Fibrohistiocytic tumors





Benign tumors


Fibrous Papule


Definition





  • A benign proliferation in the superficial dermis, composed of fibroblasts, blood vessels, and sparse inflammatory cell infiltrate in the background of a sclerotic dermal collagen



  • Multiple lesions can be associated with tuberous sclerosis and, less commonly, with Birt–Hogg–Dube syndrome



  • Similar lesions on the penis have been designated as pearly penile papules



Clinical features


Epidemiology





  • Solitary lesions most common in middle-aged adults



  • Multiple lesions in patients with tuberous sclerosis can be present at birth, but usually develop during childhood and are seen in about 90% of adult patients



  • Equal sex distribution



Presentation





  • Solitary small, skin-colored, dome-shaped papule, rarely pedunculated



  • Predilection for the nose and face



  • Size from 1 to 5 mm



  • Multiple lesions associated with tuberous sclerosis (referred to as adenoma sebaceum ) typically develop over the nose and cheeks in a “butterfly” pattern, with bilateral and symmetrical, periungual proliferations known as Koenen tumors



Prognosis and treatment





  • No treatment generally required



  • Simple excision curative



Pathology


Histology





  • Well-circumscribed proliferation in the superficial dermis composed of




    • Plump, spindle-shaped, stellate and/or multinucleated cell fibroblasts



    • Vascular channels, frequently dilated



    • Sparse inflammatory cell infiltrate, mainly composed of lymphocytes




  • Sclerotic dermal collagen in the background



  • Mitoses of spindle cells exceptional



  • Focal nuclear hyperchromatism of spindle cells can be seen



  • Additional features include




    • Perivascular and periadnexal fibrosis



    • Scattered vellus hairs



    • Lentiginous proliferation of melanocytes above the dermal lesion



    • Epidermis with hyperkeratosis, acanthosis, and flattened rete ridges pattern




  • Histological variants




    • Clear cell



    • Hypercellular




      • Marked proliferation of fibroblasts in comparison to classical fibrous papule




    • Pigmented




      • Hyperplasia of melanocytes, can be prominent



      • Dermal melanophages




    • Pleomorphic




      • Bizarre-shaped fibroblasts in the papillary dermis




    • Granular cell



    • Inflammatory




      • Predominance of inflammatory cells



      • Admixture of lymphocytes, plasma cells, and histiocytes



      • Eosinophilic and neutrophilic granulocytes rare




    • Epithelioid




      • Large cells, forming sheets and nests



      • Medium-sized centrally or peripherally located nuclei with prominent nucleoli



      • Prominent pink cytoplasm





Immunohistopathology/special stains





  • Factor XIIIa positive, may also be CD34 positive



  • Clear cell variant can be NKIC3 and CD68 positive



  • Epithelioid cell variant diffusely procollagen positive, scattered CD68 positivity (70%), factor XIIIa negative



  • Consistently negative for cytokeratins, EMA, and S100 protein



Main differential diagnoses





  • Melanocytic lesions (intradermal melanocytic nevus, Spitz nevus, lentigo maligna, balloon cell nevus)



  • Epithelioid cell fibrous histiocytoma



  • Pleomorphic fibroma



  • Granular cell tumor



  • Dermal nonneural granular cell tumor


Fig. 1


Fibrous papule.

Dermal lesion composed of fibroblasts; dilated, thin-walled vascular channels; and a mild lymphohistiocytic inflammatory cell infiltrate.



Fig. 2


Fibrous papule.

Higher magnification. Dilated vascular channels and scattered elongated or stellate cells associated with sclerotic dermal collagen.



Fig. 3


Fibrous papule

—clear cell variant. Note cells with prominent clear cell change.



Fig. 4


Fibrous papule

—clear cell variant. Higher magnification.




