Rebound hypercalcemia post-denosumab cessation in metastatic breast cancer.

Denosumab delays and prevents skeletal related events in patients with malignancy-related bony metastases. Rarely, denosumab discontinuation can lead to rebound hypercalcemia in the setting of increased bone resorption. We describe the case of a 49-year-old breast cancer survivor who developed rebound hypercalcemia after cessation of long-term denosumab. She had been treated with 42 doses of denosumab between August 2013 and March 2020 and 8 months after her last dose of denosumab developed symptomatic hypercalcemia. Parathyroid hormone levels were suppressed, and active malignancy was excluded based on biochemical and radiological testing. She required treatment with intravenous bisphosphonates on three separate occasions in order to achieve long-term normalization of her hypercalcemia. Rebound hypercalcemia post-denosumab cessation is a rare but serious complication that clinicians should be aware of.