THU537 A Case Of Aggressive NET With Extensive Bone Marrow Metastasis Presenting With Hypercalcemia

Disclosure: G. Kela: None. A.S. Gill: None. F. Kussaga: None. O. Olarinmoye: None. Introduction: A metastatic cancer that has no identifiable site of origin despite a thorough workup is termed cancer of unknown primary (CUP). Neuroendocrine tumors of unknown primary (NUP) are rare, making up less than 5% of all CUPs. This case demonstrates a rare presentation of a well-differentiated neuroendocrine tumor (NET) of an unclear primary site with extensive metastasis to the bone that initially presented with hypercalcemia of unclear cause. Case: We report a case of a 76-year-old-female with a history of depression and hypertension who was initially admitted for evaluation of hypercalcemia with a corrected calcium of 11.9, low normal PTH of 14.2, and PTHrP of 18, treated with zoledronic acid who presented again 3 months later with a 30lb weight loss for 2 months and bright red blood per rectum for one day. The patient had petechiae and flat dry purpura across her upper and lower extremities, lymphadenopathy, hepatosplenomegaly, and jaundice. Labs revealed a hemoglobin of 8.4g/dl (normocytic normochromic) and platelet count of 61,000/cm3, compared to 14.5g/dl and 269,000/cm3 respectively 4 months before admission. Colonoscopy and barium enema revealed internal hemorrhoids as the bleeding source. Laboratory workup for thrombocytopenia revealed no distinct etiology. Further workup with a right iliac bone marrow biopsy revealed a well-differentiated NET. The tumor cells expressed keratin 7, chromogranin, and synaptophysin, & variably expressed INSM1. CT scan of the chest, abdomen, & pelvis revealed a left thyroid nodule measuring 2.4 cm, suspicious for a NET on an octreotide scan. Biopsy of the left thyroid nodule, however, revealed follicular neoplasm which is not a NET. The patient was started on Lanreotide with no significant improvement. She remained severely anemic requiring multiple blood transfusions despite undergoing a hemorrhoidectomy. A repeat CT scan and MRI of the head done for increased confusion and disorganized speech 2 months later showed extensive metastatic disease involving the brain and skull. The patient could not tolerate further workup or cytotoxic chemotherapy and wished to proceed with hospice care. Discussion: NETs can arise from neuroendocrine cells located in any part of the body, most commonly in the gastrointestinal tract, lungs, or pancreas, and on extremely rare occasions from the bone. Conventionally, these tumors are indolent and accompanied by carcinoid symptoms. In this case, the bone biopsy was positive for tumor markers suggesting various primary sites including small bowel, stomach, and pancreas. However, despite extensive workup, the primary localization of the tumor was unsuccessful. The combination of the absence of a site of origin, the predominance of hematologic pathology (pancytopenia), and the highly invasive nature of this patient’s malignancy are suggestive of a well-differentiated NET with extensive metastasis into the bone. Presentation: Thursday, June 15, 2023