Endometrial stromal tumors: A clinico-histomorphological spectrum of endometrial stromal tumors with review of literature.

BACKGROUND: Endometrial stromal tumors (ESTs) are rare subset of mesenchymal uterine neoplasms. There are heterogeneous morphological, immunohistochemical, and genetic features. Approximately 50% of ESTs occur in perimenopausal women. In 2020, WHO sub-categorized ESTs into four groups: endometrial stromal nodule (ESN), low-grade endometrial stromal sarcoma (LGESS), high-grade endometrial stromal sarcoma (HGESS), and undifferentiated uterine sarcoma (UUS). OBJECTIVE: To review the morphological spectrum of endometrial stromal tumors. METHOD: This retrospective study reviewed the histomorphological features of 15 endometrial stromal tumors with respect to atypia, necrosis, mitosis, collagen bands, whorling around vessels, myometrial invasion, and inflammatory cells. Immunohistochemistry markers (CD10, SMA, and ER) along with special stains (Masson's trichrome, toluidine blue) were also studied. RESULTS: The age of the patients ranged from 32 to 60 years. Three patients were postmenopausal. The most common presenting symptom was vaginal bleeding. Five patients were operated with a clinical diagnosis of uterine fibroid. One patient presented with prolapse with no other complaint. All the 15 patients had total abdominal hysterectomy and salpingo-oophorectomy. One case showed necrosis, eight cases showed collagen bands, all the 15 cases showed whorling around vessels, one case showed vascular emboli, and seven cases showed inflammatory cells. In low-grade cases, one case showed focal atypia and one case showed focal coagulative necrosis indicating infarction. Thirteen cases were LGESS, and one case of ESN and HGESS. All cases were positive for ER and CD10. CONCLUSION: Endometrial stromal tumors demonstrate extensive permeation of the myometrium as irregular islands with frequent vascular invasion, whorling around vessels, collagen bands, and inflammatory cells. All these features should be observed thoroughly on microscopy by pathologists to clearly differentiate the low-grade and high-grade endometrial stromal tumors, and to understand the overlapping gray areas morphologically as it affects the prognosis of the patient.

An Unusual Endometrial Stromal Neoplasm With JAZF1-BCORL1 Rearrangement.

Endometrial stromal tumors represent the second most common category of uterine mesenchymal tumors. Several different histologic variants and underlying genetic alterations have been recognized, one such being a group associated with BCORL1 rearrangements. They are usually high-grade endometrial stromal sarcomas, often associated with prominent myxoid background and aggressive behavior. Here, we report an unusual endometrial stromal neoplasm with JAZF1-BCORL1 rearrangement and briefly review the literature. The neoplasm formed a well-circumscribed uterine mass in a 50-yr-old woman and had an unusual morphologic appearance that did not warrant a high-grade categorization. It was characterized by a predominant population of epithelioid cells with clear to focally eosinophilic cytoplasm growing in interanastomosing cords and trabeculae set in a hyalinized stroma as well as nested and fascicular growths imparting focal resemblance to a uterine tumor resembling ovarian sex-cord tumor, PEComa, and a smooth muscle neoplasm. A minor storiform growth of spindle cells reminiscent of the fibroblastic variant of low-grade endometrial stromal sarcoma was also noted but conventional areas of low-grade endometrial stromal neoplasm were not identified. This case expands the spectrum of morphologic features seen in endometrial stromal tumors, especially when associated with a BCORL1 fusion and highlights the utility of immunohistochemical and molecular techniques in the diagnosis of these tumors, not all of which are high grade.

ZC3H7B-BCOR Fusion High-grade Endometrial Stromal Sarcoma With Morphologic Features of Low-grade Endometrial Stromal Sarcoma. A Case Report and Review of Literature.

