Inclusion Criteria:

          -  Age ≥18 years

          -  Diagnosis of T3-T4 N0-N3 M0 or T0-T4 N1-N3 M0 squamous cell carcinoma of the head and
             neck squamous cell carcinoma of the larynx, hypopharynx, oropharynx, oral cavity, or
             carcinoma of unknown head/neck primary) based on American Joint Committee on Cancer
             guideline (AJCC; 8th edition) with measurable disease who meet at least 1 one of the
             following 3 criteria:

             1. Not a candidates for concurrent, bolus, cisplatin-based chemoradiation therapy
             based on one of the following criteria (a-e)

               1. Age ≥ 70 with moderate to severe comorbidity or vulnerability to cisplatin,
                  defined as having one or more of the following conditions within 4 week of
                  registration:

                    1. Modified Charlson Comorbidity Index ≥ 1

                    2. Adult Comorbidity Evaluation-27 (ACE-27) Index ≥ 1

                    3. ω score < 0.80

                    4. Geriatric 8 (G-8) score ≤ 14

                    5. Cancer and Aging Research Group (CARG) Toxicity Score ≥ 30%

                    6. Cumulative Illness Rating Scale-Geriatric (CIRS-G) Score ≥ 4

               2. Age < 70 with severe comorbidity or vulnerability to cisplatin, defined as having
                  two or more of the following conditions within 4 weeks prior to registration:

                    1. Modified Charlson Comorbidity Index ≥ 1

                    2. ACE-27 Index ≥ 1

                    3. ω score < 0.80

                    4. G-8 score ≤ 14

                    5. CARG Toxicity Score ≥ 30%

                    6. CIRS-G Score ≥ 4

               3. Creatinine clearance < 60 cc/min by the Cockroft-Gault formula

               4. Pre-existing peripheral neuropathy

               5. Clinical need for a hearing aid or 25+ decibel shift over 2 contiguous
                  frequencies on a pre-treatment hearing test

                  2. Patient refuses concurrent cisplatin-based chemoradiation therapy

                  3. Patient has recurrent disease after definitive surgical resection

          -  Any patient 18 years or older with primary metastatic (AJCC 8th edition T1-T4 N0-N3
             M1) squamous cell carcinoma of the head and neck

          -  Zubrod performance status 0-3

          -  Measurable primary and/or nodal tumor in the head and neck region at the time of
             radiotherapy

          -  Patients must have the psychological ability and general health that permits
             completion of the study requirements and required follow up.

          -  Ability to tolerate multiple MRIs

          -  Adequate hematologic function within 14 days prior to registration defined as follows:
             Absolute neutrophil count (ANC) ≥ 1,000 cells/mm^3, platelets ≥ 100,000 cells/mm^3,
             hemoglobin ≥ 9.0 g/dl (Note: The use of transfusion or other intervention to achieve
             Hgb ≥ 9.0 g/dl is acceptable).

          -  Adequate hepatic function within 14 days prior to registration defined as follows:
             aspartate aminotransferase (AST) or alanine aminotransferase (ALT) ≤ 2.5 times
             institutional upper limit of normal, serum bilirubin ≤ 1.5 x institutional upper limit
             of normal.

          -  For women of childbearing potential, a negative serum or urine pregnancy test within
             14 days prior to registration is required. Note: Women will be considered
             post-menopausal if they have been amenorrheic for 12 months without an alternative
             medical cause. The following age-specific requirements apply: Women < 50 years of age
             would be considered post-menopausal if they have been amenorrheic for 12 months or
             more following cessation of exogenous hormonal treatments and if they have luteinizing
             hormone and follicle-stimulating hormone levels in the post-menopausal range for the
             institution or underwent surgical sterilization (bilateral oophorectomy or
             hysterectomy). Women ≥ 50 years of age would be considered post-menopausal if they
             have been amenorrheic for 12 months or more following cessation of all exogenous
             hormonal treatments, had radiation-induced menopause with last menses >1 year ago, had
             chemotherapy-induced menopause with last menses >1 year ago, or underwent surgical
             sterilization (bilateral oophorectomy, bilateral salpingectomy or hysterectomy).

          -  Inclusion of Covid-19 positive patients will be based on standard institutional
             protocol

        Exclusion Criteria:

          -  Prior invasive malignancy within the past 3 years (except for non-melanomatous skin
             cancer, and early stage treated prostate cancer);

          -  Life expectancy less than 12 months

          -  Performance status Zubrod >3

          -  Inability to encompass all gross disease in 19 cm superior to inferior planning target
             volume to be treated on the MR LINAC

          -  MRI-incompatible foreign body

          -  Claustrophobia precluding ability to tolerate multiple MRIs

          -  MRI-incompatible pacemaker or implantable cardioverter defibrillator (ICD) placement

          -  Patients with Cochlear implant

          -  Patients with prior radiation therapy to the head and neck Note: Prior external beam
             radiotherapy is excluded, but Iodine 131 is allowed.

