The diagnostic utility of cytology specimen in a case of EWSR1::NFATC2 sarcoma.

EWSR1::NFATC2 sarcoma, a rare round cell sarcoma constituting the majority of EWSR1::non-ETS sarcomas, has recently been defined in the latest WHO classification. To date, the cytological findings of EWSR1::NFATC2 sarcoma remain undocumented. We present the case of a 25-year-old man with a history of polyostotic fibrous dysplasia in the right leg, referred to our hospital with left thigh pain. Cytological findings included metachromasia, minimally pleomorphic round cells, and eosinophilic infiltration. There was no precursor fibrous dysplasia and the initial diagnosis was undifferentiated pleomorphic sarcoma. Following histologic review, we successfully performed immunocytochemistry and fluorescence in situ hybridization (FISH) on archival cytology specimens. The tumor cells were positive for NKX2-2, NKX3-1, and PAX7 and showed amplified 5' single signals of EWSR1 gene. Reverse transcriptase-polymerase chain reaction revealed an in-frame fusion of EWSR1 and NFATC2. This report describes the cytological features of EWSR1::NFATC2 sarcoma and highlights the diagnostic utility of archival cytology specimens.

CIC-Rearranged Sarcoma.

CIC-rearranged sarcoma is a rare type of small round cell sarcoma. The tumors often affect the deep soft tissues of patients in a wide age range. They are highly aggressive, respond poorly to chemotherapy, and have a worse outcome than Ewing sarcoma. CIC-rearranged sarcoma has characteristic and recognizable histology, including lobulated growth, focal myxoid changes, round to epithelioid cells, and minimal variation of nuclear size and shape. Nuclear ETV4 and WT1 expression are useful immunohistochemical findings. CIC fusion can be demonstrated using various methods; however, even next-generation sequencing suffers from imperfect sensitivity, especially for CIC::DUX4.

Imaging findings of NTRK­rearranged spindle cell neoplasms: A case series.

NTRK-rearranged spindle cell neoplasms (NTRK-RSCNs) are a new category of soft tissue tumors with NTRK gene fusions. The present study aimed to investigate the radiological features of NTRK-RSCNs and their association with histopathological findings. The present study included six patients with NTRK-RSCNs, whose fusion genes were confirmed using next-generation sequencing. All patients underwent surgery, and their diagnosis and clinical outcomes were investigated. In addition, the magnetic resonance imaging (MRI) features of all tumors and histopathological findings of the resected specimens were assessed. The present study included three women and three men, with a mean age of 22 years (range, 2-43 years). The NTRK gene fusions included four NTRK1 and two NTRK3 fusions. Three patients were preoperatively diagnosed with solitary fibrous tumors. One patient with NTRK3 fusion experienced local recurrence and distant metastases, whereas the other five patients had no local recurrence or metastasis. MRI revealed that all tumors were highly vascular with intra- and peritumoral flow voids of differing degrees. Furthermore, a partially ill-defined border, suggesting infiltration of tumors into the surrounding tissues, particularly fat tissue, was observed in five patients, which was confirmed by histopathological findings. In conclusion, NTRK-RSCNs are highly vascular tumors that can infiltrate the surrounding tissues. These findings suggested that NTRK-RSCNs should be considered in the differential diagnosis of highly vascular-rich mesenchymal tumors, including solitary fibrous tumors and alveolar soft part sarcomas. Furthermore, wide resection may be preferred to completely resect this type of tumor, considering its invasive nature.

Dramatic response to entrectinib in a patient with malignant peripheral nerve sheath tumor harboring novel SNRNP70-NTRK3 fusion gene.

