This study is an open-label Phase II trial of non-Hodgkin lymphoma patients receiving initial treatment with the immunomodulatory agent, pembrolizumab, plus low-dose (4 Gy x 5) involved-site radiotherapy. Eligible patients will have r/r disease with at least 2 sites of measurable disease (≥1.0 cm), and must be eligible for treatment with pembrolizumab. Biosamples (blood and, where available, tumor) will be collected as outlined below. Pembrolizumab will be continued after RT until disease progression, drug intolerance, or at the discretion of the treating medical oncologist.