Background: Papillary Carcinoma Thyroid is the most common form of differentiated thyroid cancer. Its coexistence with a Clear cell Neuro endocrine tumor (NET) has not been reported yet. Clear cell carcinoma is an epithelial derived tumor, characterised by the presence of clear cells. It may arise in multiple tissues including kidneys, uterus, GIT and ovary. Though these tumours may metastasize in thyroid rarely, they have not been reported to be originating primarily in thyroid gland and metastasising elsewhere. Also calcitonin negative NET of thyroid Gland are extremely rare. In our case, clear cell tumour was an aggressive one with widespread metastasis. It was Calcitonin negative and expressing other NET markers. Clinical Case: 40 year old male, presented to endocrine OPD in September 2018, with complaints of right sided neck swelling of 4 months duration. He gave a history of similar swelling on the right side of neck 2 years ago for which he had undergone right Hemi- thyroidectomy at an outside hospital. The HPE report had mentioned the possibility of clear cell NET. Tumour cells expressed Cytokeratin(CK),Epithelial membrane antigen(EMA), CD 56,c-kit,synaptophysin. There was no expression of TTF-1,Tg,PAX 8,Chromogranin A,calcitonin,CD 5,S-100&P 63. FDG PET scan done in 2018 showed FDG avid 8 x 6.1cm soft tissue mass in the right paratracheal region along with FDG avid nodules in the left lobe. FNAC from the mass showed recurrent carcinoma with extensive hemorrhagic cystic changes. In view of the above mentioned findings, he underwent completion thyroidectomy along with bilateral modified neck dissection(MND) and central compartment clearance(FIGURE 1). HPE report mentioned left thyroid having differentiated papillary carcinoma (pT1a pN1a) with positive lymph node in lateral and central compartment. Microscopic findings of the Tissue specimen from the right modified neck dissection(MND) showed tumor cells in groups and sheets,with clear cytoplasm and fairly uniform nucleus appearance. No papillary or follicular cells, no obvious lymphovascular invasion was seen(FIGURE 2A 2B) Frequent areas of necrosis and loose fibrinous tissue were seen amidst the tumor. Mitotic count was approximately 4-5 /hpf. Immuno histochemistry (IHC)done on the specimen from right MND- patchy Epithelial membrane antigen(EMA) expression, CD 10 weakly expressed, rest markers like TTF-1/ PAX8/ RCC antigen /CEA/ Calcitonin /P 63/High molecular weight cytokeratin (HMWCK)/ CK(MNF)/ CK7/ CK20/ CK 19 were negative(figure 2C). Based on the above findings, it was reported first as CASTLE(Carcinoma with thymus like elements) tumour. Tissue specimen was sent to TATA memorial hospital, Mumbai for review. Extensive IHC profile and molecular studies were done, they suggested that it is more likely to be Ewings sarcoma with epithelial differentiation (IHC - positivity for membranous mic2, EMA, CD56 and c Kit, and presence of EWSR1 rearrangements on molecular testing). Still there was no definite consensus regarding the final diagnosis. Tissue slides were sent abroad to Professor Dr Christopher D.M Fletcher at Harvard Medical School. He termed it as unclassified clear cell malignant neoplasm.
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