Synchronous low-grade myofibroblastic sarcoma and small cell lung cancer in a patient with situs inversus totalis: a case report and literature review

Background Situs inversus totalis (SIT) is a rare congenital anomaly. Its co-occurrence with multiple primary malignancies is exceedingly uncommon. Low-grade myofibroblastic sarcoma (LGMS) and small cell lung cancer (SCLC) represent two distinct neoplasms with markedly different biological behaviors and treatment sensitivities. To our knowledge, the synchronous presentation of LGMS and SCLC in a patient with SIT has not been previously described. Case presentation A 72-year-old man with known SIT presented with respiratory symptoms. Imaging revealed a left hilar mass causing obstructive atelectasis, along with metastatic lymphadenopathy in cervical and inguinal regions. Pathological assessment confirmed synchronous extensive-stage SCLC (ES-SCLC) and cervical LGMS. Given the high chemosensitivity of SCLC, a sequential treatment strategy was implemented. The patient first received two cycles of induction chemoimmunotherapy with etoposide/cisplatin plus tislelizumab, which resulted in resolution of metastatic lymph nodes. This was followed by surgical resection of the residual cervical LGMS and four cycles of consolidation chemoimmunotherapy. Subsequent management included sequential thoracic radiotherapy and maintenance immunotherapy with tislelizumab. Conclusion This report presents the first documented case of synchronous LGMS and SCLC in a patient with SIT. Our experience suggests that a biology-driven sequential approach, prioritizing systemic therapy for chemosensitive SCLC followed by local control of LGMS, can achieve a initial favorable response. It also underscores that radiotherapy remains feasible in SIT following detailed anatomical evaluation. This case offers a clinically applicable, biology-guided sequential strategy for managing synchronous malignancies with divergent treatment sensitivities.