Abstract
Despite the wide use of immune checkpoint inhibition for the treatment of melanoma, the mechanisms leading to long-term stable disease are incompletely understood. Patients with metastatic melanoma who had received ipilimumab alone or ipilimumab plus nivolumab 2+years prior and attained at least 6 months of stable disease were identified. Positron emission tomography/computed tomography (PET/CT) was performed. Pretreatment and posttreatment biopsies of areas of stable disease were assessed for tumor, fibrosis, and inflammation. Seven patients underwent PET/CT and tissue biopsy. Fluorodeoxyglucose avid lesions on PET/CT ranged from no activity to an SUV of 22. In 6 patients, the residual stable lesions were composed of necrosis and fibrosis with a prominent pigment containing macrophages and no residual melanoma. In 1 patient, a nodal lesion demonstrated melanoma with active inflammation. In most patients with durable stable disease after treatment with ipilimumab or ipilimumab/nivolumab, residual lesions demonstrated predominantly necrosis and fibrosis consistent with resolving lesions. The presence of melanophages in these samples may suggest ongoing immune surveillance. One patient did demonstrate residual melanoma, indicating the need for ongoing monitoring of this patient population.