Neurotropic tropomyosin receptor kinase (NTRK) gene rearrangements have been reported in limited cases of sarcomas; however, to date, there has been only one report of such rearrangements in malignant peripheral nerve sheath tumors (MPNSTs). Herein, we describe a 51-year-old male patient with a buttock tumor arising from the sciatic nerve, which was diagnosed as MPNST with positive S-100 staining, negative SOX10 staining, and loss of trimethylation at lysine 27 of histone H3 (H3K27me3) confirmed by immunohistochemistry. Soon after the resection of the primary tumor, the patient was found to have pulmonary and lymph node metastases. Chemotherapy with eribulin and trabectedin showed limited effects. However, the patient responded rapidly to pazopanib, but severe side effects caused discontinuation of the treatment. RNA panel testing revealed a novel fusion gene between Small Nuclear Ribonucleoprotein U1 Subunit 70 (SNRNP70) gene and NTRK3 gene. Furthermore, loss of NF1, SUZ12, and CDKN2A genes was confirmed by DNA panel testing, which is compatible with a histological diagnosis of MPNST. SNRNP70 possesses a coiled-coiled domain and seems to induce constitutive activation of NTRK3 through dimerization. In fact, immunohistochemistry revealed diffuse staining of pan-TRK within tumor cells. Treatment with entrectinib, which is an NTRK inhibitor, showed a quick and durable response for 10 months. Although NTRK rearrangements are very rare in MPNST, this case highlights the importance of genetic testing in MPNST, especially using an RNA panel for the detection of rare fusion genes.

Loss of H3K27 trimethylation in a distinct group of de-differentiated chordoma of the skull base.

De-differentiated chordoma is defined as a high-grade sarcoma lacking notochordal differentiation, which arises in association with conventional chordoma. The mechanism underlying de-differentiation remains unclear. We immunohistochemically investigated trimethylation at lysine 27 of histone 3 (H3K27me3) in nine de-differentiated chordomas. The tumours occurred at the skull base (n = 5) or the sacrum (n = 4) in four men and five women with a median age of 50 years. De-differentiation occurred de novo in four cases and at recurrence/metastasis in five cases. Five tumours retained H3K27me3, whereas four showed complete loss of H3K27me3 only in the de-differentiated component, while the conventional chordoma component retained H3K27me3. All the H3K27me3-negative tumours showed co-loss of dimethylation at H3K27 (H3K27me2), consistent with inactivation of polycomb repressive complex 2. Two genetically analysed H3K27me3-negative tumours harboured EED homozygous deletions. All four H3K27me3-negative de-differentiated chordomas affected the skull base of young or middle-aged women. Unlike dense proliferation of highly pleomorphic spindle or epithelioid cells in the H3K27me3-positive de-differentiated chordomas, all H3K27me3-negative tumours displayed swirling fascicles of relatively uniform spindle cells with alternating cellularity and perivascular accentuation, resembling malignant peripheral nerve sheath tumour (MPNST). Rhabdomyoblastic differentiation was present in one H3K27me3-negative tumour. We identified a novel group of de-differentiated chordomas in the skull base that lost H3K27me3/me2 only in the de-differentiated component, which was associated with EED homozygous deletion and MPNST-like histology. Our data suggest a distinct 'polycomb-type' de-differentiation pathway in chordoma, similar to a recently described de-differentiated chondrosarcoma with H3K27me3 loss.

Utility of Multimodality Approach Including Systemic FGF23 Venous Sampling in Localizing Phosphaturic Mesenchymal Tumors.

