Key Clinical Message:Masson’s tumor is a rare condition that can mimic malignancy, particularly when located in atypical regions. This case emphasizes the importance of considering this diagnosis in unusual anatomical locations and highlights the critical role of histopathological evaluation in achieving an accurate diagnosis. Prompt identification and surgical management can lead to favorable outcomes, even in rare presentations.Introduction:Masson’s tumor, also known as intravascular papillary endothelial hyperplasia (IPEH), is a benign non-neoplastic lesion, first described in 1923 under the name of “vegetant intravascular hemangioendothelioma” [1]. Initially believed to be an atypical proliferation of papillary endothelial cells, it was reclassified as a reactive process in 1932 by Henschen. This tumor is rare, and accounts for 2% of benign and malignant vascular tumors of the skin and subcutaneous tissues [2].This case highlights a Masson tumor, a vascular tumor arising in the lower extremities, a region where such tumors are less commonly observed compared to their typical occurrence in the head and neck area.Case History/ExaminationA 49-year-old male hairdresser presented to an outpatient clinic in Beirut, Lebanon, with a chief complaint of a lesion on the right posterior lower extremity, which was present for the past 3 years. The lesion had grown recently, over the last 6 months and was not painful to touch. The patient’s occupation required prolonged periods of standing.On examination, a large, soft, well-demarcated subcutaneous nodule measuring approximately 4 x 4 cm in size was noted at the site (fig1). The lesion was non-tender to palpation. The patient had no significant past medical history, surgical history, or reported allergies. Family history included pilar cysts in the patient’s mother and brother.MethodsDifferential DiagnosisThe patient presented with a subcutaneous nodule with a benign clinical appearance, suggestive of a common soft tissue lesion such as a lipoma. No additional symptoms or concerning features, such as rapid growth, pain, or skin changes, were observed. Based on the clinical presentation, the differential diagnosis primarily included lipoma, epidermoid cyst, or a benign vascular lesion.InvestigationsDue to financial limitations, advanced imaging or biopsy was not performed prior to excision. The clinical evaluation supported the decision to proceed with direct surgical excision under the working assumption of a lipoma.Post-excision, the gross examination revealed a well-demarcated vascular nodule, located superficial to the subcutaneous tissue, comprising three lobules encapsulated within fibrous bands. Histologic examination of the excised tissue revealed a dilated vessel within the superficial subcutaneous tissue, showing intravascular proliferation of reactive endothelial cells. The endothelial cells formed numerous papillary structures lined by a single layer of plump endothelial cells. The lesion was enclosed within a fibrous pseudo capsule, supporting the diagnosis of a benign vascular tumor.TreatmentThe nodule was excised in its entirety during the initial surgical procedure. Complete surgical excision was achieved, as confirmed by histopathology, with no evidence of residual lesion or malignancy.Conclusion and results:Figures 2 and 3 show the histological results. The surgical approach and prognosis were explained to the patient. Informed consent was obtained for the use of his photos and clinical information in this case report.Following complete surgical excision, the patient experienced an uneventful recovery with no signs of recurrence at follow-up. Dermatologists and surgeons should maintain a high index of suspicion for IPEH when evaluating vascular tumors to optimize patient outcomes.Intravascular papillary endothelial hyperplasia (IPEH) is a rare but benign vascular lesion that poses diagnostic challenges, particularly in the clinical and pre-operative settings. Accurate diagnosis relies heavily on histopathological evaluation, as clinical presentations often mimic other soft tissue lesions. This case underscores the importance of considering IPEH in the differential diagnosis of vascular tumors to ensure timely and appropriate management.Discussion:This tumor has several names, including Masson’s hemangioma, Masson intravascular hemangio-endothelioma, intravascular papillary endothelial hyperplasia (IPEH), and reactive papillary endothelial hyperplasia [3].The precise etiopathogenesis remains unclear, though it is widely regarded as a reactive process believed to result from growth factors production, leading to excessive endothelial cell proliferation [4]. In fact, research employing northern blot and immunoblotting revealed a significant increase in the expression of fibroblast growth factor beta, suggesting its secretion by the endothelial cells through an autocrine mechanism. The condition is thought to