Radioiodine diagnosis of differentiated thyroid cancer lung metastases in children: a case report

Lung metastases (LM) in differentiated thyroid cancer in children occur in 7-30% of cases, 3-4 times more often than in adults. Early diagnosis of LM improves the results of radioiodine therapy and the rate of complete remissions. We present a clinical case demonstrating the specific features of LM diagnosis in children. Patient G., a 9-year-old boy, was for two years observed for excessive body weight. Ultrasound thyroid gland examination revealed multiple hypoechogenic nodules, cytologically verified papillary cancer. Computed tomography (CT) of the chest revealed subpleural foci up to 3 mm in both lungs, which were more consistent with intrapulmonary lymph nodes, but did not completely rule out metastatic lesions. A thyroidectomy with central and lateral neck dissections was performed in January 2022. Histologic examination revealed multifocal papillary cancer growth in the entire thyroid gland with Tall cell sites and metastases to multiple cervical lymph nodes. According to TNM classification (American Joint Committee on Cancer (AJCC), 8th edition), the tumor grade was considered as pT2N1bM0, stage I. Seven weeks after surgical treatment, the patient underwent whole body scintigraphy with I-123 (24 hours after I-123 injection for 50 MBq) followed by radioiodine therapy on endogenous TSH-stimulation (4 weeks of L-thyroxine withdrawal). Scintigraphy with I-123 (whole-body and SPECT/CT) did not detect iodine-accumulating tissue. However, post-therapy (72 hours after administration of 2 GBq I-131) scintigraphy (whole-body and SPECT/CT) revealed residual thyroid tissue and diffuse accumulation of I-131 in both lungs. The TSH-stimulated thyroglobulin was 118 ng/ml, and thyroglobulin antibodies were 21 IU/ml. Pathological accumulation of I-131 in the lungs in combination with the elevated thyroglobulin level and chest CT findings were regarded as LM, therefore the tumor process was restaged — pT2N1bM1, stage II and radioiodine therapy was planned to continue.Conclusion. In this clinical case, when diagnosing LM, the result of scintigraphy with I-123 was false-negative, and chest CT scan was uncertain, requiring dynamic monitoring. Scintigraphy after administration of I-131 therapeutic activity revealed LM, thereby changing further treatment strategy to continued radioiodine therapy and increasing the possibility of achieving complete disease remission. Factors associated with LM were miltifocal growth, massive neck lymph node involvement and high TSH-stimulated thyroglobulin levels.