Context: Tumor-induced osteomalacia (TIO) is one of the most common forms of acquired fibroblast growth factor 23 (FGF23)-related hypophosphatemia and is usually caused by phosphaturic mesenchymal tumors (PMTs). Although the complete resection of PMTs can cure TIO, preoperative localization of tumors by standard imaging modalities is often challenging. In addition to 18F-fluoro-2-deoxy-D-glucose positron emission tomography-computed tomography (FDG-PET) and 111In-pentetreotide scintigraphy (SRS), systemic FGF23 venous sampling (FGF23VS) has been used to help localize PMTs in specialized institutions. Objective: This study aimed to evaluate the diagnostic performance of each imaging test and their combinations in localizing PMTs. Methods: In an observational retrospective study of patients with adult-onset FGF23-related osteomalacia who underwent all 3 imaging studies (FDG-PET, SRS, and FGF23VS), the rate of successful preoperative localization of the tumors was evaluated only in the patients with pathological diagnoses of PMTs, considering the possibility that pathogenesis of patients without identified tumors might be due to other causes such as late-onset hereditary FGF23-related hypophosphatemia. Results: A total of 30 Japanese patients with TIO (median age, 60 years [range, 28-87 years]; 10 women [33.3%]) were included in the study. The success rate of preoperative localization for each test and combinations of 2 or 3 tests among 18 patients with PMTs was as follows: 72% (FDG-PET), 72% (SRS), 94% (FGF23VS), 89% (FDG-PET, SRS), 100% (FDG-PET, FGF23VS), 94% (SRS, FGF23VS), and 100% (FDG-PET, SRS, and FGF23VS). Conclusion: We observed the highest localization rate of PMTs in patients with identified PMTs with the combination of FDG-PET and FGF23VS.

Giant filiform polyposis with high-grade dysplasia: A case report and review of the literature.

Introduction and importance: Filiform polyposis, a rare condition also referred to as inflammatory polyposis or pseudopolyposis, is commonly observed in cases of inflammatory bowel disease, such as ulcerative colitis or Crohn's disease. It is generally considered a benign tumour characterised by multiple finger-like projections that are mostly observed in the transverse and descending colon. Case presentation: A 69-year-old woman with a history of ulcerative colitis for 18 years who underwent temporary decompression ileostomy for large bowel obstruction at another hospital was referred to our institution for further investigation. Abdominal computed tomography revealed bowel wall thickening of the transverse colon, and colonoscopy revealed stenosis in the hepatic flexure obstructing the endoscope. Although several biopsies of the tumour showed no malignancy, laparoscopic subtotal colectomy with lymph node dissection was performed. Histopathological findings revealed localised filiform polyposis with dysplasia. Clinical discussion: Filiform polyposis has been considered a benign inflammatory polyp without any risk of dysplasia. We accumulated previous cases of giant filiform polyposis and reviewed their characteristics. The presented case of filiform polyposis with ulcerative colitis complicated with high-grade dysplasia highlights the importance of considering malignancy in patients with filiform polyposis. Conclusion: In cases of giant filiform polyposis, even when no malignancy is detected, surgical resection should be considered for the possibility of a malignant component of dysplasia.

Subcutaneous axillary primary epithelioid hemangioendothelioma: report of a rare case.

BACKGROUND: Epithelioid hemangioendothelioma (EHE) is a rare and slow-growing malignant vascular neoplasm composed of epithelioid endothelial cells within a distinctive myxohyaline stroma. It most commonly involves somatic soft tissue, lungs, liver and bone. Herein, we describe a case of EHE arising in the axillary region. CASE PRESENTATION: A 61-year-old man was under observation for multiple hepatic hemangiomas. Fluorodeoxyglucose-positron emission tomography/computed tomography showed specific uptake in a right axillary tumor. The patient was referred to our department for further investigation of the axillary tumor. An elastic-soft and poorly mobile tumor was palpable in the right axilla. Contrast-enhanced computed tomography showed a right axillary tumor and enlarged hepatic hemangiomas. In addition, multiple nodules in both lungs, a left renal angiomyolipoma, and left adrenal adenoma were revealed. Ultrasonography showed masses in both lobes of the thyroid gland, and a 30-mm lobulated hypoechoic mass in the axilla with well-defined and rough borders, showing internal heterogeneity. Fine-needle aspiration cytology was performed on the thyroid and axillary tumors: the thyroid tumor was class V, raising suspicion of papillary thyroid cancer (PTC); the left superior internal jugular node was class V, raising suspicion of metastasis of PTC; and the axillary tumor was class III, raising suspicion of a mesenchymal tumor with few epithelioid cells. The multiple lung nodules were diagnosed as metastatic tumors derived from thyroid cancer. We diagnosed these diseases as PTC of T1b(m)N1bM1(lung) Stage IVB and a right axillary tumor of unclear origin. However, it was assumed to be a primary mesenchymal tumor or a lymph node metastasis from lung cancer or occult breast cancer. We performed total thyroidectomy, left cervical lymph node dissection, and right axillary tumor excision. Histopathologic examination revealed the thyroid tumor as a PTC and the axillary tumor as an EHE. The EHE showed nuclear atypia, necrosis and high mitotic figures. Hence, it was considered to be a high-risk EHE. CONCLUSIONS: We experienced a rare primary subcutaneous axillary EHE with metastatic thyroid cancer in the lung. Since our case was classified as a high-risk EHE, a close follow-up would be appropriate.

