Patients enrolled during the maintenance phase of chemoimmunotherapy treatment (standard of care) according to the IMPOWER 133 scheme which is divided as follows: 1) initial chemo-immunotherapy phase with IV treatment cycles with carboplatin AUC 5 day 1 and etoposide 100 mg/m2 days 1-3 q21 followed by a maintenance phase with immunotherapy (Atezolizumab day 1 1200 mg i.v. g1 of each cycle) until progression-toxicity.

In association with immunotherapy, all patients will undergo treatment radiotherapy with 45 Gy in 15 fractions of 3 Gy administered 5 days a week on residual disease documented on CT-PET, dose 32.5Gy (2.16Gy for 15 fractions) on lymph node sites in response complete, equivalent to (EQD2 alpha beta 10) at 30 Gy in 3 fractions.

Thoracic radiotherapy using 4D technique by irradiating the initial sites of disease (primary tumor and involved hilo-mediastinal lymph nodes). The treatments must be provided with the modulated intensity technique (IMRT or VMAT), treatments with conformal technique (3DCRT) are not permitted, where necessary, the Simultaneous Integrated Boost (SIB) technique can be used.

The local recurrence rate of the disease at one year will be evaluated, with the aim of considering a % greater than 49.9% as unacceptable, assuming that with the experimental treatment object of this study a % of local recurrences of 30% can be achieved. We will therefore test the hypothesis of a local recurrence rate >50% vs the hypothesis of a local control <30%. Assuming a power of 80% and a one-sided significance level of 5%, it will be necessary to enroll 37 patients; if 13 or fewer local recurrences are observed, the study will be considered positive.

The treatment will be considered feasible and non-toxic if severe toxicity values are found ≥ G3 similar or not higher than previous studies that used thoraco-mediastinal radiotherapy for consolidation after chemotherapy in the setting of small cell lung cancer - Extensive disease.

In the present study, considering the use of modern radiotherapy techniques (IMRT or VMAT), the association of immunotherapy with thoracic radiotherapy in the maintenance phase will be considered feasible if an overall toxicity rate ≥ G3 < 20%, a respiratory toxicity grade ≥ G2 <10%, a respiratory toxicity grade ≥ G3 <5% is found. Toxicity will be evaluated according to the CTCAE V5.0 scale.