The Spectrum of Fusions Occurring in Non-Smooth Muscle Mesenchymal Uterine Tumors: A Review of the Current Knowledge

Integrating clinicopathological and molecular data to assess the biological behavior of uterine inflammatory myofibroblastic tumors

Assessing the biological behavior of uterine inflammatory myofibroblastic tumors (IMTs) remains challenging. This study evaluated previously proposed risk schemes and features in 9 IMTs (6 indolent, 3 aggressive) by integrating clinicopathological features, immunohistochemistry, and next-generation sequencing (NGS). High-risk features (necrosis, infiltrative growth, nuclear atypia) were present in both groups, with LVSI in 1/3 of aggressive IMTs. Aberrant p16 expression and CDKN2A/2B deletions were noted in 2/3 aggressive cases. All cases harbored ALK fusions, wild-type p53, and lacked pathogenic gene mutations. Aggressive cases harbored arm-level and segmental copy number gains/losses at chr 1, 2, X, and had significantly reduced AR expression. The clinicopathological risk stratification score (CRSS) predicted the biological behavior correctly in cases with complete clinicopathological data (size, mitoses, age, infiltrative growth). Two morcellated cases (one indolent and one aggressive) would have been predicted as low risk based solely on the absence of pathogenic mutations. Hereby, the reliability of the proposed CRSS was confirmed. Aberrant p16 expression predicted malignant behavior in 2/3 aggressive cases. Absence of pathogenic mutations or presence of large scale CNVs does not seem to be a predictor of clinical behavior. Additional studies and NGS analyses of more cases may improve risk stratification for patients with incomplete clinicopathological information and may reveal additional risk stratifiers (such as the suggested large-scale CNVs or AR downregulation) for IMTs.

Uterine sarcoma with KAT6B/A::KANSL1 fusion: a molecular and clinicopathological study on 9 cases

Uterine sarcomas with KAT6B/A::KANSL1 fusion represent a new entity characterized by bland morphology, commonly with hybrid features of low-grade endometrial stromal sarcoma (LG-ESS) and tumors with smooth muscle differentiation. In our study, we performed a detailed morphological, immunohistochemical, and molecular analysis of 9 cases of these tumors. Six of those had been originally diagnosed as LG-ESS, one as leiomyoma, one as leiomyosarcoma, and the remaining case as sarcoma with the KAT6B/A::KANSL1 fusion. Seven cases showed overlapping features between endometrial stromal and smooth muscle tumors, one case resembled cellular leiomyoma, and one case resembled high-grade endometrial stromal sarcoma. Immunohistochemically, the tumors showed a common expression of smooth muscle markers and endometrial stromal markers. Molecular findings showed the KAT6B/A::KANSL1 fusion in all cases (by NGS and FISH). In addition, mutations affecting genes such as TP53, PDGFRB, NF1, RB1, PTEN, ATM, RB1, FANCD2, and TSC1 were present in all 5 cases with aggressive behavior. One patient with no evidence of disease showed no additional mutations, while another harbored a mutation of a single gene (ERCC3). Of the 8 patients with available follow-up, two died of disease, 3 are currently alive with disease, and 3 have no evidence of disease. The correct recognition of tumors with the KAT6B/A::KANSL1 fusion is essential because despite the bland morphological features of most cases, these tumors have a propensity for aggressive behavior.

Endometrial stromal tumor with whorling and GREB1::CTNNB1 fusion-a case report on a rare entity

Endometrial stromal tumors are rare lesions with a diverse morphology, which may make achieving the correct diagnosis challenging in some cases. We report a case of a uterine mesenchymal tumor diagnosed as endometrial stromal nodule with a peculiar whorled morphology and GREB1::CTNNB1 fusion confirmed by transcriptome RNA sequencing. The tumor was sharply demarcated, lacked invasive growth, and had benign behavior, as the patient remained without disease recurrence 15 years later. Immunohistochemically, the tumor cells showed diffuse nuclear expression of beta-catenin, confirming the activation of the beta-catenin pathway. Our case represents only the 4th reported case of CTNNB1-rearranged endometrial stromal tumor with extensive whorling. The biological nature of uterine tumors characterized by whorled morphology and rearrangement of CTNNB1 is not yet clear, which underscores the importance of genetic profiling for accurate diagnosis and potential targeted therapies in malignant cases.

Immunohistochemical analysis of 147 cases of low-grade endometrial stromal sarcoma: refining the immunohistochemical profile of LG-ESS on a large, molecularly confirmed series

Low-grade endometrial stromal sarcoma (LG-ESS) can present diagnostic challenges, due to its overlapping morphological features with other uterine mesenchymal tumors. Misdiagnosis rates remain significant, and immunohistochemical data for LG-ESS are limited to small series and inconsistent antibody panels. This study aimed to refine the IHC profile of LG-ESS by analyzing a large, molecularly confirmed series of 147 cases using a panel of 24 antibodies, including newer markers like transgelin and smoothelin. CD10 and IFITM1, key endometrial stromal markers, were expressed in 86% (92% of those extensively) and 69% (60% of those extensively) of cases, with fusion-positive tumors showing significantly higher expression. Smooth muscle markers (α-SMA, desmin, h-caldesmon, calponin, transgelin) were variably expressed, predominantly in focal or low-intensity patterns, with α-SMA reaching the highest frequency of expression (44%). However, the intensity of smooth muscle marker expression was usually very low. Smoothelin was rarely expressed. Hormone receptors were frequently positive, with PR showing a higher frequency (92% vs. 83%) and intensity than ER. Markers like S-100, HMB45, and CD117 were largely negative; all tumors were p53 wild-type, with preserved SMARCB1/SMARCA4 expression and ALK and ROS1 negativity. This work represents the largest molecularly validated IHC study on LG-ESS, providing a robust diagnostic profile for routine pathology. By addressing key diagnostic limitations and examining newer markers, our study supports a more standardized approach to diagnosing LG-ESS and underscores the value of immunohistochemical panels, particularly in fusion-negative tumors where diagnosis relies on morphological and immunohistochemical interpretation. These findings contribute critical data for improving diagnostic accuracy.