Multinucleate Cell Angiohistiocytoma


Definition





  • A distinctive benign dermal proliferation composed of thin-walled capillaries and veins, admixed with scattered multinucleated cells



Clinical features


Epidemiology





  • Female predominance (F:M = 3 : 1)



  • Middle-aged adult patients



Presentation





  • Slowly growing single or multiple firm, red-brown to violaceous papules



  • Multiple lesions usually distributed over the same area, occasionally bilateral



  • Surface is usually smooth, occasionally scaly



  • Size less than 1 cm in diameter



  • Most common over distal extremities, particularly dorsum of hands, wrists, thighs, and legs, with less frequent involvement of the face and trunk



  • Mucosal sites distinctly uncommon (oral cavity)



  • Usually asymptomatic, pruritic lesions rare



  • Clinical variants




    • Linear



    • Eruptive



    • Plaque-like



    • Disseminated/generalized




Prognosis and treatment





  • Spontaneous regression possible, but rare



  • Association with mycosis fungoides, diabetes mellitus, or vitiligo most likely coincidental



Pathology


Histology





  • Vascular proliferation, composed predominantly of thin-walled capillaries and veins




    • Lumina dilated or narrow



    • Lined by a single layer of bland endothelial cells



    • Each vessel surrounded by layer of pericytes




  • Scattered multinucleated cells




    • Bizarre shaped



    • Hyperchromatic nuclei



    • Angulated cytoplasm




  • Fibroblast-like and histiocyte-like mononuclear cells



  • Thickened collagen bundles, frequently hyalinized



  • Occasional inflammatory cells, predominantly lymphocytes



  • Hemorrhage absent, no hemosiderin deposition



  • Decreased elastic fibers in the dermis can be observed



  • Overlying epidermis normal, but can also be hyperplastic



  • Proliferation restricted to upper and middermis



Immunohistochemistry/special stains





  • Multinucleated cells display variable CD68 positivity



  • Vascular markers delineate endothelial cells



Main differential diagnoses





  • Atrophic dermatofibroma



  • Microvenular hemangioma



  • Angiofibroma



  • Kaposi sarcoma


Fig. 1


Multinucleate cell angiohistiocytoma.

Skin with a slight increase in the number of blood vessels and mild dermal fibrosis.



Fig. 2


Multinucleate cell angiohistiocytoma.

Another area featuring distinctive multinucleated giant cells in the dermis, proliferation of small blood vessels, mild fibrosis, and perivascular inflammation.



Fig. 3


Multinucleate cell angiohistiocytoma

—higher magnification.




Benign Fibrous Histiocytoma (Dermatofibroma)


Definition





  • Neoplastic process is favored over a reactive condition due to the absence of spontaneous regression, tendency for local recurrence(s), extremely rare locoregional and distant metastases, as well as the recent detection of fusions in protein kinase C and membrane-associated proteins in some fibrous histiocytomas



  • Represents a group of morphologically diverse proliferations with different clinical presentations and biological potential



Clinical features


Epidemiology





  • One of the most common benign soft tissue proliferations in the skin



  • Broad age distribution, most common in middle-aged adults



  • Slight female predominance



Presentation





  • Virtually any site of skin can be affected



  • Most commonly on the limbs (about 70%), followed by the trunk



  • Slowly growing, firm solitary nodule, round to oval



  • Overlying skin often reddish-brown to darkly pigmented and scaly



  • Size of the lesion is usually less than 10 mm



  • Central dimple formation on lateral compression or squeezing is a common clinical sign



  • History of previous trauma occasionally present



  • Clinical variants




    • Giant variant, including plaquelike, measuring from 35 to 300 mm



    • Eruptive variant, usually related to immunosuppression, HIV infection, and highly active antiretroviral therapy




Prognosis and treatment





  • Simple excision is curative



  • Recurrences after incomplete or marginal excision have been estimated to develop in less than 2% of cases



  • Lesions occurring on the face and certain morphological variants (cellular, aneurysmal, atypical) are associated with a higher recurrence rate (up to 26%)



  • Exceedingly rare metastases have been reported in particular variants of fibrous histiocytoma (cellular, aneurysmal, atypical); however, no histological features have been detected to predict metastatic potential



Pathology


Histology





  • Epidermis above the lesion is usually hyperplastic



  • Elongation and broadening of rete ridges associated with hyperpigmentation of basal keratinocytes especially over the tips of rete ridges (so-called dirty fingers )



