Unexpected radioactive iodine accumulation on whole-body scan after I-131 ablation therapy for differentiated thyroid cancer

Abstract

We retrospectively evaluated the frequency of unexpected accumulation of radioactive iodine on the post-therapy whole-body scan (Rx-WBS) after radioactive iodine (RAI) ablation therapy in patients with differentiated thyroid cancer (DTC). We searched our institutional database for Rx-WBSs of DTC patients who underwent RAI ablation or adjuvant therapy between 2012 and 2019. Patients with distant metastasis diagnosed by CT or PET/CT before therapy, and those had previously received RAI therapy were excluded. In total, 293 patients (201 female and 92 male, median age 54 years) were selected. Two nuclear medicine physicians interpreted the Rx-WBS images by determining the visual intensity of radioiodine uptake by the thyroid bed, cervical and mediastinal lymph nodes, lungs, and bone. Clinical features of the patients with and without the metastatic accumulation were compared by chi-square test and median test. Logistic regression analyses were performed to compare the association between the presence of metastatic accumulation and these clinical factors. Eighty-four of 293 patients (28.7%) showed metastatic accumulation. Patients with metastatic RAI accumulation showed a significantly higher frequency of pathological N1 (pN1) and serum thyroglobulin (Tg) > 1.5 ng/ml under TSH stimulation (p = 0.035 and p = 0.031, respectively). Logistic regression analysis indicated that a serum Tg > 1.5 ng/ml was significantly correlated with the presence of metastatic accumulation (odds ratio = 1.985; p = 0.033). In conclusion, Patients with Tg > 1.5 ng/ml were more likely to show metastatic accumulation. In addition, the presence of lymph node metastasis at the initial thyroid surgery was also associated with this unexpected metastatic accumulation.

Keywords: SPECT/CT; ablation; differentiated thyroid cancer; lymph node metastasis; radioactive iodine.

Fig. 1

Whole body scan (WBS) and SPECT/CT after ablation therapy A 69-year-old female with papillary carcinoma (pT3N0). WBS after ablation therapy using 1,110 MBq radioactive iodine shows a focus only in the thyroid bed (white arrow). Non-specific accumulation were also seen in nasal and oral cavity, stomach, liver and bladder. The serum thyroglobulin value under TSH stimulation was 0.1 ng/ml. (a) WBS (b) axial SPECT/CT image (c) coronal SPECT/CT image.

Fig. 2

Whole body scan (WBS) and SPECT/CT after ablation therapy A 61-year-old female with papillary carcinoma (pT2N1a). WBS after ablation therapy using 1,110 MBq radioactive iodine shows accumulation in the thyroid bed (white arrow), bilateral neck (yellow arrow), and left superior mediastinum (blue arrow). Non-specific accumulation was also seen in colon. Serum thyroglobulin value under TSH stimulation was 3.9 ng/ml. Fig. 2a: WBS. Fig. 2b: axial SPECT/CT image. Fig. 2c: coronal SPECT/CT image.

Fig. 3

Whole body scan (WBS) and SPECT/CT after ablation therapy A 26-year-old female with papillary carcinoma (pT3N1b). WBS after ablation therapy using 1,110 MBq radioactive iodine shows accumulation in the thyroid bed and bilateral neck (white arrow), and bilateral lung (yellow arrow). SPECT/CT also shows bilateral lung accumulation, while no metastatic lesions are seen on the CT image. Non-specific accumulation were also seen in liver, colon and bladder. The serum thyroglobulin value under TSH stimulation was 45.2 ng/ml. (a) WBS (b) axial SPECT/CT image (c) CT image.