Oliveira, Andre

ABSTRACT Myxoid liposarcoma (MLS) accounts for 20%–30% of all liposarcomas, with most cases harboring the fusion gene FUS::DDIT3 , while approximately 5% exhibit the EWSR1::DDIT3 fusion. We report the case of a 26‐year‐old male patient with a right upper arm mass. The tumor displayed the classic histological features of MLS, including small spindle/ovoid cells, variable univacuolated lipoblasts, and a prominent myxoid stroma with delicate arborizing vasculature. Despite these characteristic features, fluorescence in situ hybridization (FISH) revealed no apparent rearrangement of the DDIT3 locus. Next‐generation sequencing (NGS) identified a novel fusion transcript in which SMARCA2 exon 4 was fused in‐frame with DDIT3 exon 2. Chromosomal microarray analysis demonstrated the unbalanced nature of the rearrangement, with partial deletions of 0.243 and 0.176 Mb flanking the centromeric end of the DDIT3 locus on 12q13.3 (which also included GLI ) and disrupting the SMARCA2 locus on 9p24, respectively. The resultant chimeric fusion protein is predicted to lack the SMARCA2 DNA‐binding domains while retaining the DDIT3 leucine zipper dimerization domain. These findings indicate an unusual and complex rearrangement, leading to the recruitment of a novel DDIT3 partner gene. Moreover, they emphasize that the functional aspects of myxoid liposarcoma fusion genes depend on the retention of the key DDIT3 domain. Finally, this case illustrates how classic morphology can appropriately trigger reflex molecular analyses, which may, in turn, uncover novel fusion genes or other molecular alterations.

With the first series of PRRX1 -rearranged tumors published in 2019, the spectrum of these so-called fibroblastic tumors has been expanded. Since then, several smaller case series have been published; however, our understanding of them continues to be quite limited given their rarity. We herein studied 18 additional cases, the largest series to date. Eighteen tumors present in 9 male, 8 female, and 1 nonbinary patient with a median age of 35 years (range: 11 to 70 y) and involved the neck (5), the chest region (4), thigh (3), back (1), shoulder (1), forehead (1), lower leg (1), axilla (1), and the parapharyngeal region (1). Clinical follow-up (9/18 tumors; 50%; median: 10 mo; range: 4 to 40 mo) showed consistent indolent behavior without local recurrences or distant metastases. On morphology, these tumors were characterized by well-circumscription and distinctive peripheral crescent-shaped vessels. They were composed of uniform spindle and round cells growing in short fascicles within often densely hyalinized collagen lacking significant mitotic activity, necrosis, or cytologic atypia. Immunohistochemically, about half of the tested tumors expressed focal to rarely diffuse S100 with occasional co-expression of SOX10. Interestingly, almost half of the tested cases also showed complete loss of RB expression. All but 1 tumor harbored a PRRX1::NCOA1 fusion, while 1 case harbored a novel PRRX1::EP300 fusion. We herein provide additional data on these exceptionally uncommon tumors, expand their molecular spectrum, and compare them to their close morphologic mimics to aid in accurate diagnosis and avoid confusion with potentially more aggressive neoplasms.

AbstractInfantile myofibromatosis (IM) is characterized by solitary musculoskeletal nodules presenting during infancy but can manifest as multiple lesions with visceral involvement. Multicentric IM with visceral involvement carries a high risk of mortality and there is no consensus on treatment. We present a case of a patient with multicentric IM and pulmonary involvement who progressed on several chemotherapeutic regimens and subsequently had a complete response to sorafenib and later imatinib. This report describes the novel use of sorafenib and imatinib to treat generalized IM and the role of continued tyrosine kinase inhibitor therapy to maintain remission.