Abstract: Intravascular papillary endothelial hyperplasia (IPEH), also known as Masson tumor, is a rare vascular benign tumor of blood vessels. It may occur in any part of the body, especially the deep dermis and subcutaneous tissue of the head, neck, fingers and trunk. The imaging and histopathology of IPEH are similar to hemangiosarcoma, especially in the case of active vascular endothelial hyperplasia. IPEH is a reactive proliferative lesion of vascular intima. The etiology is still unclear. After some studies showed that IPEH was a benign lesion, few reports on the etiology of it were reported. IPEH is usually limited to the thrombotic vessels or lumens of vascular malformations, usually accompanied by a clear history of trauma. IPEH usually does not cause any symptoms. It looks like a slow-growing lump. Some cases have been reported with pain and swelling. Although IPEH is relatively rare, its accurate diagnosis is crucial because it may be similar to malignant angiogenic lesions in clinical practice. There were few reports of cases related to intravascular papillary endothelial hyperplasia located in the sternocleidomastoid muscle after reviewing the domestic and foreign literature in recent 10 years. This case reports that a young male, who was admitted to the hospital one month after finding a subcutaneous tumor in the left neck. After admittance, relevant preoperative examinations were completed. After multi-disciplinary discussion and elimination of surgical contraindications, a specific surgical plan was formulated. The tumor was removed under local anesthesia on the second day after admission. During the operation, it was found that the tumor was located between the sternocleidomastoid muscle bundles, and it was sent for pathologic examination. Paraffin section pathology was reported after operation. Histological examination showed that the morphology was consistent with vascular endothelial papillary hyperplasia. There were no related surgical complications and recurrence in the 3-month follow-up. The purpose of this paper is to provide clinicians with a certain understanding of this rare disease through the report of this case of IPEH, and to identify it in later clinical work, and at the same time, to avoid confusion with malignant diseases, such as hemangiosarcoma, leading to unnecessary treatment and increase the cost of treatment.

Key words: Hemangioendothelioma, Hyperplasia, Hemangiosarcoma, Differential diagnosis, Intravascular papillary endothelial hyperplasia