Update on cutaneous mesenchymal tumors in the 5th edition of WHO classification of skin tumors with an emphasis on new fusion-associated neoplasms.

The section on mesenchymal tumors in the 5th edition of WHO classification of skin tumors has undergone several changes, the most important of which is the inclusion of newly identified tumor entities, which will be the main focus of this review article. These specifically include three novel cutaneous mesenchymal tumors with melanocytic differentiation, and rearrangements of the CRTC1::TRIM11, ACTIN::MITF, and MITF::CREM genes as well as EWSR1::SMAD3-rearranged fibroblastic tumors, superficial CD34-positive fibroblastic tumors, and NTRK-rearranged spindle cell neoplasms. Some of the other most important changes will be briefly mentioned as well.

EWSR1-PATZ1-rearranged sarcoma: a report of nine cases of spindle and round cell neoplasms with predilection for thoracoabdominal soft tissues and frequent expression of neural and skeletal muscle markers.

The knowledge of clinical features and, particularly, histopathological spectrum of EWSR1-PATZ1-rearranged spindle and round cell sarcomas (EPS) remains limited. For this reason, we report the largest clinicopathological study of EPS to date. Nine cases were collected, consisting of four males and five females ranging in age from 10 to 81 years (average: 49 years).

Inflammatory leiomyosarcoma shows frequent co-expression of smooth and skeletal muscle markers supporting a primitive myogenic phenotype: a report of 9 cases with a proposal for reclassification as low-grade inflammatory myogenic tumor.

Inflammatory leiomyosarcoma (ILMS) is a very rare soft tissue tumor that usually follows an indolent clinical course, but long-term follow-up studies are lacking. Recent publications primarily focused on its genetic profile characterized by a near haploid genome. One study also showed these tumors to have upregulation of genes known to be crucial for skeletal muscle differentiation.