  • Proliferation of immature hair-follicle structures above the dermal proliferation is frequently seen and can mimic a basal cell carcinoma or an adnexal tumor



  • Papillary dermis is usually, but not always, spared (so-called grenz zone )



  • Defining histological features include




    • Spindle cells growing in a haphazard, vague, storiform or short intersecting fascicular pattern



    • Histiocytes in variable proportions, including multinucleated giant cells (Touton and foreign body type), siderophages, and foamy cells



    • Inflammatory cells in variable proportions, usually lymphocytes



    • Delicate collagenous or loosely myxoid stroma containing thin-walled blood vessels




  • Ill-defined nonencapsulated proliferation in the dermis/superficial subcutis




    • Growth into subcutis usually in the form of short extensions



    • Individual bundles of collagen surrounded by lesional cells, so-called collagen trapping, especially at the periphery of the lesion




  • Lesions restricted to subcutis or deeper structures rare



  • Several histological variants have been recognized, including cellular, epithelioid, aneurysmal, atypical (pseudosarcomatoid), lipidized (“ankle type”), clear cell, palisading, deep, signet ring cell, fibrous histiocytoma with osteoclast-like cells, cholesterolotic



  • Different variants can coexist within a single lesion



Immunohistochemistry/special stains





  • Although factor XIIIa is usually positive, it may just delineate a background population of dermal dendrocytes



  • See variants for additional immunohistochemical features



Genetic profile





  • Gene fusions involving multiple protein kinase C–encoding genes ( PRKCA, PRKCB, or PRKCD ) with a variety of other genes have been documented



  • Prevalence and distribution of these fusions in the various subtypes of benign fibrous histiocytoma are not known, but they are only present in a subset



Main differential diagnoses





  • Nodular fasciitis



  • Dermatofibrosarcoma protuberans



  • Scar



  • Xanthoma



  • Xanthogranuloma



  • Melanocytic proliferations


Fig. 1


Benign fibrous histiocytoma (dermatofibroma).

Classic appearance of an ill-defined dermal tumor associated with overlying epidermal hyperplasia.



Fig. 2


Benign fibrous histiocytoma (dermatofibroma).

Variably cellular, polymorphic dermal tumor that can involve the full thickness of the dermis.



Fig. 3


Benign fibrous histiocytoma (dermatofibroma).

Focal involvement of the subcutis is relatively common.



Fig. 4


Benign fibrous histiocytoma (dermatofibroma).

The tumor is generally composed of an admixture of spindle cells and histiocyte-like cells, albeit in variable proportions.



Fig. 5


Benign fibrous histiocytoma (dermatofibroma).

High-power view of spindle-shaped tumor cells displaying a storiform growth pattern.



Fig. 6


Benign fibrous histiocytoma (dermatofibroma).

In this example, spindle cells in a curlicue and focal storiform growth pattern are embedded in collagenous hyalinized stroma.



Fig. 7


Benign fibrous histiocytoma (dermatofibroma),

higher magnification. An admixture of spindle and histiocyte-like cells is noted. Note also the presence of an inflammatory cell infiltrate composed of lymphocytes.



Fig. 8


Benign fibrous histiocytoma (dermatofibroma).

Occasional normal mitoses are not uncommon. In addition, note the prominent deposition of hemosiderin in this example.



Fig. 9


Benign fibrous histiocytoma (dermatofibroma).

Collagen trapping is typically present at the periphery of the tumor. This is an important clue as an aid in recognizing the entity.



Fig. 10


Benign fibrous histiocytoma (dermatofibroma).

The epidermis overlying dermatofibroma is frequently acanthotic. A grenz zone separates the tumor from the epidermis.



Fig. 11


Benign fibrous histiocytoma (dermatofibroma).

Elongation and broadening of the rete ridges associated with hyperpigmentation of basal keratinocytes in the epidermis overlying the tumor.



Fig. 12


Benign fibrous histiocytoma (dermatofibroma).

Proliferation of immature hair follicle structures reminiscent of trichoblastoma is not uncommonly seen above the dermal proliferation and should not be mistaken for basal cell carcinoma.