High-grade endometrial stromal sarcomas with ZC3H7B-BCOR fusion are rare. They are predominantly located in the endomyometrium, with morphologic features characterized as haphazardly arranged fascicles of spindle cells with mild to moderate atypia, abundant myxoid matrix, high mitotic index, and tongue-like/pushing patterns of myometrial invasion. Furthermore, conventional or variant low-grade endometrial stromal sarcomas are often not present. Clinically, they present at a higher stage and are associated with worse prognosis compared with low-grade endometrial stromal sarcoma. Given the limited number of reported cases, we describe the case of a ZC3H7B-BCOR fusion high-grade endometrial stromal sarcoma initially diagnosed on the hysterectomy specimen as low-grade endometrial stromal sarcoma based on an endometrial stromal tumor showing tongue-like myometrial and lymphovascular invasion, minimal cytologic atypia, low-mitotic activity (0-1/10 high-power field), round/spindle cell component and immunohistochemical stain results (positive for CD10, estrogen receptor, progesterone receptor, and focally positive for cyclin D1). At the time of pathologic diagnosis, she was Stage Ia and managed conservatively. Subsequent molecular analysis revealed a ZC3H7B (exon 10)- BCOR (BCL-6 corepressor) (exon 7) gene fusion. On follow-up, she showed no evidence of disease at 37 months from the time of diagnosis. This case report expands the morphologic spectrum of ZC3H7B-BCOR fusion high-grade ESS, which includes an intramural location, morphologic and immunophenotypic features similar to LG-ESS, as well as the presence of round and spindle cell components. This case also underscores the value of molecular analysis in the proper classification of ESS.

Unusual morphologic features of low-grade endometrial stromal sarcoma: A case report.

BACKGROUND: Endometrial stromal tumours are uncommon tumours of the uterus. They mainly occur in perimenopausal women. Tumours with typical clinicopathological features do not usually pose diagnostic problems. However, rare clinicopathological features can occur, and clinicians without significant experience may have difficulty diagnosing these tumours and managing these patients. METHODS: Herein, we report a case of endometrial stromal sarcoma that occurred in a 25-year-old woman. The pathological features, immunophenotype, treatment and prognosis were discussed. RESULTS: The tumour revealed morphological heterogeneity, and there were similar proliferative-type endometrial stromal cells, an extensive amount of mature adipose tissue, and prominent rhabdomyoblastic and smooth muscle cells. Histopathological and immunohistochemical studies confirmed low-grade endometrial stromal sarcoma with smooth muscle, adipocytic and rhabdomyoblastic differentiation (approximately 60% were differentiated tissues). The final treatment of the tumour was total abdominal hysterectomy with bilateral salpingo-oophorectomy. There was no evidence of recurrence for 109 months postoperatively. CONCLUSIONS: We found that low-grade endometrial stromal tumours with extensive adipocytic and prominent rhabdomyoblastic differentiation are misdiagnosed because they are infrequent. They must be differentiated from rhabdomyosarcoma with accurate identification of adipocytes, and long-term follow-up is needed.

[Endometrial and other rare uterine sarcomas : Diagnostic aspects in the context of the 2020 WHO classification]. / Endometriale und weitere seltene uterine Sarkome : Diagnostische Aspekte im Kontext der WHO-Klassifikation 2020.

Uterine sarcomas are a heterogeneous group of rare malignancies. Mostly (40-50%), they are leiomyosarcomas, followed by endometrial stromal sarcomas (ESS), low-grade (LG) and high-grade (HG), as well as undifferentiated sarcoma of the uterus (UUS) and adenosarcomas (AS). Other, non-organ-specific tumours such as NTRK-rearranged spindle cell neoplasia, perivascular epithelioid cell tumour (PEComa) and inflammatory myofibroblastic tumour (IMT) are extremely difficult to differentiate.In the most recent WHO classification, endometrial stromal tumours are subdivided as follows: benign, expansively growing endometrial stromal nodule (ESN) with sharp demarcation, the histologically similar-looking LG-ESS with infiltrative growth, the highly malignant HG-ESS and, as a diagnosis of exclusion, the highly aggressive UUS lacking specific lines of differentiation. LG-ESS can be differentiated from HG-ESS in most cases histomorphologically and immunohistochemically, but molecular investigations are necessary in individual cases. HG-ESS can be divided into 4 subtypes (YWHAE/NUTM2 fusion low-grade component, YWHAE/NUTM2 fusion high-grade component, ZC3H7B-BCOR fusion or BCOR-ITD) on the basis of molecular findings. Prognostically unfavourable factors in AS are severe sarcomatous overgrowth, deep myometrial invasion, high-grade histology and lymphatic vessel invasion. Tumours with NTRK fusion are immunohistochemically positive for S100 and TRK. PEComas express cathepsin K and HMB45, as well as TFE3 when translocation is present. Almost every IMT shows an alteration in the ALK gene In the case of overlapping morphology and simultaneous therapeutic and prognostic relevance, it is becoming increasingly important to verify or confirm the suspected histomorphological diagnosis by immunohistochemical and possibly molecular investigations.

