Recurrent pleuropulmonary blastoma type III initially misclassified as embryonal rhabdomyosarcoma on limited biopsy: a case report with pathogenic DICER1 variant.

Background: Pleuropulmonary blastoma (PPB) is a rare, aggressive pediatric thoracic malignancy and a sentinel tumor for DICER1 syndrome. Advanced PPB can mimic other pediatric sarcomas and complex cystic lung lesions, creating diagnostic and management pitfalls. Case presentation: A boy aged 2 years 11 months presented with progressive respiratory distress and fever due to a rapidly enlarging right hemithorax mass with marked mediastinal shift, near-complete right lung collapse, and recurrent pneumothorax. Initial CT-guided biopsy was interpreted as embryonal rhabdomyosarcoma (ERMS) on limited material, and systemic therapy was started with VAC, later escalated to VDC because of poor response and ongoing life-threatening mass effect. The clinical course was complicated by persistent/recurrent pneumothorax requiring multiple pleural interventions and eventual transfer for urgent surgical management. He underwent near-total resection followed by repeat resection for rapid local recurrence several weeks later. Definitive resection pathology established PPB, type III, and tumor next-generation sequencing detected a pathogenic DICER1 variant (germline status not yet confirmed). Postoperatively, chemotherapy was initiated with IVADo. A complete blood count obtained locally after return home showed severe cytopenias, prompting growth-factor support and close hematology-oncology follow-up. Conclusion: This case highlights that PPB should be prioritized in the differential diagnosis of aggressive pediatric intrathoracic masses with pneumothorax and mass effect, and demonstrates the value of adequate tissue sampling, expert pathology review, and early integration of molecular testing to prevent protocol misdirection and to trigger DICER1-directed genetic counseling and surveillance.