A case of miliary tuberculosis following transurethral surgery and prostate biopsy after intravesical Bacillus Calmette-Guerin immunotherapy.

INTRODUCTION: Intravesical Bacillus Calmette-Guerin immunotherapy is known to prevent recurrence of bladder cancer, but it can cause tuberculosis infections as an adverse event. CASE PRESENTATION: A 75-year-old man visited our hospital due to hematuria. The patient was diagnosed with bladder cancer and underwent transurethral resection of the bladder tumor. Postoperatively, the patient received Bacillus Calmette-Guerin immunotherapy. One year later, we performed transurethral surgery and prostate biopsy because of cystoscopic findings showing nodulous lesions in the bladder and an elevated serum prostate-specific antigen level. The patient presented with high fever and malaise since the surgery. After careful examination, the patient was diagnosed with miliary tuberculosis caused by Mycobacterium bovis. The pathology of the bladder and prostate revealed acid-fast bacilli collection by Ziehl-Neelsen staining. CONCLUSION: The surgery exacerbated the local infection into a systemic infection. The risk of developing miliary tuberculosis should be considered at transurethral surgery or prostate biopsy in patients after intravesical Bacillus Calmette-Guerin immunotherapy.

Atypical Neurofibromatous Neoplasm with Uncertain Biologic Potential in the Posterior Mediastinum of a Young Patient with Neurofibromatosis Type 1: A Case Report.

Atypical neurofibromatous neoplasm with unknown biological potential (ANNUBP), proposed in a recent NIH consensus overview, is a rare precursor entity of malignant peripheral nerve sheath tumor (MPNST) in neurofibromatosis type 1 (NF1) patients. Only one report on imaging findings of ANNUBP is available. Herein, we present the case of a 19-year-old female, diagnosed with a mediastinal tumor by chance, who visited to our hospital. She had café-au-lait spots on her trunk and a past history of resected neurofibroma. Her family also had café-au-lait spots; therefore, an NF1-induced tumor was strongly suspected. MRI revealed a paravertebral mass of 7.5 cm in size consisting of an inner rim with low T2 signal intensity and an outer rim with high T2 intensity, which was similar to a target sign, adjacent to the pulmonary veins; the center of the tumor was well enhanced by gadolinium, and the peripheral region was myxoid and slightly enhanced. FDG-PET showed high FDG uptake, SUVmax of 8.5, although the peripheral region represented low FDG accumulation. CT-guided needle biopsy was repeated because of the suspicion of an MPNST, which resulted in the histopathological diagnosis of ANNUBP. Marginal tumor resection was performed, and the final post-resection histopathological diagnosis was ANNUBP transformed from neurofibroma; the region of ANNUBP lost p16 immunostaining, although it was retained in the peripheral region of the neurofibroma. There has been no recurrence or metastasis 1 year after treatment. In conclusion, ANNUBP could be represented as a well-enhanced homogeneous mass on MRI and a high FDG accumulated region on FDG PET/CT, as seen in MPNST, in NF1 patients.

Neuroimaging features of diffuse hemispheric glioma, H3 G34-mutant: A case series and systematic review.

