) ongoing RR defined as low or high risk according several outcomes after initial treatment.

Results: Mean age of included patients was 45.3±13 years. Both the ATA 2009-RR and the M-2009-RSS provided clinically meaningful graded estimates with regard to the status of NED at the end of follow-up in low-risk patients (84% for 2009 ATA-RR and 74% for M-2009-RSS) or the likelihood of having persistent structural disease (0% for 2009 ATA RR and 3.6% for the M-2009-RSS). When patients were classified as low risk, the positive predictive value (PPV) and negative predictive value (NPV) to predict structural disease was 0% and 88.7% for the 2009 ATA-RR, 3.6% and 86.5% for the M-2009-RSS, and 1.6% and 68.2% for the ongoing RR (p=0.022 and 0.055 of chi-square test for PPV and NPV, respectively).

Conclusions: Despite expanding the definition of low risk to include small-volume lymph node metastases, minor extrathyroidal extension, and minimally invasive follicular thyroid cancer, the M-2009-RSS predicts clinical outcomes (structural incomplete response and NED at the end of follow-up) that are very similar to the previously validated 2009 ATA RR classification system.