Endometrial stromal tumors: Diagnostic updates and challenges.

Endometrial stromal tumors are rare uterine mesenchymal tumors of endometrial stromal origin. They are classified into endometrial stromal nodule, low-grade endometrial stromal sarcoma, high-grade endometrial stromal sarcoma, and undifferentiated uterine sarcoma by the current (2020) WHO classification. Correct diagnosis of endometrial stromal tumors is critical for proper patient management. However, due to infrequent encounters, overlapping morphological features and immunohistochemical profiles, the differential diagnoses among endometrial stromal lesions and their morphologic mimics are often challenging. Partially with our own experience, here we review and summarize the tumor morphology, immunohistochemical phenotype, as well as molecular feature of endometrial stromal tumors and key differential diagnoses, emphasizing the newest developments and their utilization in daily practice.

Role of the Immunohistochemical ZEB1 Expression in Uterine Mesenchymal Neoplasms.

The distinction of mesenchymal tumors of the uterus is a frequent diagnostic challenge in gynecologic pathology. Especially, distinguishing low-grade endometrial stromal sarcoma (ESS) from leiomyoma or distinguishing low-grade ESS from high-grade ESS can be difficult. Epithelial-mesenchymal transition (EMT) is a physiological and pathological process in which epithelial cells lose their morphological features, become elongated and acquire mesenchymal traits. The signaling pathway of Zinc finger E-box binding homeobox 1 (ZEB1) is one of the most significant pathways involved in the EMT process and it has a crucial role in cancer progression, metastasis, and therapy resistance. We studied a series of 69 uterine mesenchymal neoplasms including 18 endometrial stromal sarcomas (10 cases of low grade and 8 cases of high grade endometrial stromal sarcomas), 26 leiomyosarcomas (8 cases of grade 1 and 19 cases of grade 2-3 leiomyosarcomas), 15 leiomyomas, and 10 rhabdomyosarcomas, using an antibody ZEB1. We graded the leiomyosarcomas depending on the FNCLCC grading system. It was observed that leiomyosarcoma was more intensely stained with ZEB1 than leiomyoma (P < 0.001) and high-grade ESS was significantly more intensely stained with ZEB1 protein than low-grade ESS (P < 0.004). It also was observed that high-grade leiomyosarcoma was significantly more intensely stained with ZEB1 protein than low-grade leiomyosarcoma (P < 0.000). Our data suggest that Zeb1 can be used to differentiate high-grade sarcomas from their low-grade counterparts as well as benign and malignant smooth muscle tumors of the uterus.

Low-grade Endometrial Stromal Sarcoma With Sex Cord-like Differentiation and PHF1-JAZF1 Fusion With Deletions: A Diagnostic Pitfall of JAZF1 FISH.

The molecular knowledge of endometrial stromal neoplasms has been rapidly increasing and is considered complementary to morphologic and immunohistochemical findings for better categorization of these tumors. The most common molecular alteration observed in low-grade endometrial stromal sarcomas is the JAZF1-SUZ12 fusion, whereas, low-grade endometrial stromal sarcoma with sex cord-like differentiation have been shown more commonly to have fusions involving PHF1. Herein, we present a low-grade endometrial stromal sarcoma with sex cord-like differentiation with a fluorescence in situ hybridization showing the apparent loss of one copy of JAZF1 5' and 3' signals, rather than the expected "break-apart" pattern seen in the setting of a JAZF1 fusion. The case was then further evaluated by chromosome microarray and RNA fusion analysis. Overall, the molecular findings supported a PHF1-JAZF1 fusion with deletions right before and after the JAZF1 locus, impairing probe binding and resulting in the unusual "deletion" pattern observed in the JAZF1 fluorescence in situ hybridization, which would not intuitively suggest a fusion involving JAZF1. This case illustrates the importance of integration of morphological and molecular findings as well as the limitations of fluorescence in situ hybridization in detecting fusions, particularly in the setting of more complex chromosomal alterations even though the fusion partners are well-known.

JAZF1, YWHAE and BCOR gene translocation in primary extrauterine low-grade and high-grade endometrial stromal sarcomas.

