Pilomatrixoma - Presented as Hypopigmented Tender Nodule: Diagnosed by FNAC: A Case Report with Review of Literature.

Pilomatrixoma (PMX) is a skin appendage tumor of hair matrix origin, which usually occurs on the face or upper extremities. Although the lesion can appear at any age, it is commonly seen in children and is more common in females. Despite being better defined, pilomatricomas continue to be frequently misdiagnosed and are not usually considered in differential diagnoses, either in clinical set-up or during cytological reporting. They typically present as a superficial, firm, solitary, slow-growing, painless mass in the dermis. The overlying skin may be normal or exhibit a bluish-red discoloration or ulceration. We report an 18-year-old girl presented with tender, subcutaneous nodule with overlying skin showing atrophy and hypopigmentation. Clinically, it was diagnosed as neurofibroma and sent for FNAC. We offered precise diagnosis of pilomatrixoma on cytological examination, (where chances of wrong diagnosis are very high) and it was subsequently confirmed by histopathology. We discuss the varied clinical presentations, diagnostic difficulties, and differential diagnoses of PMX.

What was known?

Pilomatrixoma-The unambiguous preoperative diagnosis is challenging; both clinically and cytologically. There are many cytology case reports in the literature where PMX is misdiagnosed as malignant tumor.

PMX usually presents as painless tumor covered by either normal skin or skin with bluish red discoloration making a clinical probable diagnosis as hemangiomas.

Introduction

Pilomatrixoma (PMX), also known as calcifying epithelioma of Malherbe, is a skin appendage tumor of hair matrix origin, which usually occurs on the face or upper extremities.[1] It presents as a solitary, slow growing dermal or subcutaneous nodule and is rarely diagnosed clinically. Although histologic features of this lesion are well recognized, pathologists continue to encounter difficulties in diagnosis on aspiration cytology. Problems arise mainly when the yield contains numerous keratinized squamous cells with scarcity of basaloid and shadow cells. The diagnostic triad of basaloid cells, ghost or shadow cells, and foreign body giant cells need not necessarily be present in all the cases.[2]

We are reporting a case presenting as tender, subcutaneous nodule with overlying skin showing atrophy and hypopigmentation. Clinically, it was diagnosed as neurofibroma, probably due to its firm consistency and tenderness and was sent for FNAC. We offered precise diagnosis of pilomatrixoma on cytological examination, (where chances of wrong diagnosis are very high), and it was subsequently confirmed by histopathology.

Case Report

An 18-year-old girl presented to the surgical outpatient department with complaints of tender swelling over left mid-arm of one-year duration. The swelling gradually increased in size to attain present size. We received requisition for cytological examination with clinical probable diagnosis of neurofibroma. On inspection, swelling measured 3 × 2 cm, seen over left arm with overlying skin showing atrophic changes and hypopigmentation, giving an appearance of a small patch of vitiligo [Figure 1]. There was no history of trauma. There was also no history of topical application of any steroids or other medications. On palpation, swelling was firm to hard, tender, mobile, and situated in the subcutaneous plane.

Figure 1

Swelling over left arm with overlying skin showing atrophic changes and hypopigmentation with vitiligo-like appearance

FNA was carried out, and all slides were issued to junior and senior faculties in the department of pathology for their independent opinion and discussion. Different diagnoses were obtained like giant cell tumor of tendon sheath, epidermal inclusion cyst with giant cell reaction, granulomatous inflammatory lesion, basi-squamous carcinoma, and pilomatricoma were obtained. The smears studied revealed cell-rich aspirate containing predominantly basaloid cells in tight clusters, scattered squamous cells and anucleate squames, shadow cells, plenty of multinucleated giant cells [Figure 2], and focal areas of calcification. In view of presence of all the diagnostic criteria required to diagnosis pilomatrixoma were present, we offered final diagnosis of PMX and requested for excision and histopathological confirmation. The tumor was excised and sent.

Figure 2

FNAC smear showing basaloid cells, anucleate squames, shadow cells, and multinucleated giant cells (Pap stain, ×10 magnification)

Grossly, the tumor measured 3 × 2 cm, well-encapsulated, nodular, with surface showing multiple chalky white powdery deposits scattered throughout [Figure 3]. The tumor was gritty to cut, showed powdery, chalky white areas of calcification. Slices were decalcified in decalcifying solution, and sections were prepared and stained with hematoxylin and eosin. Histopathological examination revealed encapsulated tumor mass, composed of nests of peripheral basaloid cells admixed with squamous cells showing transition phase to anucleate squamous cells and then to ghost cells [Figures 4 and 5]. The stroma showed multinucleated foreign body type of giant cell response [Figure 6] to the tumor keratin. Focal areas of calcifications were also seen. All the features correlated with cytological findings, and diagnosis of PMX was issued. The patient is asymptomatic during post-operative follow up with no local recurrence.

