SUN-825 - Beyond the Joint: Knee Pain Revealing Parathyroid Carcinoma

Parathyroid neoplasms are a common cause of primary hyperparathyroidism. Amongst the neoplasms, parathyroid carcinoma accounts for less than 1% of the total cases. The 5-year survival rate is 78-85%.
Patients usually present due to symptoms of hypercalcemia (bone and kidney disease) and a palpable neck mass with local invasion into adjacent structures. The parathyroid hormone levels are markedly elevated (above 2000 pg/mL). The diagnosis is usually made at the time of surgery, which is the mainstay of therapy. Hypercalcemia is controlled with hydration, bisphosphonates, cinacalcet, and denosumab. Biologic agents, immune therapy, and molecularly targeted therapies are emerging promising treatment strategies.
We present a 73-year-old healthy male who presented to the outpatient clinic for bilateral knee pain for 2 weeks. On further inquiry, he reported intentional weight loss of 10 pounds in the past few weeks. He denied any other symptoms such as abdominal pain, constipation, confusion, or memory changes. The physical examination was unremarkable. Knee x-rays were negative for any arthritic changes. Outpatient laboratory testing revealed a serum calcium level of 15.8 mg/dL (normal 8.5-10.5 mg/dL) and a creatinine of 2.95 mg/dL (0.7-1.3 mg/dL). He was eventually admitted for the management of hypercalcemia.
Work-up of hypercalcemia revealed a parathyroid hormone level of 2425pg/mL (normal 10-65pg/mL). Serum and urine electrophoresis and vitamin D levels were normal. Hypercalcemia was treated with intravenous fluids, calcitonin, and zoledronic acid. Endocrinology was consulted and recommended a thyroid ultrasound, which showed bilateral parathyroid adenomas. Pre-operative imaging showed left-sided 6th and 7th rib fractures and lytic changes. The patient underwent curative parathyroidectomy, and histopathology confirmed a 3.7 cm parathyroid carcinoma with perineural invasion.
This case reiterates that knee pain in the elderly, often attributed to degenerative joint disease, may, in rare cases, represent an underlying malignancy and should not be presumed to be arthritis without proper evaluation. Early and complete surgical removal of parathyroid carcinoma and surrounding tissues can result in a cure, as seen in this case. Recurrence is common, and therefore long-term monitoring is essential.

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