arise from an aberrant resolution of a thrombosis, unfolding through several stages [5].It typically manifests as a firm or soft, reddish-blue color nodule over the head and neck and upper extremities [6]. Occurrences on the lower extremities, such as in our case, are rare. Generally, it is asymptomatic, slow-growing and is not associated with any pulsation. The sonographic features of subcutaneous IPEH are not specific; however, consideration of the diagnosis is warranted when encountering a soft tissue mass showing a vascular pattern. This entity has a slight predilection for adult females, and IPEH can also manifest in children [7].Diagnosis mainly relies on histology since imaging such as MRI or US cannot differentiate it from other vascular lesions. Histologically, it is characterized by the presence of papillary structures lined by a single layer of endothelial cells. There will be absence of pleomorphism, mitosis or necrosis. It has been classified into three main types. The first type is the pure form, where the endothelial proliferation occurs from veins and less commonly artery without any vascular abnormalities. The second type arises from a pre-existing vascular abnormalities such as an aneurysm [5], arteriovenous malformations, hemangiomas and pyogenic granulomatosis. The third and least common type arises from an extravascular hematoma.These tumors stain positive for CD31 and CD34 [8]. Staining positivity for factor VIII, type 4 collagen and SMA and MSA varies and is not as consistent.Clinically, IPEH can mimic a variety of conditions, both benign and malignant such as pyogenic granuloma and Kaposi’s sarcoma. Moreover, it should be distinguished from malignant angiosarcoma, a differentiation typically achieved through histology and immunohistochemical staining. The primary characteristic of IPEH is its intravascular development, in contrast to angiosarcoma, which rarely remains confined within the vascular lumen. Also, CD105 is a useful immunohistochemical stain, as angiosarcoma would stain positive while IPEH doesn’t [8].Since IPEH can be mistaken for angiosarcoma and other malignant tumors, surgical excision is the treatment of choice. Since these tumors rarely occur de novo within extravascular hematomas, meticulous surgical techniques should be utilized during resection to prevent any spillage of the aneurysm-associated thrombus, given the theoretical risk of extravascular recurrence. Other alternative options include observation or using sclerosing agents such as sodium tetradecyl sulfate [9]. Chemotherapy or radiotherapy may be considered for cases of recurrence following incomplete excision, as well as for multiple intracranial lesions or suspected lesions[10].Ethical Statement:the patient in this manuscript has given written informed consent for participation in the study and the use of their de-identified, anonymized, aggregated data and their case details (including photographs) for publication.Author ContributionMariana El Hawa (1st author): Conceptualization and study design, data analysis, manuscript drafting, and critical revision.Serena Saade: Critical review of histopathological findings and drafting specific sections of the manuscript.William Abou Shahla: literature review and drafting specific sections of the manuscript.Dana Saade: Critical review of the whole manuscript and contribution to the discussion section.References:1. Masson, P., Hemangioendotheliome vegetant intravasculaire. Bull.Soc.Anat.Paris, 1923. 93 : p. 517-523.2. Fasina, O., A. Adeoye, and E. Akang, Orbital intravascular papillary endothelial hyperplasia in a Nigerian child: a case report and review of the literature. J Med Case Rep, 2012. 6 : p. 300.3. Benantar, L., et al., Peri-orbital Masson’s tumor: a case report and literature review. Pan Afr Med J, 2022. 42 : p. 147.4. Sasso, S.E., et al., Masson’s tumor (intravascular papillary endothelial hyperplasia). An Bras Dermatol, 2019. 94 (5): p. 620-621.5. Chapman, S.C., et al., Masson tumor (intravascular papillary endothelial hyperplasia) arising in a superficial temporal artery aneurysm. J Vasc Surg Cases Innov Tech, 2019. 5 (3): p. 388-391.6. Kim, O.H., et al., Subcutaneous intravascular papillary endothelial hyperplasia: ultrasound features and pathological correlation. Skeletal Radiol, 2016. 45 (2): p. 227-33.7. Huang, W.P., et al., Intravascular papillary endothelial hyperplasia of the spleen in a child: a case report. BMC Med Imaging, 2022.22 (1): p. 207.8. Akdur, N.C., et al., Intravascular papillary endothelial hyperplasia: histomorphological and immunohistochemical features. Diagn Pathol, 2013. 8 : p. 167.9. De Maria, L., et al., Sclerotherapy for Venous Malformations of Head and Neck: Systematic Review and Meta-Analysis. Neurointervention, 2020. 15 (1): p. 4-17.10. Dryden, S.C., et al.,Intravascular Papillary Endothelial Hyperplasia of the Orbit: A Case of Masson’s Tumor. Cureus, 2019. 11 (12): p. e6266.