Alterations in kinase genes such as NTRK1/2/3 , RET , and BRAF underlie infantile fibrosarcoma (IFS), the emerging entity ‘ NTRK ‐rearranged spindle cell neoplasms’ included in the latest WHO classification, and a growing set of tumors with overlapping clinical and pathological features. In this study, we conducted a comprehensive clinicopathological and molecular analysis of 22 cases of IFS and other kinase gene‐altered spindle cell neoplasms affecting both pediatric and adult patients. Follow‐up periods for 16 patients ranged in length from 10 to 130 months (mean 38 months). Six patients were treated with targeted therapy, achieving a partial or complete response in five cases. Overall, three cases recurred and one metastasized. Eight patients were free of disease, five were alive with disease, and two patients died. All cases showed previously reported morphological patterns. Based on the cellularity and level of atypia, cases were divided into three morphological grade groups. S100 protein and CD34 were at least focally positive in 12/22 and 14/22 cases, respectively. Novel PWWP2A::RET , NUMA1::RET , ITSN1::RAF1 , and CAPZA2::MET fusions, which we report herein in mesenchymal tumors for the first time, were detected by RNA sequencing. Additionally, the first uterine case with BRAF and EGFR mutations and CD34 and S100 co‐expression is described. DNA sequencing performed in 13 cases uncovered very rare additional genetic aberrations. The CNV profiles showed that high‐grade tumors demonstrate a significantly higher percentage of copy number gains and losses across the genome compared with low‐ and intermediate‐grade tumors. Unsupervised clustering of the tumors’ methylation profiles revealed that in 8/9 cases, the methylation profiles clustered with the IFS methylation class, irrespective of their clinicopathological or molecular features. © 2024 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

In our routine and consultative pathology practices, we have repeatedly encountered an unusual subcutaneous fatty tumor with notable anisocytosis, single-cell fat necrosis, and patchy, often mild, adipocytic nuclear atypia. Because of the focal atypia, consultative cases have most often been received with concern for a diagnosis of atypical lipomatous tumor. Similar tumors have been described in small series under the designations “subcutaneous minimally atypical lipomatous tumors” and “anisometric cell lipoma.” Sixty-six cases of this tumor type were collected and reviewed. Immunohistochemistry for p53, MDM2, CDK4, Retinoblastoma 1 (RB1) protein, CD34, S100, and CD163 was performed. Cases were tested for MDM2 gene amplification and RB1 gene deletion with fluorescence in situ hybridization (FISH) and for TP53 mutations by Sanger sequencing. Next-generation sequencing analysis using a panel of 271 cancer-related genes, including TP53, RB1, and MDM2, was also carried out. Our patient cohort included 57 male patients, 8 female patients, and 1 patient of unstated sex, who ranged in age from 22 to 87 years (mean: 51.2 y). All tumors were subcutaneous, with most examples occurring on the upper back, shoulders, or posterior neck (86.4%). Ten patients had multiple (2 to 5) lipomatous tumors, and the histology was confirmed to be similar in the different sites in 4 of them, including 1 patient who had a retinoblastoma diagnosed at age 1. The tumors were generally well circumscribed. At low magnification, there was notable adipocytic size variation with single-cell fat necrosis in the background associated with reactive histiocytes. Adipocytic nuclear atypia was typically patchy and characterized by chromatin coarsening, nuclear enlargement, and focal binucleation or multinucleation. Focal Lochkern change was frequent. In most instances, the degree of atypia was judged to be mild, but in 3 instances, it was more pronounced. Spindle cells were sparse or absent, and when present, cytologically bland. Thick ropy collagen bundles were absent. In all cases, p53 immunoexpression was noted (range: 2% to 20% of adipocytic nuclei), characteristically highlighting the most atypical cells. Twenty of 50 cases had MDM2 immunoreactivity, usually in 45% of the cells by FISH (our threshold value for reporting a positive result) with an additional 3 cases being very close to the required cutoff value. MDM2 gene amplification was absent in all 60 cases tested, including those with the greatest MDM2 immunoexpression and most pronounced atypia. All 5 tested cases showed no TP53 mutation with Sanger sequencing. Because of material quality issues, next-generation sequencing analysis could be performed in only 3 cases, and this did not reveal any recurrent mutations. All tumors were managed by simple local excision. Follow-up was available for 47 patients (range: 1 to 192 mo; mean: 27 mo) and revealed 2 local recurrences and no metastases. Dysplastic lipoma is a distinctive atypical fatty tumor variant that has p53 overexpression and RB1 gene abnormalities and lacks MDM2 gene amplification by FISH. These tumors have a strong male predominance and a notable tendency to involve the subcutaneous tissue of the shoulders, upper back and posterior neck. Multifocality is frequent (18.9% of patients with follow-up information), and there is a rare association with retinoblastoma. This tumor warrants separation from ordinary lipoma with fat necrosis, fat-rich spindle cell lipoma and the conventional form of atypical lipomatous tumor that features MDM2 gene amplification.