Fig. 13


Benign fibrous histiocytoma (dermatofibroma).

Trichoblastoma-like basaloid proliferation, high-power magnification.



Fig. 14


Benign fibrous histiocytoma (dermatofibroma).

Prominent hemosiderin deposition is typical of the hemosiderotic variant. Note the presence of multinucleated giant cells.




Cellular Fibrous Histiocytoma


Definition





  • Distinctive variant of a benign fibrous histiocytoma (dermatofibroma), characterized by highly cellular monomorphic proliferation of spindle cells growing in a more fascicular and focally storiform pattern



Clinical features


Epidemiology





  • Represents about 5% of benign fibrous histiocytomas



  • Approximately equal sex distribution, although slight male predominance observed in the initial publication



  • Wide age range, but most common in young to middle-aged adults (between 33 and 42 years)



Presentation





  • Solitary asymptomatic nodule



  • Multifocal growth and familial occurrence in a single patient



  • Preoperative duration of the lesion variable (2 weeks to 2 years)



  • Upper limb/limb girdle followed by lower limb/limb girdle, head and neck, and trunk



  • Propensity for unusual sites, like face, ears, hands, and feet



Prognosis and treatment





  • Complete excision usually curative



  • Recurrence rate as high as 26% after incomplete/marginal excision



  • Locoregional (lymph nodes) and systemic metastases (lungs) in exceptional cases; no histological features have been detected to predict metastatic potential



Pathology


Histology





  • Twο growth patterns recognized on low-power examination




    • Nonexophytic and ill defined (more common)



    • Exophytic and circumscribed




  • Defining features include




    • Cellular proliferation of plump spindle cells with tapering nuclei containing small eosinophilic nucleoli and relatively abundant ill-defined, pale eosinophilic cytoplasm



    • Short fascicular growth predominates over storiform growth pattern



    • Foci of more epithelioid cells occasionally present and represent a minor component of the lesion



    • Normal mitoses common (up to 10 mitoses per 10 high-power fields)



    • Abnormal mitoses not present



    • Focal necrosis (in up to 12%), generally not associated with surface ulceration



    • Surface ulceration uncommon




  • Most frequently arise in superficial dermis and subsequently infiltrate the deep dermis, but mid dermal and deep dermal origin not uncommon



  • Extension into the subcutis present in roughly one-third of the cases




    • Fascicular growth along the septa (perpendicular to the epidermis)



    • Lacelike growth between the fat cells




  • Infiltration beyond subcutis into the underlying skeletal muscle rare, usually in the head and neck area, most likely due to the more superficial localization of skeletal muscles at this particular site



  • Areas of a classical benign fibrous histiocytoma consistently identified at least focally, usually at the periphery of the cellular proliferation



Immunohistochemistry/special stains





  • Smooth muscle actin–positivity in over 90%



  • Cytoplasmic desmin positivity in 32%




    • Consistent with myofibroblastic differentiation




  • Genuine CD34 positivity in 6%




    • Usually seen at the periphery of the lesion or in a patchy pattern



    • Diffuse CD34 staining is an exception




  • Consistently negative for cytokeratins, S100 protein, and EMA



  • Factor XIIIa reveals nonneoplastic cells in the background



Main differential diagnoses





  • Atypical dermal smooth muscle tumor



  • Dermatofibrosarcoma protuberans


Fig. 1


Cellular fibrous histiocytoma.

Note an ill-defined, highly cellular dermal tumor. Extension into subcutis is present in about one-third of the cases, mainly along the fibrous septa.



Fig. 2


Cellular fibrous histiocytoma.

This example shows extensive infiltration of the subcutis.



Fig. 3


Cellular fibrous histiocytoma.

Highly cellular proliferation of monomorphic fascicles of myofibroblast-like spindle cells associated with lymphocytes in the background.



Fig. 4


Cellular fibrous histiocytoma.

Note a more polymorphic area containing spindle-shaped cells, histiocyte-like cells, and lymphocytes.



Fig. 5


Cellular fibrous histiocytoma

—higher magnification. Bland spindle cells growing in short fascicles. Note focal storiform growth pattern.



