Inflammatory Leiomyosarcoma and Histiocyte-rich Rhabdomyoblastic Tumor: a clinicopathological, immunohistochemical and genetic study of 13 cases, with a proposal for reclassification as Inflammatory Rhabdomyoblastic Tumor

Inflammatory leiomyosarcoma (ILMS), defined as a malignant neoplasm showing smooth muscle differentiation, a prominent inflammatory infiltrate, and near-haploidization, is a very rare soft tissue tumor with a generally favorable prognosis. The morphologic features of histiocyte-rich rhabdomyoblastic tumor (HRRMT) are similar to those of ILMS, although this lesion shows by definition a skeletal muscle phenotype. Recent gene expression profiling and immunohistochemical studies have also suggested that ILMS and HRRMT may be related. We studied the clinicopathologic, immunohistochemical and genetic features of four cases previously classified as ILMS and nine classified as HRRMT. Tumors from both groups tended to occur in the deep soft tissues of the extremities of young to middle-aged males and exhibited indolent behavior. Morphologically, all were well-circumscribed, often encapsulated, and showed a striking histiocyte-rich inflammatory infiltrate admixed with variably pleomorphic tumor cells showing spindled and epithelioid to rhabdoid morphology, eosinophilic cytoplasm, and prominent nucleoli, but few, if any, mitotic figures. Immunohistochemically, the tumor cells expressed desmin, alpha-smooth muscle actin, and the rhabdomyoblastic markers PAX7, MyoD1, and myogenin. H-caldesmon expression was absent in all cases, using the specific h-CD antibody. Karyotypic study (1 HRRMT) and genome-wide copy number analysis (7 HRRMT, OncoScan SNP assay), revealed near-haploidization in four cases, with subsequent genome doubling in one, an identical phenotype to that seen in ILMS. We propose reclassification of ILMS and HRRMT as inflammatory rhabdomyoblastic tumor, a name which accurately describes the salient morphologic and immunohistochemical features of this distinctive tumor, as well as its intermediate (rarely metastasizing) clinical behavior.

Automated summary

Inflammatory leiomyosarcoma (ILMS) and histiocyte-rich rhabdomyoblastic tumor (HRRMT) share similar morphological features and gene expression profiles, with both types of tumors showing indolent behavior and primarily affecting young to middle-aged males in the deep soft tissues of the extremities. The proposal to reclassify ILMS and HRRMT as inflammatory rhabdomyoblastic tumor reflects the relationship between these two rare soft tissue tumors and their distinctive clinical characteristics.