A Solitary Asymptomatic Tumor on the Knee.

CASE REPORT

A 67-year-old male patient suffering from diabetes and chronic renal failure consulted our outpatient department of dermatology for the onset of a 6-month asymptomatic cutaneous nodular lesion on the right knee. Dermatological examination revealed the presence of a dome-shaped, skin-colored, firm noninfiltrated nodule measuring 2 cm in diameter, located on the lateral aspect of the right knee [Figure 1]. The tumor was excised under local anesthesia. The histopathological study revealed these aspects [Figures 2–4].

Figure 1

A firm skin-colored sessile nodule on the right aspect of the knee

Figure 2

Multiple cords and islands of basaloid cells set in a cellular stroma (H and E, ×100)

Figure 4

Monomorphic basaloid cells with numerous mitosis (H and E, ×400)

Nests showing an abortive of hair follicle differentiation (H and E, ×200)

What is your diagnosis?

Diagnosis Trichoblastoma.

DISCUSSION

Trichoblastoma (TB) is a rare, benign skin tumor of rudimentary hair follicles.[1] It was first introduced by Headington.[2] Different clinical appearances are reported.[2] It can range from a skin-colored papule to the large nodular lesion, as well as our case[2] or more rarely as an infiltrative plaque.[3] It occurs on any hair follicle-bearing location.[3] However, the preferred sites are the head, neck, and more rarely trunk and proximal extremities.[3] Some cases of TB are reported with a preferential site location in the perianal area, buttocks, groin, and upper thigh.[2345] As far as, we could ascertain to date, no previous case of TB located on the knee has been reported yet. TB can arise at any age.[3] However, it seems to be more common in adults (fifth to seventh decades),[3] as well as our patient. Moreover, there is no sex predilection.[3] In the majority of cases, the tumor is >2 cm in size.[23] However, there are some reported cases of giant TBs reaching several centimeters (up to 10 cm).[23] Some cutaneous neoplasma can be confused with TB especially basal cell caricoma. Thus, the importance of the histopathological study to confirm the diagnosis. Based on the Ackerman's classification.[2] We distinguish multiple types of TB depending on the predominant architectural feature. Hence, TB can be categorized as large nodular, small nodular, retiform, cribriform, racemiform, or columnar and adamantinoid.[12] However, the unifying histopathologic characteristic among all of these types is the presence of well-circumscribed, symmetrical dermal tumors composed of lobules of basaloid cells with peripheral palisading of cells, with no epidermal connection.[123] A moderate mitotic activity can be noted.[2] Central necrosis can be seen within larger nodules.[2] TB may also be focally or widely pigmented with melanin and immunostains.[2] The most important differential diagnosis is the basal cell carcinoma (BCC) because treatments and prognosis are different. Therefore, there are some histological criteria that are for the diagnosis of TB such as the lack of epidermal origin, more conspicuous stroma with prominent papillary mesenchymal bodies, and absence of retraction artefact between the tumor epithelium and stroma.[23] In some cases, it is not easy to differentiate between these two diagnoses. Thus, some authors suggest the role of the immunohistochemistry to enhance the histopathology study.[6] They propose the use of Ki-67, Cytokeratins 6, and 34BE12 antibodies as diagnostic aids to differentiate between TB and BCC, Ki-67 and CK6 are expressed in BCC and 34BE12 stains intensely in BCC and mildly positively in TB.[6] Although TB are benign tumors, few cases of recurrent TB and transformation to a frank carcinoma after many years of quiescence[57] are reported in the literature. In view of these rare cases of local recurrence and malignant transformation while simple excision is curative in benign TB, as was the case of our patient with 1-year period of follow-up, complete excision, with narrow margins, should be the preferred treatment for any mitotically active TB with long-term follow-up.[23]

In conclusion, we emphasize through our original presentation the clinical variety of this benign tumor, and we report an atypical anatomical site, which has not been reported yet through literature. Thus, the histological examination is primordial in these cases to have the right diagnosis.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.