64Cu-DOTA-pembrolizumab PET for the Study of PD1 Expression

Inclusion Criteria: * All subjects must have the ability to understand and the willingness to sign a written informed consent * Ability to undergo standard PET imaging * Plan to be treated with immunotherapy * Karnofsky performance status (KPS) \>= 50% * Absolute neutrophil count (ANC) \>= 1000/mm\^3 * Platelet count \>= 50/mm\^3; platelet transfusions to help patients meet eligibility criteria are not allowed within 7 days before study enrollment * Total bilirubin =\< 1.5 x the upper limit of normal (ULN) * Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) =\< 3 x ULN * Calculated creatinine clearance \>= 30 mL/min * Woman of childbearing potential must be practicing a highly effective method of birth control consistent with local regulations regarding the use of birth control methods for subjects participating in clinical studies: e.g., established use of oral, injected or implanted hormonal methods of contraception; placement of an intrauterine device or intrauterine system; barrier methods; condom with spermicidal foam/gel/film/cream/suppository or occlusive cap (diaphragm or cervical/vault caps) with spermicidal foam/gel/film/cream/suppository; male partner sterilization; true abstinence (when this is in line with the preferred and usual lifestyle of the subject) during and after the study (6 months after the last dose of 64Cu-anti-PD1 \[pembrolizumab\]-N-hydroxysuccinimide \[NHS\]-DOTA for women) * A man who is sexually active with a woman of childbea…