O-20 BIOCHEMICAL AND RADIOLOGICAL REMISSION IN A METASTATIC MEDULLARY THYROID CASE UNDER SELPERCATINIB TREATMENT PRECEDED BY SUCCESSFUL CONVENTIONAL APPROACHES

Abstract Introduction Medullary thyroid carcinoma (MTC) is a rare tumour derived from parafollicular C cells of the thyroid gland accounting about 3-5% of all thyroid malignancies. It may be asymptomatic both at early and advanced stages. Solitary bone metastases can be detected. In addition to resection of metastases, selective RET inhibitors has provided significant improvement in management. Herein, we present a case of metastatic MTC who has achieved remission with the addition of selpercatinib, a highly selective RET-inhibitor following resection of solitary bone metastasis. Clinical Case A 41-year-old female patient was referred for further evaluation of a pretracheal lesion that had been reported as MTC metastasis following resection. She had the story of right thyroid lobectomy that had been reported as micropapillary thyroid carcinoma, classical variant without lymphovascular invasion two years before. F-18 FDG-PET scan demonstrated residual uptake around paratracheal region and a lesion at right femoral diaphysis with mild uptake (SUVmax:3.42). Due to its sclerotic and spherical nature, femoral lesion was initially considered as benign bone tumour. However, MRI of right femur confirmed a nodular lesion in diaphysis, suggestive of metastasis. GA-68 DOTA-TATE-PET showed uptake only in bilateral cervical lymph nodes and pretracheal region. She had elevated serum CEA, 129,83ng/mL (0.0-5.0) and calcitonin, 172 pg/mL(0.0-18.2), supporting the diagnosis of metastatic MTC. 24-hour urinary catecholamine and calcium metabolism were normal. She underwent left completion thyroidectomy and left cervical neck dissection. Histopathological analysis showed no malignancy. Because of the location, bone lesion was considered not suitable for biopsy. At third post-thyroidectomy month her CEA and calcitonin exhibited downwards trend; 12,64ng/mL and 150 pg/mL, respectively. Femoral bone lesion began to slightly show Ga-68 uptake. She persistently complained about hip pain, a bone scintigraphy also showed increase radiopharmaceutical uptake at right femoral diaphysis, the same location where 18F FDG-PET pointed.At the sixth month of admission, she was referred to surgery. Histopathology confirmed MTC metastasis and molecular analysis identified RET M918T mutation (c2753T>C p.Met918Thr) in tissue. Palliative radiotherapy was performed to right femoral area.RET inhibitor selpercatinib was introduced subsequently. Serum calcitonin and CEA levels exhibited significant reduction following selpercatinib (Figure 1). Ga-68 DOTA-TATE PET imaging at the sixth month of selpercatinib showed no pathological uptake along with biochemical normalization, indicating complete remission. Conclusion Treatment of MTC is mainly dependent on surgery. As shown in our case, successful surgery of isolated bone metastasis followed by targeted therapy may enable remission. Selpercatinib provided significant radiological and biochemical response, supporting its use in advanced RET-mutated MTC.Figure 1:Changes in CEA and calcitonin levels during follow-up