Role of diagnostic 131Iodine SPECT-CT in guiding radioiodine ablation therapy in patients with differentiated thyroid cancer

Full text: The aim of the study was to determine the incremental role of 131I SPECT-CT in deciding the dose for radioablation. We retrospectively analysed data of 53 patients (40F; 13M) Age range 19 to 65 yrs of differentiated thyroid cancer (DTC). These included 46 with papillary thyroid cancer (PTC), 6 with follicular thyroid cancer (FTC) and 1 with hurthle cell cancer (HTC). At 6 weeks after total thyroidectomy all patients underwent planar WBS 48 hours after oral administration of 2mCi of 131I. SPECT-CT of the abnormal foci of 131I uptake was performed after planar imaging. WBS showed foci of tracer uptake in the anterior neck in 51 patients (average uptake 2.1%, range 0.5 to 10.4%). 18 patients of PTC showed additional foci of tracer uptake in anterior neck. 3 patients (2 PTC,1FTC) showed abnormal tracer uptake (1 patient with diffuse and two with distinct foci) in the chest. 5 patients showed foci of tracer uptake in suspicious for skeletal metastases. SPECT-CT images showed tracer uptake only in thyroid bed in 27 patients, thyroid bed plus lymph nodes in 19 patients (level VI in 16, level I in one patient) one patient in superior mediastinal lymph nodes and one patient in left supraclavicular lymph node. In 5 patients who showed uptake in thyroid bed and suspicious foci for bone uptake-SPECT/CT localised the abnormal foci in 3 patients to bones confirming bony metastases (1 in skull,1 in sternum, 1 in acetabulum) and excluded bony metastases in 2 patients by accurately localising abnormal uptake to caecum.2 patients with focal uptake in chest were confirmed to have lung metastases as SPECT/CT localised tracer uptake to lung metastases. It is concluded that on planar WBS, it is difficult to differentiate tracer uptake in thyroid bed from level VI lymph nodes and the exact anatomical sites of distant foci cannot be known. SPECT-CT had incremental diagnostic value in 48% (24/51) of patients by providing anatomical co-relates to the sites of abnormal 131I uptake. Clinical relevance statement: As per ATA guidelines empirical dose for ablation is 30 to 100mCi of 131I for remnant, 100 to 150mCi for cervical lymph nodes and 150 to 300mCi for distant bone metastasis. Addition of SPECT-CT to planar WBS guides in deciding the appropriate dose for 131I ablation following total thyroidectomy