Inclusion Criteria:

          -  Histologically or cytologically confirmed stage IV NSCLC (includes M1a, M1b, and M1c
             stage disease, AJCC 8th edition). Patients with T4NX disease (Stage IIIB) with nodule
             in ipsilateral lung lobe are eligible if they are not candidates for combined
             chemotherapy and radiation. Patients with Large Cell Neuroendocrine Carcinoma (LCNEC)
             are not eligible.

          -  Confirmed EGFR and ALK mutation-negative disease

          -  Patients must have a PD-L1 test result from a certified laboratory indicating PD-L1
             expression Tumour Proportion Score (TPS) ≥ 1%.

          -  No prior systemic chemotherapy or immunotherapy for advanced metastatic NSCLC.
             Chemotherapy for non-metastatic disease (e.g. adjuvant therapy) or immunotherapy for
             locally advanced Stage III disease is allowed if at least 6 months have elapsed since
             the prior therapy and enrollment. Local therapy, e.g. palliative radiation, is allowed
             as long as a period of 2 weeks has passed since completion.

          -  Patients must have recovered to ≤ grade 1 from all reversible toxicity related to
             prior systemic or radiation therapy.

          -  Previous major surgery is permitted provided that surgery occurred at least 28 days
             prior to patient enrollment and that wound healing has occurred.

          -  Eligible to receive treatment with pembrolizumab. Reimbursement of pembrolizumab may
             not be uniform across all sites. In the event that the site/investigator is unable to
             provide access to the drug, the patient will not be eligible for this trial.

          -  Must be ≥ 18 years of age.

          -  ECOG performance status 0 or 1.

          -  Clinically and/or radiologically documented disease with at least one lesion
             measurable as defined by RECIST 1.1.

          -  Imaging investigations including CT of the chest, abdomen and pelvis and MRI of the
             brain (if known brain metastases) or other scans as necessary to document all sites of
             disease must be done within 28 days prior to enrollment.

          -  Adequate hematology and organ function as defined below (must be done within 14 days
             prior to enrollment).

               -  White Blood Cells ≥ 2.0 x 10^9/L (2000/μL)

               -  Absolute neutrophils ≥ 1.5 x 10^9/L (1500/μL)

               -  Platelets ≥ 100 x 10^9/L (100 x 10^3/μL)

               -  Bilirubin ≤ 1.5 x ULN (upper limit of normal)*

               -  AST and/or ALT ≤ 3 x ULN, < 5 x ULN for patients with liver metastases

               -  Serum creatinine or Creatinine clearance ≤ 1.5 x ULN OR ≥ 40 mL/min

          -  Patients must consent to provision of, and investigator must confirm access to and
             agree to submit, a representative archival formalin-fixed paraffin block of tumour
             tissue for correlative analyses when tumour tissue is available.

        Patients must consent to collection of liquid biopsy (blood) samples for ctDNA analysis by
        CLIA central laboratory and for correlative analysis by a research central laboratory.

          -  Patient consent must be appropriately obtained in accordance with applicable local and
             regulatory requirements. Each patient must sign a consent form prior to enrollment to
             the trial to document their willingness to participate.

          -  Patients must be accessible for treatment and follow-up. Investigators must assure
             themselves the patients enrolled on this trial will be available for complete
             documentation of the treatment, adverse events, collection of blood samples, response
             assessments and follow-up. Patients must agree to return to their primary care
             facility for response assessments as well as any adverse events which may occur
             through the course of the trial.

          -  In accordance with CCTG policy, protocol treatment with pembrolizumab is to begin
             within 2 working days of patient enrollment.

          -  Women/men of childbearing potential must have agreed to use a highly effective
             contraceptive method. Women of childbearing potential will have a pregnancy test to
             determine eligibility as part of the Pre-Study Evaluation.

        Exclusion Criteria:

          -  Patients with a prior malignancy whose natural history or treatment does not have the
             potential to interfere with the safety or efficacy assessment of the investigational
             regimen are eligible for this trial.

          -  Patients with brain metastases or leptomeningeal metastases are eligible if these have
             been treated and follow up brain imaging shows no evidence of progression or if the
             treating physician determines that immediate CNS specific treatment is not required
             and is unlikely to be required during the first cycle of therapy. There must also be
             no requirement for immunosuppressive doses of systemic corticosteroids (> 10 mg/day
             prednisone equivalents).

          -  Patients who are not suitable candidates for treatment with pembrolizumab according to
             the current guidance/indications described in the Product Monograph (Canada) or Drug
             Label (U.S.) including but not limited to patients with active infection, autoimmune
             disease, conditions that require systemic immunosuppressive therapy (such as
             transplant patients) and patients with a history of severe immune-mediated adverse
             reactions, or known hypersensitivity to pembrolizumab or its components. Patients with
             pre-existing conditions such as colitis, hepatic impairment, respiratory or endocrine
             disorders (such as hypo or hyperthyroidism or diabetes mellitus), can be considered
             for enrollment to this study provided pembrolizumab is administered with caution and
             patients are closely monitored

          -  History of significant neurologic or psychiatric disorder which would impair the
             ability to obtain consent or limit compliance with study requirements.

          -  Concurrent treatment with other anti-cancer therapy or other investigational
             anti-cancer agents

          -  Pregnant or lactating women.

          -  Men who are sexually active with women of childbearing potential and women of
             childbearing potential must agree to use adequate contraception.