Fig. 6


Cellular fibrous histiocytoma.

Areas of necrosis are present in about 10% of the tumors and have no prognostic implication.



Fig. 7


Cellular fibrous histiocytoma.

Mitotic activity can be brisk. Atypical mitoses are generally absent.



Fig. 8


Cellular fibrous histiocytoma.

This example displays scattered large atypical cells indicating a tumor combining features of cellular and atypical fibrous histiocytoma.



Fig. 9


Cellular fibrous histiocytoma.

This example is characterized by granular cell change.



Fig. 10


Cellular fibrous histiocytoma.

A rare finding of pseudovascular invasion shows tumor cells protruding into the lumen of a small vein.



Fig. 11


Cellular fibrous histiocytoma

—CD34 immunohistochemistry. Patchy positivity can frequently be observed at the periphery of the tumor. Focal positivity of tumor cells for this marker can be seen in other areas of the tumor, but diffuse positivity as seen in dermatofibrosarcoma protuberans is not usually a feature.




Aneurysmal Fibrous Histiocytoma


Definition





  • Distinctive variant of a benign fibrous histiocytoma (dermatofibroma), characterized by formation of variably sized, blood-filled spaces developing in the background of an ordinary common benign fibrous histiocytoma



Clinical features


Epidemiology





  • Represents less than 2% of fibrous histiocytomas



  • Develops most frequently in the fourth decade of life (mean age 37 years)



  • Shows female predominance, with male-to-female ratio of 1 : 1.5



Presentation





  • Broad anatomical distribution with predilection for the lower limbs/limb girdle (55%), followed by upper limbs/limb girdle (17%), trunk (12%), and head and neck (4%)



  • Solitary papule of variegated color from dark brown to red or blue



  • Diameter of the lesion from 5 to 40 mm



  • Multiple lesions develop exceptionally



  • Rapid growth due to hemorrhage within the preexistent long-standing lesion is a common presenting symptom



Prognosis and treatment





  • Simple excision is usually curative



  • Recurrences after incomplete/marginal excision are common (up to 20%)



  • Metastatic disease exceptional; no histological features have been detected to predict metastatic potential



Pathology


Histology





  • Epidermis overlying the lesion demonstrates variable degrees of acanthosis



  • Generally located in the dermis; extension into the subcutis not uncommon, but infiltration into the underlying skeletal muscle rare



  • The lesion can be ill defined



  • Defining features include blood-filled spaces ranging from slitlike to large cavities, similar to cavernous hemangioma



  • Blood-filled spaces




    • Lack of endothelial lining



    • Are lined by the lesional cells, including histiocytes, fibroblasts, and giant cells



    • Are most frequently located within the most cellular parts of the lesion, usually central, which are devoid of collagen and elastic fibers



    • Can represent the predominant component of the lesion




  • Surrounding stroma contains numerous small capillaries, prominent interstitial hemorrhage, and hemosiderin deposition



  • Classical histological features of benign fibrous histiocytoma can usually be recognized at the periphery of the lesion



Immunohistochemistry/special stains/cytogenetics





  • Lesional cells can stain focally for smooth muscle actin



  • Factor XIIIa–positive and CD34-positive cells may represent reactive cells within the lesion



  • Lesional cells are consistently negative for desmin, CD34, CD31, CD68, and factor XIIIa



Main differential diagnoses





  • Angiomatoid fibrous histiocytoma



  • Spindle cell hemangioma



  • Nodular Kaposi sarcoma



  • Angiosarcoma


Fig. 1


Aneurysmal fibrous histiocytoma.

Tumor with variably sized, blood-filled spaces mimicking blood vessels in the background of an ordinary fibrous histiocytoma.



Fig. 2


Aneurysmal fibrous histiocytoma.

Note numerous pseudovascular spaces with prominent hemorrhage and artifactual slitlike spaces.



Fig. 3


Aneurysmal fibrous histiocytoma.

Blood-filled pseudovascular spaces lack an endothelial lining and are surrounded by lesional cells.



Fig. 4


Aneurysmal fibrous histiocytoma.

