Dual INI1-BRG1-Deficient Pericardial Tumor With Rhabdoid Morphology and Clinical Benefit From DNA-Damaging Therapy.

Primary pericardial SMARCB1-deficient tumors with rhabdoid morphology are exceptionally rare. We present an adult case of a primary SMARCB1-deficient pericardial tumor showing concurrent loss of INI1 (SMARCB1) and BRG1 (SMARCA4) expression, representing the third documented tumor with co-loss of these SWI/SNF subunits across tumor sites. Targeted RNA sequencing detected no pathogenic sequence variants or fusion transcripts and showed reduced SMARCB1 and SMARCA4 transcript abundance. DNA-based whole-exome sequencing (WES) identified no pathogenic or likely pathogenic sequence variants in either gene. High-resolution SNP microarray demonstrated a homozygous SMARCB1 deletion with intact SMARCA4, establishing biallelic SMARCB1 inactivation; BRG1 loss was accompanied by marked SMARCA4 transcript downregulation in the absence of a detectable SMARCA4 genomic alteration. In the clinicopathological context, the overall findings supported classification as a SMARCB1-deficient pericardial tumor with rhabdoid morphology. Following resection, postoperative imaging suggested residual or recurrent disease. Combined doxorubicin-ifosfamide chemotherapy and radical radiotherapy achieved a complete radiologic response. However, imaging 18 months after resection suggested multifocal pericardial recurrence, with disease control regained following similar treatment. This course is hypothesis-generating and may suggest sensitivity to DNA-damaging therapy in the setting of dual SWI/SNF deficiency.