In 1988, Santa Cruz and Aronberg  first described targetoid hemosiderotic hemangioma (THH), which is a benign vascular neoplasm that some have postulated may represent a reactive condition.  Also termed hobnail hemangioma, targetoid hemosiderotic hemangioma typically presents as a solitary papule affecting the limbs or trunk of young or middle-aged people. Recognition of characteristic features is important to avoid misdiagnosis, as clinically and histopathologically, targetoid hemosiderotic hemangioma can mimic disorders such as melanoma, Kaposi sarcoma (KS), and angiosarcoma. Note the images below. Because the histology of targetoid hemosiderotic hemangioma is reproducible, its clinical features variable, and the hobnail phenomenon not specific, some authors favor the designation of "superficial hemosiderotic lymphovascular malformation" instead of HH or targetoid hemosiderotic hemangioma. 
The exact pathogenesis of targetoid hemosiderotic hemangioma (THH) is unknown, but some authorities have postulated trauma to lymphatic vessels with development of lymphatic-vascular microshunts play a key role in the pathogenesis of the defining features of targetoid hemosiderotic hemangioma: dilated vascular spaces with hobnailed endothelial cells, extravasation of red blood cells and hemosiderin deposits, fibrosis, and inflammation. Recent studies have demonstrated that THH vessels are lymphatic in origin. The absence of Wilms tumor 1 antigen expression supports a malformative rather than neoplastic etiology. [4, 5]
One theory states that targetoid hemosiderotic hemangioma is the result of trauma to a preexisting hemangioma that subsequently is altered by thrombosis and recanalization; however, reports fail to document a preexisting vascular tumor, such as lobular capillary hemangioma or congenital vascular malformation, in the 100-plus cases reported to date. However, a 2015 case series by AbuHilal M et al reported three congenital pediatric cases or 3 (50%) of 6. 
A second theory suggests that trauma disrupts vasomotor enervation. This leads to an inability to regulate blood flow and results in progressive vascular ectasia, which is characteristic of targetoid hemosiderotic hemangioma. This scenario also has been postulated as a mechanism contributing to the development of acquired port-wine stains. No documented aberration in nerve fiber density in targetoid hemosiderotic hemangioma exists to support this theory.
A third theory proposes that trauma obstructs or destroys draining lymphatics or veins. This leads to proximal compensatory vasodilation and subsequent formation of a hemolymphangioma. This theory is supported by microscopic findings, such as fibrosis and inflammation (scar formation), extravasation of red blood cells and hemosiderin deposition (bruising), telangiectases (secondary to venous bed blockage or destruction), and lymphangiectases (consequence of destruction of lymphatic drainage). Expression of vascular endothelial cell growth factor receptor 3 (VEGFR-3) and D2-40, lymphatic endothelial cell markers, by targetoid hemosiderotic hemangioma endothelial cells confirms the primarily lymphatic origin of targetoid hemosiderotic hemangioma. 
These studies examining vessel origin have revealed microshunts between lymphatic channels and small blood vessels that may well explain many of the histologic features of targetoid hemosiderotic hemangioma, such as aneurysmatic microstructures (telangiectases/lymphangiectases) and erythrocytes within histologically apparent lymphatic spaces. The hemolymphatic nature of targetoid hemosiderotic hemangioma would also explain the clinicopathologic overlap with solitary angiokeratoma, an acquired vascular tumor that is also suspected to arise secondary to trauma and is characterized by superficial dermal dilated vessels (telangiectases).
No clear indication exists that targetoid hemosiderotic hemangioma is a true neoplastic process. Incomplete biopsies and complete excisions typically have not been followed by recurrence. To the contrary, reports of waxing and waning lesions suggest a reactive nature. In some cases, this may be the result of the hormonal influence on vasomotor stability that occurs during pregnancy and the menstrual cycle. [8, 9, 10, 11] Spontaneous resolution can be followed by recurrence and is not associated with any scarring. [5, 12, 13]
Prevalence is unknown. Targetoid hemosiderotic hemangioma represents less than 0.1% of solitary pigmented lesions presenting to the Albany Medical College pigmented lesion clinic. From a dermatopathologic perspective, the Albany Medical College Dermatopathology service identified 33 cases out of more than 90,000 (0.1%) dermatopathologic accessions over a 3-year period.
Global incidence of targetoid hemosiderotic hemangioma is unknown but presumably is similar to that observed in the United States.
No known racial associations exist, although THH has been reported only in whites.
Males appear to be affected more frequently by targetoid hemosiderotic hemangioma than females (male-to-female ratio of 1.4:1). Episodically changing targetoid hemosiderotic hemangiomas have been described more frequently in females than in males.
In targetoid hemosiderotic hemangioma patient age varies from 5-72 years, and most patients present during their 20s and 30s.
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