Numerous multinucleated giant cells containing hemosiderin pigment are seen.



Fig. 5


Aneurysmal fibrous histiocytoma.

This example resembles nodular Kaposi sarcoma.




Epithelioid Fibrous Histiocytoma


Definition





  • Although currently regarded as a variant of benign fibrous histiocytoma (dermatofibroma), it likely represents a distinctive and unrelated tumor type



  • Characterized histologically by proliferation of rounded or epithelioid cells comprising at least 50% of the lesional cell population



  • Typically displays cytoplasmic ALK positivity on immunohistochemistry and ALK gene rearrangements by fluorescence in situ hybridization (FISH)



Clinical features


Epidemiology





  • Most frequent in the fifth decade of life



  • Slight male predominance with M:F ratio of 1.4 : 1



Presentation





  • Solitary lesion, multicentric growth exceptional



  • Slightly raised or polypoid, symmetrical



  • Reddish papule/nodule of 5 to 20 mm in diameter



  • Most frequent sites of origin are lower extremities (60%), followed by upper extremities (20%), trunk (10%), and head and neck (less than 10%)



Prognosis and treatment





  • Simple excision is curative



  • Nondestructive recurrences after incomplete excision are rare



Pathology


Histology





  • Two patterns at low-power magnification




    • Well-demarcated exophytic proliferation in the papillary and superficial reticular dermis (more common)



    • Less circumscribed and frequently more cellular proliferation extending into deep dermis but also into the superficial fatty tissue




  • Epidermal collarette frequently surrounds the lesion and can extend along its base



  • Epidermis above the lesion hyperplastic, atrophic, or both



  • Tumor-free zone between the epidermis and dermis, typical of common fibrous histiocytoma, is generally lacking



  • Rounded or polygonal epithelioid cells with abundant eosinophilic cytoplasm containing round-to-oval vesicular nuclei with small eosinophilic nucleoli should represent at least 50% of the lesional cells



  • Mild nuclear pleomorphism



  • Variable proportions of binucleated or trinucleated cells scattered among epithelioid cells; multinucleated giant cells, however, are rare



  • Occasional regular mitoses (average 1 per 10 high-power fields)



  • Numerous small blood vessels dispersed among epithelioid cells are a constant feature. They can form a predominant component of the proliferation, thus mimicking a vascular lesion



  • Delicate collagenous stroma, hyalinized in older lesions



  • Myxoid change can be prominent and can even predominate



  • Adnexal structures usually not seen within the lesion



  • Typical histological features of benign fibrous histiocytoma are usually seen at least focally



Immunohistochemistry/special stains





  • Admixture of factor XIIIa–positive dendritic histiocytes and CD34-positive dermal fibroblasts



  • EMA positivity can be present in roughly two-thirds of cases and D2-40 positivity in about 50%



  • Consistently negative for low- and high-molecular-weight cytokeratins, S100 protein, smooth muscle actin, desmin, and CD31



  • ALK expression (usually cytoplasmic) reported in most cases



Genetic profile





  • ALK rearrangements are present in most cases



  • PRKCA, PRKCB, and PRCKD rearrangements are seen in a subset of other benign fibrous histiocytoma subtypes



  • Clonality detected in a limited number of cases



Main differential diagnoses





  • Spitz nevus



  • Reticulohistiocytoma



  • Juvenile xanthogranuloma



  • Epithelioid perineurioma


Fig. 1


Epithelioid fibrous histiocytoma.

Polypoid well-circumscribed dermal tumor with flattened overlying epidermis. Note an epidermal collarette.



Fig. 2


Epithelioid fibrous histiocytoma.

Epidermal collarette frequently surrounds the lesion and can extend along its base.



Fig. 3


Epithelioid fibrous histiocytoma.

Dense dermal proliferation of epithelioid cells with a rich vascular network in the background.



Fig. 4


Epithelioid fibrous histiocytoma.

Epithelioid cells are associated with a variably collagenized stroma. Capillaries within the tumor frequently contain thickened and hyalinized walls.



Fig. 5


Epithelioid fibrous histiocytoma.

