Randomized Phase II/III Trial of First Line Platinum/Etoposide with or without Atezolizumab (NSC#783608) in Patients with Poorly Differentiated Extrapulmonary Small Cell Neuroendocrine Carcinomas (NEC)

For a comprehensive list, see section 5.0 of the protocol.

-Participants must have histologically-confirmed (local site pathological confirmation sufficient) extrapulmonary poorly differentiated, small cell neuroendocrine carcinoma (NEC)
-Participants must have radiologically evaluable disease, measurable or non-measurable, per Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 criteria. All measurable and non-measurable lesions must be assessed by computed tomography (CT) scan within 28 days prior to registration. For patients who received one cycle of platinum + etoposide prior to registration, at least 21 days must have elapsed between day 1 of platinum + etoposide and the pre-registration CT scan. All known sites of disease must be assessed and documented on the Baseline Tumor Assessment Form
-Participants must have brain magnetic resonance imaging (MRI) (or CT head with contrast if there is contraindication to MRI brain) within 28 days prior to registration. -Participants with asymptomatic central nervous system (CNS) metastases are eligible if one or more of the following apply:
-Participants who have received treatment for brain metastases must have:
-No evidence of radiological progression (by MRI brain or CT head with contrast if there is contraindication to MRI brain) within 28 days prior to registration
-Discontinued all corticosteroids at least 14 days prior to registration
-Participants with treatment-naïve brain lesions must have:
-No lesion measuring > 2.0 cm in size in any axis
-MRI brain or CT head with contrast (if there is contraindication to MRI brain) demonstrating no evidence for mass effect, edema, or other impending neurological compromise within 28 days prior to registration
-No evidence of radiological progression (by MRI brain or CT head with contrast if there is contraindication to MRI brain) within 28 days prior to registration
-Participants with prior history of non-metastatic (localized/locally advanced disease) extrapulmonary poorly differentiated small cell NEC may have had prior platinum-based therapy +/- radiation +/- surgery provided that all therapy was completed >= 6 months prior to registration
-Participants must discontinue denosumab prior to study registration and plan to replace with a bisphosphonate while on the study