Tumor burden of persistent disease in patients with differentiated thyroid cancer: correlation with postoperative risk-stratification and impact on outcome

Background In patients with differentiated thyroid cancer (DTC), tumor burden of persistent disease (PD) is a variable that could affect therapy efficiency. Our aim was to assess its correlation with the 2015 American Thyroid Association (ATA) risk-stratification system, and its impact on response to initial therapy and outcome. Methods This retrospective cohort study included 618 consecutive DTC patients referred for postoperative radioiodine (RAI) treatment. Patients were risk-stratified using the 2015 ATA guidelines according to postoperative data, before RAI treatment. Tumor burden of PD was classified into three categories, i.e. very small-, small- and large-volume PD. Very small-volume PD was defined by the presence of abnormal foci on post-RAI scintigraphy with SPECT/CT or(18)FDG PET/CT without identifiable lesions on anatomic imaging. Small- and large-volume PD were defined by lesions with a largest size < 10 or >= 10 mm respectively. Results PD was evidenced in 107 patients (17%). Mean follow-up for patients with PD was 7 +/- 3 years. The percentage of large-volume PD increased with the ATA risk (18, 56 and 89% in low-, intermediate- and high-risk patients, respectively,p < 0.0001). There was a significant trend for a decrease in excellent response rate from the very small-, small- to large-volume PD groups at 9-12 months after initial therapy (71, 20 and 7%, respectively;p = 0.01) and at last follow-up visit (75, 28 and 16%, respectively;p = 0.04). On multivariate analysis, age >= 45 years, distant and/or thyroid bed disease, small-volume or large-volume tumor burden and(18)FDG-positive PD were independent risk factors for indeterminate or incomplete response at last follow-up visit. Conclusions The tumor burden of PD correlates with the ATA risk-stratification, affects the response to initial therapy and is an independent predictor of residual disease after a mean 7-yr follow-up. This variable might be taken into account in addition to the postoperative ATA risk-stratification to refine outcome prognostication after initial treatment.