AIMS: JAZF1 translocation is the most common genetic change in low-grade (LG) endometrial stromal sarcoma (ESS), and YWHAE and BCOR translocations are common in high-grade (HG) ESS. Primary extrauterine ESS is rare, and there are limited data on molecular alterations in these tumours. METHODS AND RESULTS: Cases of primary extrauterine ESS, comprising eight LG-ESS cases and five HG-ESS cases were collected. Haematoxylin and eosin and immunohistochemical staining were used to observe the histomorphology and analyse related protein expression. JAZF1, YWHAE and BCOR rearrangements were explored with fluorescence in-situ hybridisation (FISH). In LG-ESS, the tumour cells resembled normal proliferative-phase endometrial stromal cells; CD10, oestrogen receptor and progesterone receptor were expressed in all eight cases. In HG-ESS, the tumour cells had uniform HG round and/or spindle morphology, sometimes with an LG component; CD10 was fully expressed in one case and focally expressed in four cases; BCOR was expressed in all five cases, and cyclin D1 in four of five cases. FISH analysis showed JAZF1 translocation in one of eight LG-ESS cases (12.5%). YWHAE translocation occurred in four of five HG-ESS cases, with a positivity rate of 80%. BCOR translocation was absent in all five cases. CONCLUSIONS: In extrauterine LG-ESS, the rate of JAZF1 rearrangement was significantly lower than in uterine LG-ESS. This result limited the value of JAZF1 translocation for diagnosis. YWHAE rearrangement is a common genetic change in extrauterine HG-ESS. Further studies are required to confirm these findings, especially in LG-ESS.

IFITM1, CD10, SMA, and h-caldesmon as a helpful combination in differential diagnosis between endometrial stromal tumor and cellular leiomyoma.

BACKGROUND: The differential diagnosis of endometrial stromal tumor (EST) and uterine cellular leiomyoma (CL) remains a challenge in clinical practice, especially low grade endometrial stromal sarcoma (ESS) and CL, suggesting the need for novel immunomarkers panels for differential diagnosis. Interferon-induced transmembrane protein 1 (IFITM1) is a novel immunomarker for endometrial stromal cells, h-caldesmon is an immunomarker for smooth muscle cells and has a higher specificity than smooth muscle actin (SMA). So this study aimed to evaluate whether IFITM1, cluster of differentiation 10(CD10), SMA, and h-caldesmon are useful biomarker combinations for the differential diagnosis of EST and CL. METHODS: Tissue microarrays were used to detect IFITM1, CD10, SMA, and h-caldesmon immunohistochemical staining in 30 EST and 33 CL cases. RESULTS: The expressions of IFITM1 and CD10 were high in EST (86.7 and 63.3%, respectively) but low in CL (18.2 and 21.2%), whereas those of h-caldesmon and SMA were high in CL (87.9 and 100%) and low in EST (6.9 and 40%). In diagnosing EST, IFITM1 shows better sensitivity and specificity (86.7 and 81.8%, respectively) than CD10 (63.3 and 78.8%). The specificity of h-caldesmon in diagnosing CL was significantly higher (93.1%) than that of SMA (60%). When all four antibodies were combined for the differential diagnosis, the area-under-the-curve (AUC) predictive value was 0.995. The best combination for diagnosing EST was IFITM1 (+) or CD10 (+) and h-caldesmon (-) (sensitivity 86.7%, specificity 93.9%). CONCLUSION: The best combination for diagnosing CL were h-caldesmon (+) and SMA (+) (sensitivity 87.9%, specificity 100%). IFITM1, CD10, SMA, and h-caldesmon are a good combination for the differential diagnosis of EST and CL.

Vaginal Low-Grade Endometrial Stromal Sarcoma: An Extremely Rare Case Report and Review of the Literature.

