Cytomorphology of pleomorphic fibroma of skin: A diagnostic enigma.

Pleomorphic fibroma (PF) is a benign, polypoid, or dome-shaped cutaneous neoplasm with cytologically atypical fibrohistiocytic cells. We describe the cytomorphological features of PF retrospectively with histopathological diagnosis in a 38-year-old male who presented with 3 × 1.5 cm swelling in the soft tissues of the thigh for 6 months. This lesion is benign despite the presence of pleomorphic or bizarre cells. We review the differential diagnosis of PF with other mesenchymal tumors. To the best of our knowledge, cytomorphological features on fine needle aspiration cytology of this tumor are not yet documented in literature.

Introduction

Pleomorphic fibroma (PF) of the skin is a rare benign fibrous tumor.[1] The lesion is usually polypoid, located in the dermis, and is formed by coarse collagen bundles with sparse cells. It is also characterized by the presence of marked cellular atypia and pleomorphism without mitosis.[1] We describe the cytomorphological features on fine needle aspiration (FNA) smears of a histologically and immunohistochemically proven case of PF. Cytomorphological features on FNA of this tumor are not well documented in literature.

Case Report

A 38-year-old male presented with a superficial, painless, polypoidal, soft tissue mass since 6 months, measuring about 3 × 1.5 cm in size, and involving the back of the thigh. Fine needle aspiration cytology (FNAC) was done and slides were stained with Giemsa stain. The aspirate yielded cellular smears. Background showed metachromatic stromal fragments. Cells were pleomorphic having very large nuclei (monster cells) with scanty cytoplasm. Few of the nuclei showed single nucleoli [Figure 1]. Nuclear membranes frequently showed notches, creases, or folds. Cells were lying singly and occasionally forming clusters. These were admixed with the spindle cell component along with few multinucleated cells. Necrosis and mitotic figures were not seen in smears. On FNA, the swelling was diagnosed as fibrohistiocytic lesion with atypical cells and the mass was subsequently excised.

Figure 1

Cellular smears containing pleomorphic cells having very large nuclei (monster cells) with scanty cytoplasm. Few of the nuclei revealing nucleoli (Giemsa, ×400)

On gross examination, the mass was well circumscribed measuring 3 × 1.5 cm. The cut section was solid, grayish-white, and fibrous. Routine processing and hematoxylin and eosin staining were done. Microscopically, tumor showed thick bundle of haphazardly arranged collagen in the dermis. Spindle- and irregularly shaped stellate or multinucleate cells were scattered between the collagen bundles. These cells were markedly atypical showing scant cytoplasm and large, pleomorphic, enlarged hyperchromatic nuclei with small nucleoli [Figure 2]. No mitosis or necrosis was evident in multiple sections examined. Based on these features, diagnosis of PF was made.

Figure 2

Section showing pleomorphic cells scattered between the collagen bundles (H and E, ×200)

Immunoprofile of lesional cells revealed positive staining for vimentin [Figure 3] and CD34 and negative staining for S-100.

Figure 3

Lesional cells revealed positive staining for vimentin (IHC, ×200)

Discussion

PF of skin was first described by Kamino et al., in 1989.[1] PF is a benign fibrous lesion which is located mainly in the lower extremities, followed by trunk, head, and neck and rarely in the subungual region. Most of these lesions occur in adults, with a peak incidence in the fifth decade of life.[1]

Role of FNAC in diagnosis of soft tissue tumor has been fairly documented as well as debated but the cytomorphological features on FNAC of PF are not yet documented in literature.[2] In contrast to the histological findings (which is hypocellular), the FNA smear in our case was cellular. Majority of cells were pleomorphic with very large nuclei like monster cells of dermatofibroma and few of them possessed single nucleolus. Nuclear membrane showed notches, creases, or folds. Few cells were multinucleated. Cells were having scanty cytoplasm or were devoid of cytoplasm. Small fragments of metachromatic stroma were present in the background representing collagen bundles. Necrosis and mitotic figures were not seen in the smears. Thus, on FNA, PF can easily be misdiagnosed as malignant soft tissue tumor because it contains bizarre pleomorphic cells showing considerable atypia. This lesion has been shown to be benign despite the presence of pleomorphic or bizarre cells, although lesion may locally recur when incompletely excised.[1] This also reveals the importance of mitosis and necrosis in making a diagnosis of malignant soft tissue tumor on cytology, as both these features were not seen in this tumor.

Histological examination of PF of the skin reveals a dermal tumor with striking nuclear atypia. The neoplastic cells show large pleomorphic and hyperchromatic nuclei with small nucleoli. In addition to these mononuclear cells, the atypical nuclear features can also be observed in multinucleated giant cells.[13] Degeneration, ischemia, or the paracrine influence of mast cells may create the cytological atypia in PF.[145] Two variants have been described: Myxoid and sclerotic.[67] In fact, some investigators have postulated that the PF of the skin is actually a variant of the sclerotic fibroma.[48] Other researchers have called these as pleomorphic sclerotic fibromas.[9]

Lesional cells in PF are always positive for vimentin and negative for S-100 and cytokeratin. Positive expression of muscle-specific actin, CD34 and rarely alpha-1 antichymotrypsin, has also been observed.[1348-10]

The cytological differential diagnosis of PF of skin has not been discussed in literature. However, histological differential diagnosis includes: Atypical fibroxanthoma, dermatofibroma with monster cells, giant cell fibroblastoma, desmoplastic Spitz nevus, and desmoplastic melanoma.[13910]

In our case, bundles of collagen were present with low cellularity, thus ruling out atypical fibroxanthoma and dermatofibromas with atypical or monster cells. Dermatofibromas with atypical or monster cells also contains foam cells and hemosiderin-laden macrophages which were not seen in our case. Giant cell fibroblastoma resembles PF as both contain atypical fibroblastic cells. Giant cell fibroblastoma is usually seen in childhood and is characterized by infiltrative growth and features sinusoidal structures lined by atypical cells, whereas our case was an adult without infiltrative growth and sinusoids. Desmoplastic Spitz nevus and desmoplastic melanoma show some areas of melanocytic differentiation and cells are positive for S-100 protein, both these features were absent in our case. Thus, the final diagnosis of PF was made.

Thus, from a clinical as well as pathological standpoint, recognition of a benign lesion of this type is very important since an incorrect cytological interpretation could result in inappropriate treatment.