¹⁸FDG PET/CT Delayed Scan in Diagnosis of Thyroid Disorders. Incidental Finding. Case Study.

Objectives To presents the clinical significance of focal increased ¹⁸F-FDG PET/CT uptake in delayed scans for differentiating benign from malignant thyroid disorders

Methods Two cancer female patients; a 68y old with Non-Hodgkin’s Lymphoma (NHL) of Maxilla, and a 54y old with papillary ovarian adenocarcinoma underwent ¹⁸F-FDG PET/CT scans; IV injection dose 300 MBq, and (7-8) bed/130sec. Early scan performed at 60-65 min., and delayed scan at 170-180 min. from ¹⁸F-FDG injection time. Early and delayed SUV_max of each lesion was determined and retention index (RI%) calculated: RI% = 100 x ( SUV_{max delayed}  - SUV_{max early} ) / SUV_{max early}

Results ¹⁸F-FDG PET/CT scan for the patient with NHL of Maxilla showed a focal hyper metabolic mass lesion in the left lung/mid zone posteriorly; and enlarged thyroid with multi nodular changes detected on CT scan. One lesion showed increased ¹⁸F-FDG metabolism with SUV_{max early} = 6.11. We further took a delayed image resulting in SUV_{max delayed} = 4.02; RI% = -34.2%. The patient was referred for thyroid function test: TSH = 2.4 IU/L; Tg < 10 ng/ml. The histopathology of US-guided FNA biopsy samples suggested thyroid multi nodular goiter of benign cells. The negative RI% value indicated that the benign lesion was unable to retain ¹⁸F-FDG inside its normal cells. ¹⁸F-FDG PET/CT scan for the patient with ovarian adenocarcinoma showed focal hyper metabolic lymph node lesion visible in the left lower posterior deep cervical region. Thyroid was enlarged, with focal nodules of irregular hyper metabolic ¹⁸F-FDG uptake in Lt lobe with SUV_{max early =} 12.57, SUV_{max delayed =} 16.28; and RI%=29.5. Thyroid Function Test was advised: TSH = 7.9 IU/L, Tg = 18.9 ng/ml, and final diagnosis was determined by thyroid tissue biopsy. The histopathology of US-guided FNA biopsy samples suggested Hashimoto’s thyroiditis and papillary carcinoma, classical type. The patient underwent total thyroidectomy. The positive RI% value indicated that malignant lesion retained ¹⁸F-FDG inside its abnormal cells.

Conclusions Positive RI% of incidentally detected thyroid lesion by ¹⁸F-FDG PET/CT delayed scan can be a sensitive tool to indicate malignancy. Further work is required to assure its utility as a complementary protocol for differentiating thyroid benign lesion from malignant lesions. ¹⁸F-PDG PET/CT computer software can be modified to project the differentiation.