Plexiform fibrohistiocytic tumor

ARTICLE

Auteur(s) : Osamu MORI, Takashi HASHIMOTO

Department of Dermatology, Kurume University School of Medicine, 67 Asahimachi, Kurume, Fukuoka, 830-0011, Japan

Article accepted on 29/10/2003

Plexiform fibrohistiocytic tumor was first described by Enzinger and Zhang in 1988 [1]. This rare tumor, which predominantly involves the upper limbs of infants and children, has an immunophenotype suggestive of myofibroblastic differentiation [1]. It involves the superficial soft tissue, often arising at the dermal-subcutaneous junction [2]. Local recurrence is common, usually because of incomplete removal. We herein present an additional case of this tumor in a Japanese boy.

Case report

A 6-year-old boy presented with a painless nodule on the right perioral region that had appeared spontaneously 8 months earlier. It had grown very slowly, and had never ulcerated. He was otherwise healthy, and his family history was noncontributory. Examination revealed a round, erythematous, firm nodule, 5 mm in size, with teleangiectasia on the surface (Fig. 1a). However, this tumor was not tethered to deeper tissue, and no mandibular and cervical lymph nodes were palpable. This nodule was excised, and follow-up for nine months revealed no recurrence or metastasis.

Pathological findings

At scanning magnification, the lesion was located in superficial to deep dermis. At low power magnification, many tiny cellular nodules that occupied the dermis were observed (Fig. 1b), and in some areas isolated nodules were seen (Fig. 1c). A higher magnification showed that these nodules contained a mixture of two components: spindle-shaped fibroblasts and epithelioid macrophages (Fig. 1d). The cells in these nodules were well differentiated and did not display atypia or significant levels of mitotic activity. The nodules were circumscribed by short fascicles of fibroblastic cells. Multinucleated giant cells were absent.

Immunohistochemistry

All lesions were negative for S-100 protein, lysozyme, NK1/C3 (CD57) and factor XIIIa. Macrophages expressed CD68 (KP1) (Fig. 2a), while the spindle cells expressed smooth muscle actin (Fig. 2b).

Discussion

Enzinger and Zhang first reported 65 cases of plexiform fibrohistiocytic tumor [1], and two subsequent studies made clear its distinctive clinicopathological features [2, 3]. In the series of Enzinger and Zhang [1], the average age of occurrence was 14.5 years (~ 70% of the patients were younger than 20 years) and the female-to-male ratio was ~ 3: 1. Results by Remstein et al. [3] were very similar: the average age of occurrence was 14.6 years and ~ 77% of the patients were younger than 20 years. However, the female-to-male ratio was 6: 1, and: ~ 65% of the tumors involved soft tissues of the upper extremities. Plexiform fibrohistiocytic tumors rarely affect the head and neck region: 10% of the tumors in Enzinger and Zhang’ s series [1], 7% in the Hollowood et al. series [2], and none in Remstein et al. series [3].
Plexiform fibrohistiocytic tumors usually are located in subcutaneous adipose tissue and frequently extend into dermis [1]. All 22 tumors in Remstein et al. series involved subcutaneous adipose tissue, and ~ 30% of them extended into dermis [3]. Less frequently, they extended into skeletal muscle.
The local recurrence rate was 12.5% in Remstein et al. series [3], which is low compared with that reported by Enzinger and Zhang (37.5%) [1] and Hollowood et al. (40%) [2]. Regional lymph node metastasis was documented in one of the Remstein et al. series (6%) [3], similarly, 2 of 32 (6%) patients in Enzinger and Zhang’s report [1], and no case in Hollowood et al. series [2]. Remstein et al. [3] stated that it is important to recognize that the plexiform fibrohistiocytic tumor is capable of producing systemic metastasis and that these lesions may be present at the time of the initial diagnosis or may develop subsequently.
Enzinger and Zhang [1] proposed the histological differential diagnoses, which included cutaneous fibrous histiocytoma, plexiform neurofibroma, fibromatosis and benign and malignant giant cell tumors. In the present case, as multinucleated giant cells were not observed, we excluded giant cell tumors. Plexiform neurofibroma was excluded because S-100 protein was completely negative in the tumor cells. The so-called cellular benign fibrous histiocytoma [4] often extend into the subcutis; they sometimes recur locally after incomplete surgical removal. Normal mitotic figures are common in the cellular benign fibrous histiocytoma, and 12% of cases show central necrosis. Up to 60% of cases show focal positivity for smooth muscle actin in a minority of the cells. However, our case had the arrangement of cellular nodules and fascicles, which showed the fibrohistiocytic aggregate composed of macrophages forming nodules or irregular shaped clusters, interspersed among fascicles of spindle cells.
Benign fibrous histiocytoma (dermatofibroma) of the face deserves attention. Mentzel et al. [5] reported 34 cases. Histologically, fibrous histiocytoma of the face is composed of an admixture of CD68 positive histiocytoid cells and actin positive spindle-shaped myofibroblasts. In many cases cellular fascicles and bundles of spindle-shaped tumor cells were noted in addition to classical morphological features of fibrous histiocytoma. In contrast with typical cases, fibrous histiocytoma in this location is characterized by an infiltrative growth with involvement of deep soft tissues including striated muscles in many cases, and a more aggressive clinical course.
One other entity which deserves further attention is cellular neurothekeoma. Requena and Sangeza [6] already stressed the striking similarity to plexiform fibrohistiocytic tumor in 1995. Cellular neurothekeoma usually occurs on the upper trunk or head and neck of children or young adults and exhibits a plexiform pattern composed of nests and fascicles of eosinophilic epithelioid or spindle-shaped cells with common multinucleated cells. Both entities only differ (a) by the predominantly subcutaneous location of plexiform fibrohistiocytic tumor, although a dermal variant of plexiform fibrohistiocytic tumor was recently described [7]; and (b) by the greater prominence of (osteoclast-like) giant cells in plexiform fibrohistiocytic tumor which may also be seen in various subtypes of dermatofibroma such as pseudosarcomatous [8], elusive [9] and aneurismal (fibrous) histiocytomas [10].
In our case, there are exceptions from the usual appearance of the plexiform fibrohistiocytic tumor described by Enzinger and Zhang [1]; localization on the face, lack of multinucleated cells, solely intradermal presentation. The location was not characteristic, and the tumor was located in superficial to deep dermis although Zelger et al. also reported two cases that showed an intradermal location [7].
Another peculiar histologic feature was the absence of multinucleated osteoclast-like giant cells in our case. In most reported cases, a minor component of multinucleated giant cells was interspersed among the mononuclear cells. Like our case, Salamanca et al. [11] recently reported a plexiform fibrohistiocytic tumor without multinucleated osteoclast-like giant cells, the third classic cellular component of this mesenchymal neoplasm.
Unlike plexiform fibrohistiocytic tumor, cellular neurothekeoma shows frequent mitotic figures and immunohistochemical positivity for neuron specific enolase and NK1-C3 [11]. In cellular neurothekeoma, CD68 strongly outlined the giant cells, while the reactivity of mononuclear cells was negative [12]. On the other hand, in our case, the tumor cells did not show frequent mitotic figures, and the reactivity of mononuclear cells was positive for CD68, and negative for NK1-C3. Consequently, we made a diagnosis of plexiform fibrohistiocytic tumor.
The tumor presented here is relatively small and superficial, and has behaved in a benign fashion. However, since some plexiform fibrohistiocytic tumors exhibit an aggressive behavior, careful follow-up is necessary. n

References

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