Phase I/II Trial in ES-SCLC to Enhance Response to Atezolizumab Plus Chemotherapy With Total Body Irradiation

Inclusion Criteria: * Age ≥ 18 years at time of informed consent * Histologically documented or cytologically confirmed diagnosis of extensive stage small-celllung cancer with evaluable disease per RECIST v1.1 criteria. Patients may be considered extensive-stage based on M1 disease per AJCC 8th edition, OR may be clinically staged as extensive-stage disease based on anatomical extent that would preclude the use of standard radiotherapy fields as assessed by the treating radiation oncologist. * Eastern Cooperative Oncology Group (ECOG) performance status of 0-2 * Absolute neutrophil count (ANC) ≥ 1.5 x 10\^9/L * Platelet count ≥ 100 x 10\^9/L * Lymphocyte count ≥ 0.5 x 10\^9/L * Hemoglobin ≥ 9 g/dL * Total bilirubin ≤ 1.5 x upper limit of normal (ULN) * Aspartate aminotransferase (AST) ≤ 2.5 x ULN, alanine aminotransferase (ALT) ≤ 2.5 x ULN and alkaline phosphatase ≤ 2.5 x ULN * Creatinine ≤ 1.5 x ULN or calculated creatinine clearance (CrCl) ≥ 50 mL/min if creatinine (Cr) \> 1.5 x ULN. GFR can also be utilized. If no local calculation guidance on CrCl, should be calculated according to Cockcroft-Gault Method * International normalized ratio (INR) or prothrombin time (PT) ≤ 1.5 x ULN and activated partial thromboplastin time (aPTT) ≤ 1.5 x ULN unless participant is receiving anticoagulation therapy. For patients receiving therapeutic anticoagulation: stable anticoagulant regimen. * Negative HIV test at screening, with the following exception: patients with positive HIV test at screening are eligible provided they are stable on anti-retroviral therapy, have a CD4 count ≥ 200, and have an undetectable viral load * Negative Hepatitis B surface antigen at screening * Presence of brain metastases allowed (should undergo management with surgery and/or radiation therapy if symptomatic prior to TESSERACT radiation regimen; upfront cranial irradiation not mandatory on protocol if asymptomatic) * Contraceptive use should be initiated or continued per guidance in labeling for approved chemotherapies * Female patients must be non-pregnant and not breastfeeding * Women of childbearing potential (WOCBP) must remain abstinent or use contraceptive methods with a failure rate of \<1% per year during the treatment period and for 5 months after the final dose of atezolizumab. A woman is considered to be of childbearing potential if she is post-menarchal, has not reached a post-menopausal state (12 months of amenorrhea with no identified cause other than menopause), and has not undergone surgical sterilization (removal of ovaries and/or uterus). The definition of childbearing potential may be adapted for alignment with local guidelines or requirements. * Examples of contraceptive methods with a failure rate of \<1% per year include, bilateral tubal ligation, male sterilization, hormonal contraceptives that inhibit ovulation, hormone-releasing intrauterine devices, and copper intrauterine devices. * The reliability of sexual abstinence should be evaluated in relation to the duration of the clinical trial and the preferred and usual lifestyle of the patient. Periodic abstinence (e.g. calendar, ovulation, symptomthermal or postovulation methods) and withdrawal are not adequate methods of contraception. * With a female partner of childbearing potential who is not pregnant, men who are not surgically sterile must remain abstinent or use a condom plus an additional contraceptive method that together result in a failure rate of \< 1% per year during the treatment period and for 5 months after the final dose of atezolizumab. Men must refrain from donating sperm during this same period. * With a pregnant female partner, men must remain abstinent or use a condom during the treatment period and for 5 months after the final dose of atezolizumab to avoid exposing the embryo. * The reliability of sexual abstinence should be evaluated in relation to the duration of the clinical trial and the preferred and usual lifestyle of the patient. Periodic abstinence (e.g., calendar, ovulation, symptothermal, or postovulation methods) and withdrawal. * Eligible for immunotherapy-based systemic regimens per judgment of patient's study physician * Able to submit written informed consent Exclusion Criteria: * Major surgery (requiring general anesthesia or at discretion of study physician) within 4 weeks prior to study enrollment that would prevent treatment with TESSERACT regimen * Known clinically significant (per study physician) acute or chronic infections including HIV (per inclusion criteria above), hepatitis B virus (HBV), hepatitis C virus (HCV) or active tuberculosis (testing not required for tuberculosis \[TB\]). Patients with HCV must be on stable dose of antiviral therapy on study entry. Current treatment with antivirals for HBV is not allowed on study * Significant cardiovascular disease (such as New York Heart Association Class II or greater cardiac disease, myocardial infarction, or cerebrovascular accident) within 3 months prior to initiation of study treatment, unstable arrhythmia, or unstable angina * Uncontrolled hypertension (average systolic blood pressure greater than or equal to 140 or average diastolic blood pressure greater than or equal to 90 despite optimal medical therapy). Patients with hypertension (systolic blood pressure \[SBP\] ≥ 140 and/or diastolic blood pressure \[DBP\] ≥ 90) may enroll provided that an effective anti- hypertensive regimen is initiated. * History of prior malignancy within 3 years of enrollment, except for adequately treated basal or squamous cell carcinoma of the skin, adequately treated carcinoma in situ (e.g. cervix or non- invasive bladder cancer) or other malignancy with minimal risk of metastasis or death (survival \> 90% at 5 years) * Receipt of prior courses of cytotoxic chemotherapy or anti-neoplastic biologic/immunotherapy for current malignancy (other than the current course of therapy). Patients may have undergone 1 cycle of 1st line of systemic therapy for SC