This case report refers an additional case to the 10 previous reported cases of myointimoma in children and adolescents [1, 3,4,5], (Table 1). In our case, like in all previously reported cases in adolescents and adults [1, 6,7,8,9,10], the myointimoma affects uniquely the glans penis. Likewise, no reported case was associated with pain, dysuria or signs of lower urinary tract obstruction. In our case, the patient reported a relatively fast-growing mass. The history of initial rapid growth is common in the literature; later however, the formation may remain stable over time. Monsalves  described a case of myointimoma that remained unchanged 10 months after an incomplete excision. Fetsch  described the same experience with a 6-month stable residual mass in a patient after an incisional biopsy. In one case, complete regression of myointimoma at 10-years follow-up was described . Local aggressive growth or distant metastases were never reported.
There are currently no guidelines describing the extent of imaging in adolescents with penile tumors. The existing literature does not deal with the scope of imaging; both existing series [1, 3] of cases are based on a retrospective re-evaluation of stored hematoxylin and eosin-stained slides of penile tumors over the last few decades only. Therefore, we adhered to EAU guidelines for penile cancer in the adults and performed penile Doppler Ultrasonography and MRI to exclude corporal invasion. The examinations confirmed the solid nature of the tumor, excluded cystic lesion and multiple involvement of cavernosal tissue. In a case of penile tumor in adolescents, the main concern was to exclude clinically aggressive conditions, thus an excisional biopsy was decided. Since the boy was confirmed to have benign findings on histopathological examination and had clinically normal findings on the inguinal nodes, we did not perform staging (abdominal, pelvic and thoracic CT).
The diagnosis of myointimoma and its differential diagnosis based on morphology only may be confusing. There are several types of mesenchymal tumors with plexiform or nodular structure. Immunohistochemistry is a key to exact diagnosis. Myointimomas always express alpha-smooth muscle actin (αSMA). Desmin may be absent or show only focal reactivity. There is no reactivity for S-100 protein, CD31, CD34, ERG, epithelial membrane antigen (EMA) or neuron specific enolase (NSE). The plexiform growth pattern can be found in plexiform histiocytic tumor (PFHT) . Unlike myointimoma, it contains a mixture of two components: a differentiated spindle fibroblastic/myofibroblastic cells and a round histiocytic cell component containing multinucleated giant cells (osteoclast-like giant cells). Immunohistochemically, the histiocytes and multinucleated giant cell express CD68, whereas the spindle cells express αSMA. PFHT may recur and has a low risk of metastases (lymph node, lung). A plexiform or nodular growth pattern can we see in some nerve sheet tumors such as plexiform schwannoma  or neurofibroma. Immunohistochemical expression for S-100 protein is then helpful in differential diagnosis.
The myointimoma structure may resemble myofibroma, a more common tumor in children. In contrast, it does not exhibit the exclusive intravascular growth; the growth is rather concentric around the small vessels. The tumor is composed of oval or spindle myoid cells . Myopericytomas characterized by a distinctive biphasic growth pattern, with central hypercellular zone composed of spindle tumor cells, hyalinization and myoid cell nodules visible towards the periphery of the tumor. In contrast to myofibroma, intravascular growth is more common in myopericytoma, but it does not indicate a malignant neoplatic process .
Epithelioid hemangioma and hemangioendothelioma can be distinguished from myointimoma by immunostaining as the endothelial nature of the lesional cells can be confirmed by CD31, CD34 and ERG positivity. Another structurally similar pathology is a late phase of intravascular fasciitis (intravascular nodular fasciitis). Histologically, intralesional inflammatory cells between spindle myofibroblast cells, mucoid pools, a less compact stroma with more eosinophilic hyalinization, and obvious mitotic figures were observed . Intravascular spindle cells lesion such as intravascular leiomyoma or leiomyomatosis can be easily distinguished by immunohistochemistry, with αSMA, desmin and h-caldesmon antibodies, which are typically strongly positive. Last but not least, the possibility of sarcoma with angioinvasive spread must be ruled out at the histological examination.
Both clinician and pathologist must be aware of this rare benign entity. The key to a correct diagnosis is a careful histological examination of the specimen, including immunohistochemistry. Local excision is safe and effective treatment modality.