Figure 3

Gross specimen: Well-encapsulated tumor with nodular surface showing multiple chalky white powdery deposits

Figure 4

Histopathology section showing nests of peripheral basaloid cells admixed with squamous cells (H and E stain, ×10 magnifications)

Figure 5

Microscopy showing transition phase where basaloid cells are converting into anucleate squamous cells and then to ghost cells (H and E stain, ×40 magnification)

Figure 6

Microscopy showing multinucleated foreign body giant cell response of the stroma to tumor keratin (H and E stain, ×40 magnifications)

Discussion

Pilomatricoma, a benign neoplasm of the hair follicle, was initially thought to arise from sebaceous glands and was called calcifying epithelioma of Malherbe by Malherbe and Chenantais in 1880. In 1961, after histochemical and electron microscopic analysis of 228 such tumors, Forbis and Helwig found the cell of origin to be the outer root sheath cell of the hair follicle and proposed the name, pilomatrixoma, now called pilomatricoma. In 1973, Moehlenbeck reviewed 140,000 skin tumors and noted that pilomatricoma represented 0.12% of cases. Although pilomatricoma can develop in patients of any age, it occurs most often in children and young adults.[3]

Despite being better defined, pilomatricomas continue to be frequently misdiagnosed and are not usually considered in differential diagnoses. The overlying skin changes described in many cases of pilomatricoma include bluish-red discoloration that may confuse the clinician for wrong interpretation of these tumors as hemangiomas.[4] Similarly, presence of tenderness in a firm subcutaneous swelling may lead to make a clinical diagnosis as neural tumor like neurofibromas, as seen in our case. They are characterized by calcification within the lesion, which makes it feel firm to hard, and often results in an irregular angulated shapes when stretched; this is referred as ‘tent sign,’ which indicates several facets and irregular angles of pilomatricoma. Positive tent sign should alert the diagnosis of PMX.[56]

FNAC smears from a tumor located in the head, neck, or upper extremities presenting in a young adult should be examined carefully to exclude PMX. When the aspirate happens to be from the periphery of the tumor or from an early lesion, it will show a marked predominance of basaloid cells at times to the extent of absence of other components.[7] In many cases, PMX has been erroneously diagnosed cytologically as benign lesions like epidermal inclusion cyst, benign cystic lesion, giant cell tumor of tendon sheath, or fibrohistiocytic lesion.[7]

Certain cytological features of PMX like high cellular yield, the presence of small primitive-appearing basaloid cells with a high nuclear-cytoplasmic ratio, a background rich in debris, foreign body multinucleated giant cells, and inflammatory cells resembling tumor necrosis are likely to cause confusion even with the malignancies like squamous cell carcinoma or malignant adnexal tumor, depending on the different components of the tumor, which may be aspirated blindly during the procedure of FNAC. Problems arise mainly when the yield contains numerous keratinized squamous cells with scarcity of basaloid and shadow cells. The diagnostic triad of basaloid cells, ghost or shadow cells, and foreign body giant cells need not necessarily be present in all the cases. Spectrum of characteristic cellular components and diagnostic trap arising most commonly due to predominance of one component over the others should always be considered while examining FNAC smears to avoid incorrect diagnosis of PMX on cytology.[8] Pilomatrix carcinoma is the rare malignant counterpart of PMX, usually arises de novo, but can arise from previously excised PMX. Pilomatrix carcinoma show male predominance and is common in elderly.

In conclusion, the cytological features of PMX are characteristic and allow a conclusive diagnosis provided the smears be examined keenly bearing in mind the diagnostic traps that can mislead a cytopathologist. Although they are usually non-tender swellings, neither the tenderness nor the skin changes should be taken into account to rule out diagnosis of pilomatricoma. Probably, tenderness may be related to size of the tumor where bigger swelling can cause compression of underlying nerve fibers, and skin hypopigmentation may be due secondary degenerative changes that can cause loss of melanocytes.

What is new?

Preoperative cytological diagnosis is challenging as PMX usually will not be considered in the differential diagnoses. We offered the precise diagnosis as PMX, in view of presence of all the diagnostic criteria.

PMX usually presents as painless tumor covered by either normal skin or skin with bluish-red discoloration, making a clinical probable diagnosis of hemangiomas. But, in our case, it was painful lesion with overlying skin showing atrophic changes and loss of pigmentation. Hence, we conclude that, neither the tenderness nor the skin changes should be taken into account to rule out diagnosis of pilomatricoma.