64Cu-DOTA Pembrolizumab for Imaging Metastatic Solid Tumors in Patients Receiving Stereotactic Body Radiation

Inclusion Criteria:

* Documented informed consent of the participant and/or legally authorized representative.

  * Assent, when appropriate, will be obtained per institutional guidelines
* Age: \>= 18 years
* Karnofsky performance status (KPS) \>=70
* Patients with metastatic disease from a solid-tumor malignancy currently being treated with single-agent pembroilizumab, who have been referred for stereotactic body radiation therapy (SBRT) for consolidative local therapy or for oligo resistant/progressive disease
* Sites that amenable to SBRT are located in lymph nodes, bone/spine, or lung
* Brain metastases or cases with intra-cranial progression are allowed, but an additional extra-cranial site planned for SBRT is required
* 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 \>= 30mL/min
* Contraception:

  * 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 \[pembrolizumab\]-NHS-DOTA for women);
  * A man who is sexually active with a woman of childbearing potential and has not had a vasectomy must agree to use a barrier method of birth control, e.g., either condom with spermicidal foam/gel/film/cream/suppository or partner with occlusive cap (diaphragm or cervical/vault caps) with spermicidal foam/gel/film/cream/suppository, and all men must also not donate sperm during the study and for 6 months after receiving the last dose of study drug

Exclusion Criteria:

* Patient planned to stop Pembrolizumab at time of referral for SBRT
* Patient unable to tolerate positron emission tomography (PET) scan even with anxiolytic medications
* Any other condition that would, in the Investigator's judgment, contraindicate the patient's participation in the clinical study due to safety concerns with clinical study procedures
* Vaccination with live attenuated vaccines within 4 weeks of study agent administration except forthcoming COVID-19 vaccines
* Subject is currently using or has used immunosuppressive medication within 14 days prior to the study agent administration with the exception of:

  * Intranasal, topical, inhaled, or local steroid injections (e.g., intra-articular injection);
  * Chronic systemic corticosteroids at physiologic doses not to exceed 10 mg/day of prednisone or equivalent;
  * Steroids as premedication for hypersensitivity reactions (e.g., infusion-related reactions, CT scan premedication)
* Female patients who are lactating or have a positive pregnancy test during the screening period
* Infection requiring systemic antibiotic therapy within 14 days prior to start of study treatment
* Subject has plasma cell leukemia, Waldenstrom's macroglobulinemia, POEMS syndrome (polyneuropathy, organomegaly, endocrinopathy, monoclonal protein, and skin changes), or primary amyloidosis
* Prospective participants who, in the opinion of the investigator, may not be able to comply with all study procedures (including compliance issues related to feasibility/logistics)