Inclusion Criteria:

          -  Signed informed consent form

          -  Biopsy-confirmed cancer diagnosis indicated to received anti-PD-1 therapy

          -  Has not received prior anti-PD-1 therapy (i.e., anti-PD-1 naïve) or is currently
             receiving anti-PD-1 therapy and can be considered an anti-PD-1 non-responder (per SITC
             guidelines)

          -  Has at least one tumor lesion that can be accurately measured according to RECIST 1.1.
             and is amenable for intratumoral injection

          -  ECOG performance status 0-2

          -  Life expectancy >12 weeks

          -  Adequate organ and bone marrow function

          -  Negative pregnancy test ≤ 7 days prior to NBTXR3 injection in all female participants
             of child-bearing potential

        Exclusion Criteria:

          -  History of severe immune-related adverse events related to administration of anti-PD-1

          -  Symptomatic central nervous system metastases and/or carcinomatous meningitis

          -  Active autoimmune disease that has required systemic treatment in the past 2 years

          -  Known HIV or active hepatitis B/C infection

          -  Active infection requiring systemic treatment

          -  Received a live virus vaccine within 30 days prior to study treatment

          -  History of pneumonitis that required steroids or with current pneumonitis

          -  Extensive metastatic disease burden considered to be unamenable for radiation
             treatment

          -  Locoregional recurrent HNSCC with ulceration

          -  Has received prior therapy with a checkpoint inhibitor, within 4 weeks prior to NBTXR3
             injection

          -  Has received prior systemic anti-neoplastic therapy, including investigational agents,
             within 4 weeks prior to NBTXR3 injection

          -  Has not recovered from AEs due to previous anti-neoplastic therapies and/or
             interventions (including radiation) to ≤ Grade 1 or baseline at screening

          -  Clinically significant cardiac arrhythmias

          -  Class III or IV Congestive Heart Failure as defined by the New York Heart Association
             functional classification system < 6 months prior to screening

          -  A pregnant or nursing female, or women of child-bearing potential and men who are
             sexually active and not willing/able to use medically acceptable forms of
             contraception

          -  Any condition for which participation would not be in the best interest of the
             participant