BACKGROUND AND PURPOSE: Diffuse hemispheric gliomas, H3 G34-mutant (DHGs-G34m), are newly recognized malignant brain tumors characterized by histone gene mutations. However, the neuroradiologic characteristics of these tumors require elucidation. We reviewed the demographic, clinical, and neuroradiological features of DHGs-G34m. METHODS: Data were extracted using a database search in MEDLINE, SCOPUS, and Google Scholar in June 2021. Studies assessing pathologically proven DHGs-G34m with each patient's information and neuroradiological findings were included. After screening and reviewing 332 abstracts, 12 articles including 56 cases met the criteria. We also added the findings for three patients evaluated in our hospital. Two board-certified radiologists reviewed all demographic, clinical, and neuroradiological findings of each study. One board-certified pathologist reviewed all pathological data of each study. Kaplan-Meier analyses with log-rank tests were performed to compare the survival between patients with different tumor margin characteristics (well-delineated and ill-defined). RESULTS: The median patient age at diagnosis was 19 years (range, 6-66 years), and 31/59 patients (52.5%) were men. Supratentorial tumors were observed in all patients (59/59, 100%). Frequent contact with leptomeninges (92.3%) and ependymal regions (87.5%) was observed. The 1- and 2-year survival rates after initial surgery were 66.7% and 40.0%, respectively. DHGs-G34m with ill-defined and well-delineated margins showed significant differences in survival (p = .04). CONCLUSIONS: DHGs-G34m occur most often in the supratentorial regions of adolescents. Prognosis varies among patients. Evaluation of tumor margins may provide prognostic value.

Scapular Angiomatoid Fibrous Histiocytoma with EWSR1-CREB1 Fusion in an Adult Patient.

BACKGROUND: Angiomatoid fibrous histiocytoma (AFH) is a rare intermediate malignant tumor that arises mainly in soft tissues, especially in the superficial extremities of patients younger than 30 years. There have been a few reports of AFH arising from sites other than soft tissue, including bone, and unusual site and age make it difficult to diagnose this rare tumor. Case Presentation. Here, we present a case of a 54-year-old man who was examined for chest pain, and computed tomography (CT) incidentally detected a bone tumor at the scapula with destruction of cortical bone and invasion into soft tissue. Magnetic resonance imaging revealed multiple cystic components with fluid-fluid levels. FDG-PET showed uptake at the axillary lymph node. The CT-guided needle biopsy revealed spindle cell sarcoma on histopathology. After neoadjuvant chemotherapy, a scapulectomy was performed. The final postresection histopathological diagnosis was the same as the preoperative diagnosis, and no obvious chemotherapeutic effect was observed. Next-generation sequencing of RNA isolated from paraffin-embedded tumor tissue revealed that these lesions harbored the EWSR1-CREB1 fusion gene, and the tumor was diagnosed as AFH. C-reactive protein level, which was elevated preoperatively, decreased after the operation, and there was no recurrence or metastasis 5 years after the treatment. CONCLUSION: The diagnosis of AFH is difficult when the tumor has an atypical presentation. Comprehensive genomic analysis, especially RNA sequencing, is efficient in diagnosing this rare tumor. Moreover, magnetic resonance imaging findings identical to AFH in soft tissue, the presence of paraneoplastic symptoms such as elevated inflammatory markers, and lymph node swelling were clues towards suspecting this tumor.

Hemangioma of the Rib Mimicking Chondrosarcoma: A Report of Two Cases and Literature Review.

