Added value of single photon emission computed tomography (SPECT) / Computed Tomography (CT) with radioiodine whole body scan in follow up of patients with differentiated thyroid cancer

Aim: To evaluate the added value of single-photon emission computerized tomography/computerized tomography (SPECT/CT) in whole body radioiodine (WBI) scan in patients with differentiated thyroid cancer.

Material and Methods: All patients who underwent WBI scan with SPECT/CT from October 2016 to May 2018 were consecutively included. The study included 48 patients (average age of 46.9±18.7 years; 30 females). Data collected included patients’ demographics, TSH levels, TG and TG antibodies level, prior radioiodine ablation, surgical and histopathological findings as well as planer and SPECT/CT findings. Findings other than obvious physiological uptake were interpreted as positive, negative or equivocal in thyroid bed (TB), cervical lymph nodes (LN) or distant metastases (DM). For positive-negative classification, equivocal results were included with the negative group. We used Student T-test to compare mean values of continuous variables between study groups. Fissure Exact test was used to compare the frequency of added SPECT/CT findings between study groups based on TG level or age. The cutoff for TG and age that best differentiate positive from negative scans was set based on ROC analysis. Pearson correlation was used to assess the relationship between number of SPECT/CT added lesions and TG level or age. Logistic regression was used to detect independent predictors of SPECT/CT additional lesions.

Results: The study population included 43 papillary, 4 follicular and 1 hurthel cell carcinoma. Sixteen patients (33.3%) had at least one prior ablative dose of I-131. WBI scans included 15 I-123 scan and 33 I-131 post ablation scan. Thyrogen preparation was used in 26 (54%) patients. Based on both the planar and SPECT/CT images there were 43 (89.6%) positive patients and 5 (10.4%) negative patients. Positive patients included 38 with TB uptake, 18 with LN uptake and 12 with DM. SPECT/CT was able to detect or confirm additional 47 lesions (4 in TB, 22 LN and 21 DM) in 20 patients. Patients group with SPECT/CT added lesions had higher mean serum TG level (1190.5±1872.9 vs 46.8±225.4; p<0.001) and patients age (58.3±16.9 vs 38.8±15.6; p<0.001). Both TG and age were positively correlated to number of SPECT/CT added lesions (0.601; p<0.0001 and 0.375; p=0.009 respectively). Cutoff values of ˃3.4 ng/ml for TG and >45 years old for age were the best to predict additional lesions on SPECT/CT with odds ratio of 43.6 and 6.6 respectively and sensitivity, specificity, PPV, NPP and accuracy of 95, 75, 73.1, 95.5 & 87.5% (p<0.0001) and 80, 71.4, 66.7, 83.3 & 75% (p=0.001) respectively. Adding SPECT/CT reduced equivocal results by accurate localization of uptake sites, differentiating lymph nodes from thyroid bed uptake and detecting false positive due to underlying unexpected physiological uptake. Adding the SPECT/CT to planar imaging decreased equivocal results by 87% from 54 (2 TB, 33 LN and 19 DM) to only 7 (5 LN and 2 DM) and achieved definitive diagnosis in 41/48 patients (85.4%). Conclusion: Adding SPECT/CT to WBI scan detected new lesions and significantly reduced equivocal results by either confirming or excluding the presence of LN involvement or DM, decreased false positive results due to unexpected physiological uptake and achieved definitive results in most studies. Addition of SPECT/CT could particularly be more useful in patients with serum TG >3.4 and/or >45 years of age.