- Patients must have been treated with at least one prior systemic treatment for incurable advanced or metastatic SCCA of the anal canal; prior treatment for metastatic disease is not required for patients who develop new metastatic lesions during or within 6 months of completion of chemoradiation for limited-stage disease; patients who receive chemotherapy for incurable advanced or metastatic SCCA of the anal canal must wait a minimum >= 28 days (6 weeks for nitrosoureas or mitomycin C) after the date of completion of chemotherapy prior to initiating treatment with nivolumab on this study; patients who undergo radiotherapy to a site of tumor must wait a minimum >= 3 months from the date of completion of radiotherapy prior to initiating treatment with nivolumab on this study