PSUN377 Use of Novel Therapy in Management of Metastatic Parathyroid Cancer

INTRODUCTION: Parathyroid cancer (PC) is a rare malignant tumor which represents <1% of patients with primary hyperparathyroidism (PHPT). With less than 1000 reported cases, PC is the rarest endocrine cancer. Complicating its rarity, is the known resistance to radiation and chemotherapy, and complete surgical resection is the only curative treatment available. Genomic sequencing targeted therapy has shown encouraging results. We hereby present a case where Pembrolizumab was used in treatment of metastatic PC. CASE: A 50 year old lady with a history of nephrolithiasis, hypercalcemia(HC) presented for evaluation of neck mass. She had a recent admission to outside hospital for HC, dehydration, bone pain, polyuria, constipation and workup was suggestive of PHPT with Calcium(Ca) 13.2mg/dL and PTH 1,500pg/ml. Neck ultrasound demonstrated a 3 cm nodule in the left thyroid and fine needle aspiration was suspicious for malignancy. She underwent left hemithyroidectomy, and left parathyroidectomy. PTH levels pre-, intra- and postoperatively were 680, 220, 20 pg/ml respectively with normalization of Serum Ca. Histopathology revealed high grade PC with lymphovascular and perineural invasion. Postoperatively she received adjuvant radiotherapy. Ten months later CT Neck surveillance revealed a 4cm Left prevertebral(PV) mass with recurrence of bone pain, PTH 110pg/ml, and Ca-11.2mg/dL. Parathyroid scan showed Left PV mass and Left level 2 lymph node. PET/CT later revealed fluorodeoxyglucose avidity of the recurrent mass, as well as bilateral lung, right ventricle, and pelvic lesions. Incisional biopsy confirmed carcinoma consistent with PC. Her neck lesions were deemed unresectable. Germline mutation testing confirmed Next Generation Sequencing: PDL1 5% by CPS, TMB 10 mut/Mb, IDH2, JAK1, KMT2C, NF1, PIK3RI. On the basis of TMB >10 mut/Mb Pembrolizumab was then initiated with good response, noting improvement in symptoms, and normalization of Ca 8.7mg/dL and PTH 35pg/ml. Within 4 months of therapy Surveillance CT showed an approximate 40% reduction in tumor burden noting decrease in size of neck mass and lesions in lungs, right ventricle, and pelvis. DISCUSSION: Approximately 10%-30% of PCs have metastasis at presentation, most commonly to the lung, bone, or liver. However they can often develop local recurrence and or distant metastases despite early intervention. Surgical resection remains the mainstay of treatment, and is often used to decrease tumor burden which has been shown to improve morbidity. However when surgical resection is not an option, newer approaches using gene sequence targeted therapy may be an attractive option in the future as was demonstrated in this patient. Presentation: Sunday, June 12, 2022 12:30 p.m. - 2:30 p.m.