          -  Prior systemic therapy, including cytotoxic chemotherapy, biologic/targeted therapy,
             or immune therapy for the study cancer

          -  Major surgery within 28 days prior to registration

          -  Body weight ≤ 30 kg

          -  Any of the following severe laboratory abnormalities within 14 days of registration,
             unless corrected prior to it: Sodium < 130 mmol/L or > 155 mmol/L; Potassium < 3.5
             mmol/L or > 6 mmol/L ;Fasting glucose < 40 mg/dl or > 400 mg/dl;Serum calcium (ionized
             or adjusted for albumin) < 7 mg/dl or > 12.5 mg/dl; Magnesium < 0.9 mg/dl or > 3 mg/dl

          -  Unstable angina and/or congestive heart failure requiring hospitalization within 3
             months prior to Step 1 registration

          -  Transmural myocardial infarction within 3 months prior to Step 1 registration

          -  Respiratory illness requiring hospitalization at the time of Step 1 registration

          -  Note: If the respiratory illness is resolved and the patient meets the eligibility
             status above, then the patient can be considered for the trial.

          -  Idiopathic pulmonary fibrosis or other severe interstitial lung disease that requires
             oxygen therapy or is thought to require oxygen therapy within 1 year prior to Step 1
             registration

          -  History of (non-infectious) pneumonitis that required steroids or current pneumonitis

          -  Clinically apparent jaundice and/or known coagulation defects

          -  Active or prior documented autoimmune or inflammatory disorders (including
             inflammatory bowel disease [e.g., colitis or Crohn's disease], diverticulitis [with
             the exception of diverticulosis], systemic lupus erythematosus, Sarcoidosis syndrome,
             or Wegener syndrome [granulomatosis with polyangiitis, Graves' disease, rheumatoid
             arthritis, hypophysitis, uveitis, etc.]). The following are exceptions to this
             criterion: Patients with vitiligo or alopecia; Patients with hypothyroidism (e.g.,
             following Hashimoto syndrome) stable on hormone replacement; Any chronic skin
             condition that does not require systemic therapy; Patients without active disease in
             the last 5 years may be included but only after consultation with the medical oncology
             study chair; Patients with celiac disease controlled by diet alone.

          -  History of active primary immunodeficiency including, but not limited to Acquired
             Immune Deficiency Syndrome (AIDS) based upon current Centers for Disease Control (CDC)
             definition; Note: HIV testing is not required for entry into this protocol. The need
             to exclude patients with AIDS from this protocol is necessary because the treatment
             involved in this protocol may be immunosuppressive. Patients with known HIV, cluster
             of differentiation 4 (CD4) cell counts ≥ 200/μL, and undetectable viral loads who are
             stable on an antiretroviral regimen may be included.

          -  Current or prior use of immunosuppressive medication within 14 days before
             registration, with the exceptions of intranasal and inhaled corticosteroids or
             systemic corticosteroids at physiological doses, which are not to exceed 10 mg/day of
             prednisone, or an equivalent corticosteroid

          -  Receipt of live attenuated vaccination within 30 days prior to registration

          -  Medical or psychiatric illness which would compromise the patient's ability to
             tolerate treatment or limit compliance with study requirements

          -  Pregnancy or women of childbearing potential and men who are sexually active and not
             willing/able to use medically acceptable forms of contraception during treatment and
             for 6 months after the last dose of atezolizumab, this exclusion is necessary because
             the treatment involved in this study may be significantly teratogenic. Women who are
             breastfeeding are also excluded.

          -  Prior allergic reaction or hypersensitivity to atezolizumab or any of study drug
             excipients.

          -  History of allogenic organ transplantation

          -  Uncontrolled hypertension

          -  Uncontrolled cardiac arrhythmia

          -  Uncontrolled serious chronic gastrointestinal condition associated with diarrhea

          -  Active infection including tuberculosis (clinical evaluation that includes clinical
             history, physical examination and radiographic findings, and tuberculosis (TB) testing
             in line with local practice), hepatitis B (known positive hepatitis B viral (HBV)
             surface antigen (HBsAg) result), hepatitis C. Patients with a past or resolved HBV
             infection (defined as the presence of hepatitis B core antibody (anti-HBc) and absence
             of HBsAg) are eligible. Patients positive for hepatitis C (HCV) antibody are eligible
             only if polymerase chain reaction is negative for HCV RNA.