Durvalumab After Chemoradiotherapy in Limited Stage Small Cell Lung Cancer.

Inclusion Criteria: * Female or male patients aged ≥18 years at the time of signing the Informed Consent Form (ICF). * Written informed consent obtained from the patient/legal representative prior to performing any protocol-related procedures. Additionally, signed and dated written genetic and/or biomarker informed consents, respectively, to collect baseline tissue and blood samples for future translational biomarker assessment at baseline, at the initiation of durvalumab maintenance treatment (cycles 1, 2, 3, 4, 7, 10, 13, 19, 26), and at end of treatment/progression. * Patients must have histologically or cytologically documented limited stage SCLC (stage I-III SCLC \[T any, N any, M0\] according to the AJCC Cancer Staging Manual, \[8th Edition\] or the IASLC Staging Manual in Thoracic Oncology \[2016\]), i.e., patients whose disease can be encompassed within a radical radiation portal. Patients who are stage I or II must be medically inoperable as determined by investigator. * WHO/ECOG PS of 0, 1 or 2 at enrolment, after CRT. A maximum of 20% of patients with ECOG 2 is allowed. * Received an appropriate first-line concurrent or sequential chemoradiotherapy regimen as defined below, unless after consultation with the study medical team an alternative is acceptable: * Received 3-4 cycles of platinum-based chemotherapy concurrent or sequential with radiotherapy, which must be completed within 1 to 90 days prior to the first dose of durvalumab. * The chemotherapy regimen must contain platinum and IV etoposide, administered as per local SoC regimens. * Received a total dose of radiation of 60 to 66 Gy (±10%) over approximately 6 weeks for standard QD radiation schedules or 45 Gy (±10%) over approximately 3 weeks for hyperfractionated BID radiation schedules. * Patients must have achieved CR, PR, or SD and not have progressed following definitive, platinum-based chemoradiotherapy. NOTE: PCI may be delivered at the discretion of investigator and local standard of care and must be conducted after the end of CRT and completed between 1 to 90 days to first dose of IP. * Adequate organ and marrow function (independent of transfusion, infusion, or growth factor support for at least 14 days prior to obtaining screening labs), defined as below: * Haemoglobin ≥9.0 g/dL * Absolute neutrophil count ≥1.5 x 109/L * Platelet count ≥100 x 109/L * Serum bilirubin ≤1.5 x the ULN. This will not apply to patients with confirmed Gilbert's syndrome (persistent or recurrent hyperbilirubinemia \[predominantly unconjugated bilirubin\] in the absence of evidence of hemolysis or hepatic pathology), who will be allowed in consultation with their physician. * ALT and AST ≤2.5 x ULN * Measured creatinine clearance (CL) \>40 mL/min or calculated CL \>40 mL/min as determined by Cockcroft-Gault (using actual body weight) Males Creatinine CL (mL/min) = (Weight (kg)×(140-Age))/(72×serum creatinine (mg/dL)) Females Creatinine CL (mL/min) = (Weight (kg)×(140-Age))/(72×serum creatinine (mg/dL) )× 0.85 * Must have a life expectancy of at least 12 weeks. * Body weight \>30 kg. Exclusion Criteria: * Mixed SCLC and NSCLC histology. * Extensive-stage SCLC. * Any history of grade ≥2 pneumonitis. * History of allogeneic organ transplantation. * Active or prior documented autoimmune or inflammatory disorders (including inflammatory bowel disease \[e.g., colitis or Crohn's disease\], diverticulitis \[except for diverticulosis\], 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 study physician. * Patients with celiac disease controlled by diet alone. * Uncontrolled intercurrent illness, including but not limited to, ongoing or active infection, symptomatic congestive heart failure, uncontrolled hypertension, unstable angina pectoris, uncontrolled cardiac arrhythmia, active ILD, serious chronic GI conditions associated with diarrhea, or psychiatric illness/social situations that would limit compliance with study requirements, substantially increase risk of incurring AEs or compromise the ability of the patient to give written informed consent. * History of another primary malignancy except for: * Malignancy treated with curative intent and with no known active disease ≥5 years before the first dose of durvalumab and of low potential risk for recurrence. * Adequately treated non-melanoma skin cancer or lentigo maligna without evidence of disease. * Adequately treated carcinoma in situ without evidence of disease. * History of leptomeningeal carcinomatosis. * History of active primary immunodeficiency. * Active infection including tuberculosis (clinical evaluation that includes clinical history, physical examination and radiographic findings, and tuberculosis testing in line with local practice), hepatitis B (known positive HBsAg result), hepatitis C (HCV), or human immunodeficiency virus (positive HIV 1/2 antibodies). * NOTE: Patients with a past or resolved HBV infection (defined as the presence of hepatitis B core antibody and absence of HBsAg) are eligible. Patients positive for HCV antibody are eligible only if polymerase chain reaction is negative for HCV RNA. * Any unresolved toxicity NCI Common Terminology Criteria for Adverse Events (CTCAE) Grade ≥2 from previous CRT except for alopecia, vitiligo, and the laboratory values defined in the inclusion criteria: * Patients with grade ≥2 neuropathy will be evaluated on a case-by-case basis after consultation with the study physician. * Patients with irreversible toxicity not reasonably expected to be exacerbated by treatment with durvalumab may be included only after consultation with the study physician. * Brain metastases or spinal cord compression. All patients will have an MRI (preferred) or CT, preferably with IV contrast of the brain, prior to study entry, after CRT. * Mean QT interval corrected for heart rate using Fridericia's formula (QTcF) ≥470 ms calculated from 3 ECGs (within 15 minutes at 5 minutes apart). * Known allergy or hypersensitivity to any of the study drugs or any of the study drug excipients. * Patients whose conditions have progressed while on CRT. * Major surgical procedure (as defined by the Investigator) within 42 days prior to the first dose of IP. * Current or prior use of immunosuppressive medication within 14 days before the first dose of durvalumab. The following are exceptions to this criterion: * Intranasal, inhaled, topical steroids, or local steroid injections (eg, intra articular injection). * Systemic corticosteroids at physiologic doses not to exceed 10 mg/day of prednisone or its equivalent. * Steroids as premedication for hypersensitivity reactions (eg, CT scan premedication). * Participation in another clinical study with an investigational product administered in the last 4 weeks. * Concurrent enrolment in another clinical study unless it is an observational (non-interventional) clinical study or during the follow-up period of an interventional study. * Involvement in the planning and/or conduct of the study (applies to both AstraZeneca staff and/or staff at the study site). * Female patients who are pregnant or breastfeeding and male or female patients of reproductive potential who are not willing to employ effective birth control from screening to 3 months after the last dose of durvalumab. * Judgment by the investigator that the patient should not participate in the study because the patient is unlikely to comply with study procedures, restrictions, and requirements.