P-111 MEDIASTINAL METASTASIS OF PAPILLARY THYROID CARCINOMA UNDETECTED BY RADIOIODINE SCINTIGRAPHY: A CASE REPORT

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Abstract Introduction Papillary thyroid carcinoma (PTC) accounts for more than 90% of thyroid malignancies and generally has a good prognosis as a slow growing, differentiated thyroid cancer. In the postoperative follow-up of patients, serum thyroglobulin (Tg) and anti-Tg antibody levels, neck ultrasonography and radioiodine (RAI) scintigraphy play an important role in detecting recurrence and residual disease. However, in cases where serum Tg levels are elevated but RAI scintigraphy is negative, 18F-FDG PET/CT may help identify residual disease or distant metastases. Clinical Case A 46-year-old male patient with a history of PTC was referred for further evaluation of a mediastinal mass. Thyroid fine needle aspiration biopsy performed previously at another center was reported as malignant. The patient had undergone total thyroidectomy and neck dissection, and pathology revealed a multifocal classic type papillary thyroid carcinoma with 31 positive lymph nodes out of 81 dissected. In the postoperative follow-up, neck ultrasonography demonstrated no recurrence, while serum Tg levels remained between 68.5–71.1 ng/mL and anti-Tg antibodies were negative. RAI scintigraphy showed no abnormal uptake. Due to the possibility of distant metastasis, 18F-FDG PET/CT was performed, which demonstrated a 6 × 3.5 cm hypermetabolic lesion (SUVmax ≈ 3.6) in the anterior mediastinum. Thorax CT showed this lesion as a lobulated, contrast-enhancing nodular mass. The patient underwent mediastinal mass excision and thymectomy. Pathology confirmed the diagnosis of mediastinal metastasis from papillary thyroid carcinoma. Four months after surgery, Tg levels decreased to 2.51 ng/mL and anti-Tg antibody levels to 17.1 IU/mL. The patient subsequently received 200 mCi RAI therapy. Although the clinical course of PTC is usually favorable, cervical and mediastinal lymph node metastases are observed in a significant number of cases. Elevated serum Tg levels after thyroidectomy and RAI therapy with negative imaging findings indicate an increased risk of recurrence and metastasis. RAI scintigraphy has high sensitivity for recurrence and metastasis detection; however, in some patients with recurrent disease, it may be negative. Especially in cases where serum Tg levels exceed 10 ng/mL, 18F-FDG PET/CT is a useful method for detecting locoregional and distant metastases. In this case, the mediastinal mass detected by PET/CT was surgically removed and subsequent RAI therapy provided a significant decline in Tg levels. Conclusion In PTC patients with negative RAI scintigraphy but elevated serum Tg levels, 18F-FDG PET/CT plays an important role in detecting recurrent or metastatic lesions. Mediastinal masses in such patients should raise suspicion of metastatic PTC, and surgical resection followed by appropriate RAI therapy can achieve biochemical remission and allow long-term disease-free surveillance.Figure 1:Thorax CT- Mediastinal MassAxial thoracic CT image demonstrating a lobulated nodular lesion in the right anterior-superior mediastinum, measuring up to 5.5 cm in diameter.

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