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.