Endometrial stromal sarcoma (ESS) is a malignant tumor of the uterus that has been described as the second most common malignant uterine mesenchymal tumor. Primary extrauterine ESS (EESS) is an extremely uncommon occurrence. We hereby report a new bona fide case of low-grade EESS in a 74-yr-old woman arising in the vagina, presenting as a polypoid mass associated with irregular vaginal bleeding. On examination, a 6×2×2 cm polypoid mass was found in the left vaginal wall. Consequently, the patient underwent partial vaginectomy and repair. No ESS or endometriotic lesion was found in the endometrium and bilateral adnexa. The diagnosis of ESS performed by typical pathologic and immunohistochemical evaluation was as follows: beta-catenin (+++), estrogen receptor (+++), progesterone receptor (++), vimentin (++), and uniformly negative for CD10, EMA, CD31, CD34, CD117,CD99, SMA, desmin, h-caldesmon, S-100, MelanA, and HMB45. She has remained disease free with no signs or symptoms of recurrent or advanced disease for 46 mo. Although CD10 is the most useful immunohistochemical marker for the diagnosis of this tumor, negative CD10 staining can be encountered with underfixation. Therefore, it is important to use a panel of immunostains that includes CD10, beta-catenin, and smooth muscle markers. The present study describes the clinical and pathologic features of low-grade EESS through a case report and literature review. To the best of our knowledge, this is the eighth report of EESS arising from the vagina.

Practical roles for molecular diagnostic testing in ovarian adult granulosa cell tumour, Sertoli-Leydig cell tumour, microcystic stromal tumour and their mimics.

Within the last decade, molecular advances have provided insights into the genetics of several ovarian sex cord-stromal tumours that have otherwise been enigmatic. Chief among these advances are the identification of FOXL2, DICER1 and CTNNB1 mutations in adult granulosa cell tumours, Sertoli-Leydig cell tumours (SLCTs), and microcystic stromal tumours (MCSTs), respectively. As access to molecular diagnostic laboratories continues to become more widely available, the potential roles for tumour mutation testing in the pathological diagnosis of these tumours merit discussion. Furthermore, links to inherited cancer susceptibility syndromes may exist for some women with SLCT (DICER1 syndrome) and MCST [familial adenomatous polyposis (FAP)]. This review will address practical issues in deciding when and how to apply mutation testing in the diagnosis of these three sex cord-stromal tumours. The pathologist's role in recommending referral for formal risk assessment for DICER1 syndrome and FAP will also be discussed.

Retroperitoneal low grade endometrial stromal sarcoma with florid endometrioid glandular differentiation: Cytologic-histologic correlation and differential diagnosis.

Low grade endometrial stromal sarcoma (LGESS) is a rare neoplasm that typically arises in the uterine corpus and accounts for less than 1% of uterine sarcomas. Infrequently, extra-uterine LGESS can occur. Histologically, LGESS is characterized by a monotonous population of cells that resemble the proliferative phase of endometrial stroma and in their classic form they exhibit tongue-like growth pattern of infiltration and/or lymphovascular invasion. Infrequently LGESS can demonstrate various morphologic differentiation patterns, including endometrioid-type glands. We report the first fine needle aspiration (FNA) case of a periduodenal mass that was incidentally discovered on Computed Tomography (CT) scan of a 60-year-old female. The cytomorphologic and histologic findings and the immunohistochemical staining were consistent with a LGESS with endometrioid glandular differentiation. We are presenting the correlation between the cytologic, radiologic and pathologic features.

Low-grade endometrial stromal sarcoma presenting as multiple pulmonary nodules: A potential pitfall in fine needle aspiration and core biopsy specimens - A Cytological - Pathological Correlation.

Low-grade endometrial stromal sarcoma (LGESS) is the second most common malignant mesenchymal tumor of the uterus. The most common location is the uterine corpus, but it can also primarily arise in a variety of extrauterine locations such as pelvis, ovary, abdominal cavity, vagina, and vulva. We are reporting a case of a 47-year-old female with no significant medical history who presented with multiple pulmonary nodules. Fine needle aspiration (FNA) specimen revealed spindle cell neoplasm consistent with the diagnosis of LGESS. The differential diagnosis included neuroendocrine tumor, synovial sarcoma, solitary fibrous tumor, smooth muscle tumors, and peripheral nerve sheath tumors. The clinical, cytological, and histopathologic details of this case, as well as a discussion of the potential pitfalls and differential diagnosis of spindle cell lesions of the lung are described.

Endometrial Stromal Sarcoma With Hyalinizing Giant Rosettes, Mimicking Low-Grade Fibromyxoid Sarcoma.

We highlight a rare variant pattern of low-grade endometrial stromal sarcoma showing extensive collagenous rosette formation, closely mimicking low-grade fibromyxoid sarcoma. Additionally, this neoplasm showed diffuse and strong expression of muscle markers, favoring an initial diagnosis of leiomyosarcoma. Reverse transcription-polymerase chain reaction showed the presence of JAZF1-SUZ12 fusion transcripts, and this highlights the broad morphologic and immunophenotypic spectrum of endometrial stromal sarcoma.

