18F-FDG-PET SUV AS A PROGNOSTIC MARKER OF INCREASING SIZE IN THYROID CANCER TUMORS

Objective: Positron emission tomography/computed tomography (PET/CT) scans with 2-[fluorine-18] fluoro-2-deoxy-D-glucose (18F-FDG) are used in high-risk thyroid cancer patients to identify metastasis. The prognostic significance of increases in standardized uptake values (SUVs) has not been clearly defined. This pilot study investigated the correlation between SUV increases and subsequent changes in individual lesion size. Methods: A retrospective chart review of patients with histologically confirmed thyroid cancer who were monitored with serial 18F-FDG-PET/CT scans from 2008 to 2013 was performed. Forty-seven patients were selected for analysis. A mixed-effects statistical model was used after data normalization. Results: For a 10% increase in SUV, a 6% increase in tumor area was observed (P <. 0001). Analysis on cube root-transformed data from serial scans was significant in 4 of 5 groups: scans 1 to 2 (P =.0001), scans 2 to 3 (P =.0005), scans 3 to 4 (P=.008), scans 4 to 5 (P =.66), and overall (P <. 0001). After exclusion of outliers, for a 10% increase in SUV, the expected percentage increases in area on subsequent scans were found to be 3.4% (P =.0006), 2.6% (P =.005), 4% (P =.074), and 4.1% (P =.27) for the second, third, fourth, and fifth scans, respectively. The association was similarly significant in cases with a = 25% increase in SUV. Secondary analysis showed a significant association of SUV with thyroglobulin (Tg) level (P=.035) but not with thyroid-stimulating hormone (TSH) level ( P =.85). Conclusions: A significant positive correlation was noted between the increase in lesional SUV and subsequent increase in lesion area. An increase in lesional SUV in subsequent scans may portend tumor growth and could prompt consideration for earlier or more aggressive intervention.