Locally advanced undifferentiated small round cell sarcoma of the lung with novel SDCCAG8-AKT3 fusion and type II tumor immunity in the microenvironment: a rare case report

Background: Despite significant recent advances in characterizing the molecular pathogenesis of undifferentiated small round cell sarcoma (USRCS), rare cases without reported gene alterations remain unclassified. To date, the efficacy and prognostic biomarker of immunotherapy in the treatment of unresectable USRCS has not been demonstrated, especially when these cases occurring in uncommon thoracic visceral organs with a novel gene fusion. Case Description: We report a case of locally advanced and unresectable USRCS of the lung (cT4N1M0) with SDCCAG8-AKT3 fusion identified by RNA-based next-generation sequencing ( NGS). He initially admitted to our hospital chiefly complained of cough and dyspnea without any intervention. Imaging examinations, positron emission tomography/computed tomography (PET/CT), tumor biopsy, and a series of molecular tests based on tumor specimens were conducted for diagnosis. The molecular tests supplied more information delineating the case's molecular characteristics including PMS2 mutation, CD274 amplification, high tumor mutational burden ( TMB-H), and high microsatellite instability (MSI-H). Multiple immunofluorescence (mIF) staining further revealed a specific immune-microenvironment phenotype with a 100% programmed death ligand 1 (PD-L1) expression and type II tumor immunity in the microenvironment (type II TIME) of this case. This 31-year-old non-smoking male received vincristine sulfate, dactinomycin, and cyclophosphamide (VAC) regimen chemotherapy combined with pembrolizumab and sequential radiotherapy. He had maintained a partial response (PR) according to response evaluation criteria in solid tumors (RECIST) 1.1 and a good quality of life for almost 14 months except for mild loss of appetite and hair loss after chemotherapy to the latest follow-up date. Conclusions: Our study showed a rare case of lung USRCS harboring a novel SDCCAG8-AKT3 fusion. And we indicated that a comprehensive treatment including the combination of systemic VAC chemotherapy and anti-programmed cell death protein 1 (PD-1) immunotherapy, and sequential radiotherapy could be considered for similar cases, prophylactic managements of chemotherapy-related myelosuppression and urotoxicity should be administrated along with chemotherapy as well. Tumor immune microenvironment analysis and gene sequencing are recommended to obtain more prognostic biomarkers in addition to routine pathologic examinations in diagnosis and treatment of USRCS.