Rounded or polygonal epithelioid cells with abundant eosinophilic cytoplasm containing round-to-oval vesicular nuclei with a single basophilic nucleolus are characteristic.



Fig. 6


Epithelioid fibrous histiocytoma.

Mild nuclear pleomorphism may be seen.



Fig. 7


Epithelioid fibrous histiocytoma.

Occasional normal mitoses are common.



Fig. 8


Epithelioid fibrous histiocytoma.

Variable proportions of binucleated cells are scattered among epithelioid cells. In addition, a mild inflammatory cell infiltrate composed of lymphocytes is common.



Fig. 9


Epithelioid fibrous histiocytoma.

The tumor cells show diffuse cytoplasmic ALK positivity.




Atypical (Pseudosarcomatous) Fibrous Histiocytoma


Definition





  • A rare variant of benign fibrous histiocytoma, characterized by proliferation of variably pleomorphic spindle and epithelioid cells in the background of an ordinary benign fibrous histiocytoma



  • Associated with increased tendency for local recurrences as well as slight potential for local and distant metastases



  • Similar lesions also designated dermatofibroma with monster cells, but the term atypical “pseudosarcomatous” fibrous histiocytoma preferable



Clinical features


Epidemiology





  • Most frequently occurs in young to middle-aged adults in the fourth to fifth decade of life (mean age about 40 years)



  • Roughly equal gender distribution



Presentation





  • Solitary non-painful nodule most common, polypoid lesion or a slightly raised plaque much rarer



  • Size of 4 to 80 mm in largest diameter (median 1.5 cm)



  • Lower limb/girdle (44%), followed by upper limb/girdle (32%), and trunk (11%), few cases in the head and neck area and genital region



Prognosis and treatment





  • Complete excision usually curative



  • Incomplete/marginal excision associated with recurrences in up to 14%



  • Two patients from the largest series developed distant metastases, one of whom died due to metastatic disease



  • No histological features have been detected to predict metastatic potential



Pathology


Histology





  • Pleomorphic cells represent the hallmark of the lesion




    • Spindle (fibroblast-like) and polygonal/epithelioid (histiocyte-like) pleomorphic cells in variable proportions, admixed with multinucleated giant cells



    • Nuclei are irregular, large, hyperchromatic; bizarre-shaped forms not uncommon



    • Nucleoli prominent



    • Cytoplasm abundant, eosinophilic with indistinct borders, occasionally contains hemosiderin



    • Frequent regular mitoses (1–15 per 10 high-power fields [HPF], mean 3.4 per 10 HPF), atypical forms can also be observed



    • Pleomorphism varies from mild to moderate to severe and can be focal or diffuse




  • Geographical necrosis, usually found in lesions with moderate/marked degree of pleomorphism



  • Large areas of central collagen degeneration, surrounded by lesional cells, reminiscent of necrobiotic granuloma



  • Mainly localized in the dermis, but focal and superficial extension in the subcutis present in about one-third of the cases



  • Background proliferation




    • Ordinary fibrous histiocytoma in 100% of the cases




      • Proliferation of spindle cells and histiocyte-like cells in a vague storiform pattern with collagen trapping



      • Usually found at the periphery of pleomorphic component



      • Represents an important clue for correct recognition of the proliferation




    • Aneurysmal fibrous histiocytoma in roughly 25% of the cases




  • Epidermis can be hyperplastic with elongated rete ridges pattern



Immunohistochemistry/special stains





  • Focal positivity for smooth muscle actin and CD34 in limited number of cases



  • Consistently negative for S100 protein, low- and high-molecular-weight cytokeratins, desmin, CD68, and EMA



Main differential diagnoses





  • Pleomorphic fibroma



  • Dermatofibrosarcoma protuberans



  • Atypical fibroxanthoma



  • Spindle cell squamous cell carcinoma



  • Melanoma



  • Atypical intradermal smooth muscle neoplasm



  • Leiomyosarcoma


Fig. 1


Atypical (pseudosarcomatous) fibrous histiocytoma.

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Oct 29, 2019 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Fibrohistiocytic tumors
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