CASES: Case 1 was a 58-year-old man who presented with an incidentally detected, slowly growing mass in the right hypochondrium area. An imaging study showed the mass arising from the 11th rib, with ill-defined margins and cortical destruction. Differential diagnoses included chondrosarcoma and metastatic malignant tumor. Open biopsy was associated with moderate bleeding (300 mL) despite small incision. Microscopic findings showed numerous irregular, dilated, and thin-walled vessels, consistent with the diagnosis of hemangioma of bone, and en bloc excision was performed with no surgical complication. Case 2 was a 49-year-old man who presented with an incidentally detected 4th rib mass with calcification on computed tomography scan. Chondrosarcoma was suspected according to imaging features. An open biopsy was considered to have a risk of tumor seeding because the tumor was located behind the scapula. En bloc excision of the tumor without biopsy was performed. The pathological findings were consistent with hemangioma of bone. CONCLUSION: We reported two cases of rare hemangioma arising from the rib, which mimicked chondrosarcoma. The preoperative diagnosis was challenging, both clinically and radiologically. Because biopsy for hemangioma of the rib is associated with a bleeding risk, the en bloc excision without biopsy can be a practical treatment option.

Skeletal EWSR1-NFATC2 sarcoma previously diagnosed as Ewing-like adamantinoma: A case report and literature review emphasizing its unique radiological features.

Ewing-like adamantinoma (EAD) is a rare bone tumor. It remains unclear whether EAD belongs to adamantinoma, Ewing sarcoma (ES), or an independent category. Herein, we present a case of femoral sarcoma previously diagnosed as EAD in a 26-year-old woman. We observed amplified EWSR1 and NFATC2 fusion signals using fluorescence in situ hybridization. Prompted by its unique radiological features, we reviewed the current literature on skeletal EWSR1-NFATC2 sarcoma (ENS) and EAD. In addition to the similar histological features, we found that both ENS and EAD displayed similar characteristic radiological features, such as the tendency to occur in the diaphysis of long bones, cortical expansion and buttressing-type thickening, and bone surface involvement with saucer-like erosion without cortical destruction. We believe that these unique radiological features were related to its indolent behavior. Altogether, it is possible that previously reported EAD cases may be neither ES nor the classic adamantinoma but ENS. Further studies are needed to clarify the relationship between EAD and ENS.

Fat-forming solitary fibrous tumor of the sacrum: A case report and literature review.

Fat-forming variant of solitary fibrous tumor (SFT) is a rare mesenchymal neoplasm. Here we report the case of a 33-year-old woman who developed pain and muscle weakness from the posterior aspect of the right hip to lower extremity. Imaging examinations revealed a mass with fatty components and hypervascular solid components filling the sacral spinal canal and sacral foramen. The sacral mass was resected and histological examination of the specimens revealed patternless proliferation of short spindle-shaped cells with staghorn blood vessels. A number of mature adipocyte-like cells were also observed. The tumor cells were positive for STAT6 and the nuclei of the adipocytes were also positive, which was diagnostic for fat-forming SFT.

Clinicopathologic Characterization of Epithelioid Hemangioendothelioma in a Series of 62 Cases: A Proposal of Risk Stratification and Identification of a Synaptophysin-positive Aggressive Subset.

Epithelioid hemangioendothelioma (EHE) is a rare vascular endothelial neoplasm with characteristic histology and distinctive fusion genes. Its clinical presentation and outcome are heterogeneous, and the determinants of survival are controversial. In this study, we aimed to identify clinicopathologic prognostic factors of EHE in a retrospective cohort of 62 cases with CAMTA1/TFE3/WWTR1 alterations. The tumors were of the CAMTA1 subtype for 59 cases, TFE3 subtype for 2 cases, and variant WWTR1 subtype (WWTR1-ACTL6A) for 1 case. Twenty-two tumors (35.5%) demonstrated atypical histology, defined by having at least 2 of the following 3 findings: high mitotic activity (>1/2 mm2), high nuclear grade, and coagulative necrosis. During a median follow-up of 34 months, 11 patients (18%) died, and the 5-year overall survival rate was 78.8%. Survival did not correlate with such clinical parameters as age, sex, tumor sites, multifocality, and multiorgan involvement. Conversely, based on both univariate and multivariate analyses, large tumor size (>30 mm) and histologic atypia were significantly associated with a shorter survival. A proposed 3-tiered risk assessment system using these 2 parameters significantly stratified the patients into low-risk, intermediate-risk, and high-risk groups with 5-year overall survival rates of 100%, 81.8%, and 16.9%, respectively (P<0.001). Four tumors (6.4%) expressed synaptophysin, which all belonged to the high-risk group and pursued an aggressive course. The present study demonstrated the independent prognostic significance of large tumor size and atypical histology in EHE, as well as the value of risk stratification using these 2 factors. Moreover, we revealed a small EHE subset with aberrant synaptophysin expression, which may have potential prognostic and diagnostic implications.