Pitfalls of Frozen Section in Gynecological Pathology: A Case of Endometrial Tumor With Sex Cord-Like Elements.

Endometrial stromal tumor with sex cord-like elements (ESTSCLE) is a rare entity that shares similar histological features with uterine tumors resembling ovarian sex cord tumors (UTROSCT). Differentiating the 2 entities involves ample sampling of the tissue to distinguish the percentage of sex cord components within the tissue, genetic studies, and immunohistochemical staining. Frozen section provides limited information for exclusion of either tumor; and the tumor is rare enough that the diagnosis may not be considered with the limited sampling; therefore, deferral of diagnosis to permanent sections may be appropriate.

Unusual Presentations of Gynecologic Tumors: Primary, Extrauterine, Low-Grade Endometrioid Stromal Sarcoma.

CONTEXT: - Low-grade endometrial stromal sarcomas, when uterine in location, are relatively easy to diagnose because of characteristic morphology and patterns of myometrial invasion. However, when they occur at extrauterine sites, they fall under the broad umbrella of small round blue cell tumors, making diagnosis challenging, especially when they have variant morphologic features and lack the characteristic pattern of invasion. OBJECTIVES: - To provide an insight into the sites of occurrence of low-grade endometrioid stromal sarcomas, the variant morphologic patterns, clues to diagnosis, and the usefulness of immunohistochemistry as an aid to facilitate correct diagnosis. The outcome of these tumors, in comparison with their uterine counterpart, is also discussed. DATA SOURCES: - Existing peer-reviewed literature was reviewed. CONCLUSIONS: - Low-grade endometrioid stromal sarcoma is an uncommon neoplasm that can be misdiagnosed because of its rarity, unusual location, and presence of numerous variant histologic patterns that mimic other tumors. Knowledge of those features; consideration of this tumor in the differential diagnosis of small, round blue cell tumors at any location in a woman; and an appropriate use of immunohistochemistry can help facilitate the diagnosis.

Endometrial Stromal Neoplasm in the Placenta: Report of a Case and Review of the Literature.

Placental neoplasms involving the fetal membranes are exceptionally rare. In leiomyomas and endometrial stromal neoplasms, a uterine origin for nodules in the membranes is also a possibility. We present a case of an incidental endometrial stromal nodule found in the decidua of the free membranes and review the literature.

IFITM1 Outperforms CD10 in Differentiating Low-grade Endometrial Stromal Sarcomas From Smooth Muscle Neoplasms of the Uterus.

Distinguishing between uterine neoplasms of smooth muscle and endometrial stromal origin is a frequent diagnostic challenge. We investigated the staining pattern of interferon-induced transmembrane protein-1 (IFITM1), a novel endometrial stromal marker, in endometrial and smooth muscle uterine neoplasms and compared it with CD10 in its ability to differentiate between these two groups. Immunohistochemistry for IFITM1 and CD10 was performed in 20 cases of smooth muscle neoplasms (10 cases leiomyoma, 10 cases leiomyosarcoma), 14 cases of endometrial stromal sarcoma (ESS) (12 cases of low grade and 2 cases of high grade) and 12 cases of carcinosarcoma. Staining was scored in terms of intensity and distribution (0=absent, 1=weak/<50%, 2=moderate/50%-75%, 3=strong/>75%). A total score was obtained by adding intensity and distribution scores and classified as positive (score 3-6) or negative (score 0-2). IFITM1 was positive in 10 of 12 (83%) low-grade ESSs, 6 of 20 (30%) smooth muscle tumors (leiomyomas and leiomyosarcomas) and 11 of 12 carcinosarcomas (91.6%). The 2 cases of high-grade ESS were IFITM1 negative. While both IFITM1 (83%) and CD10 (91%) had high sensitivity in differentiating low-grade ESSs from smooth muscle neoplasms, IFITM1 (70%) had higher specificity compared with CD10 (45%). In this study IFITM1 appears to be a more specific marker of endometrial stromal differentiation compared with CD10 in differentiating low-grade ESSs from smooth muscle neoplasms. Thus, IFITM1 may be a valuable tool as part of an immunohistochemical evaluation panel in this diagnostic scenario.