An autopsy case of prostatic rhabdomyosarcoma with DICER1 hotspot mutation.

Somatic hotspot DICER1 mutations, which frequently coexist with germline inactivating mutation (i.e., DICER1 syndrome), have been identified in various types of benign and malignant conditions. Herein, we report an autopsy case of prostatic rhabdomyosarcoma (RMS) with a hotspot DICER1 c.5125G>A (p.D1709N) mutation. A 26 year-old man presented with a prostatic mass, hematuria, and urinary retention. He underwent total pelvic exenteration, colostomy, ileal conduit construction and partial urethrectomy. Five months postoperatively, he developed multiple metastases to the lungs, brain, iliopsoas muscles and bones. He died of respiratory failure, and autopsy was performed. Microscopically, the tumor was primarily composed of uniform primitive mesenchymal cells infiltrating to the prostate with cambium layer. Rhabdomyoblasts and anaplastic cells were focally observed. Immunohistochemically, tumor cells were positive for desmin, myogenin, PAX7, HMGA2. Multinodular goiter was detected at autopsy. Because the morphology is similar to pleuropulmonary blastoma and DICER1-mutant RMS of the female genital tract, we tested and identified a hotspot DICER1 mutation with Sanger sequencing. Recognizing DICER1-mutant tumor is important because of its frequent association with germline DICER1 inactivation and potential therapeutic implication. Further research is needed to clarify whether this case can be classified as embryonal RMS with anaplasia or 'DICER1-associated sarcoma'.

Recurrent FOS rearrangement in proliferative fasciitis/proliferative myositis.

Proliferative fasciitis (PF) and proliferative myositis (PM) are rare benign soft tissue lesions, usually affecting the extremities of middle-aged or older adults. Presenting as poorly circumscribed masses, they histologically show bland spindle cell proliferation in a myxoid to fibrous background and a hallmark component of large epithelioid "ganglion-like" cells in various numbers, which may lead to their misdiagnosis as sarcoma. PF/PM has been long considered as reactive, akin to nodular fasciitis; however, its pathogenesis has remained unknown. In this study, we analyzed the FOS status in 6 PF/PMs (5 PFs and 1 PM). Five PF/PMs occurred in adults, all showing diffuse strong expression of c-FOS primarily in the epithelioid cells, whereas spindle cell components were largely negative. Using fluorescence in situ hybridization (FISH), all 5 c-FOS-immunopositive tumors showed evidence of FOS gene rearrangement in the epithelioid cells. RNA sequencing in 1 case detected a FOS-VIM fusion transcript, which was subsequently validated by reverse transcriptase-polymerase chain reaction, Sanger sequencing, and VIM FISH. The one pediatric PF case lacked c-FOS expression and FOS rearrangement. c-FOS immunohistochemistry was negative in 45 cases of selected mesenchymal tumor types with epithelioid components that may histologically mimic PF/PM, including pleomorphic sarcoma with epithelioid features and epithelioid sarcoma. Recurrent FOS rearrangement and c-FOS overexpression in PF/PM suggested these lesions to be neoplastic. FOS abnormality was largely restricted to the epithelioid cell population, clarifying the histological composition of at least 2 different cell types. c-FOS immunohistochemistry may serve as a useful adjunct to accurately distinguish PF/PM from mimics.