Plexiform Soft Tissue Tumors



Plexiform Soft Tissue Tumors





INTRODUCTION

The term plexiform refers to a pattern of local infiltration in which multiple, interconnected tumor nodules ramify in adjacent stroma or normal tissues and appear discrete in cross section. Thus, plexiform lesions are not circumscribed. Many tumor types can develop a plexiform pattern, and some of these form distinct clinicopathologic subtypes. Most are benign but can recur or regrow locally, due to failure to excise all outlying branches of the lesion. In addition, plexiform fibrohistiocytic tumor has metastatic potential. The differential diagnosis is summarized in Table 22.1.


PLEXIFORM FIBROHISTIOCYTIC TUMOR


Clinical Features

Plexiform fibrohistiocytic tumor (PFHT) was first described in 1988 with 65 cases in the original report by Enzinger and Zhang.1 Subsequent series of 142 and 223 cases, and more recently, a further series of 664 cases have confirmed the original description. PFHT is a slow-growing tumor that occurs mainly in adolescents and young adults, originally with a female predominance but with more equal sex distribution in later series. It forms a painless dermal/subcutaneous nodule, often tethered to adjacent tissues, and mostly on the upper limbs and head and neck region. One case arose in an irradiation field 7 years after treatment for malignant hemangiopericytoma.5

In the 2013 World Health Organization classification, PFHT is categorized as a so-called fibrohistiocytic tumor of intermediate (rarely metastasizing) biologic potential. A total of 37% of the original series of PFHT recurred, usually within 2 years, and two cases (3%) metastasized to regional lymph nodes. The overall recurrence rate is 33%, but only 2/16 (12.5%) recurred in one series of 22 patients.3 However, in this series, one case metastasized to lymph node and two additional cases metastasized to lung. One further example with apparent lymph node metastasis has been reported.6 There seems to be no correlation between histologic features and behavior. A case with cytologic atypia has not recurred after 10 years.7











TABLE 22.1 Differential Diagnosis of Plexiform Soft Tissue Tumors

Clinical Features

Microscopic Features

Ancillary Investigations


Plexiform fibrohistiocytic tumor

Young adults, mostly upper limb, head and neck


One-third recur, rare cases metastasize

Nodules based on dermal-subcutaneous junction


Rounded cells, multinucleated cells, lymphocytes


Some have branching bundles of fibroblasts


Rarely myxoid or atypical

SMA+, CD68+, CD34−, S100 protein−


Giant cell tumor of soft parts of low malignant potential

Mostly upper limb


Subcutaneous or subfascial


Multinodular or circumscribed


Can recur, rarely metastasizes

Uniformly distributed large osteoclast-like giant cells, bland mononuclear cells


Hemorrhage, peripheral osteoid, or bone formation


Vascular invasion

CD68+ in giant cells


Cellular neurothekeoma

Young adults, upper extremity, head and neck


Recurrence rare, no metastases

Dermal


Rounded nodules of uniform rounded cells


Multinucleated cells rare


Often myxoid


Can show focal nuclear atypia

SMA+, NKIC3+, MITF+, S100 protein−, HMB45−


Dermal nerve sheath myxoma

Circumscribed multilobulated myxoid lesion, low cellularity

Spindle cells can show focal pleomorphism or multinucleation

S100 protein+ in myxoid areas, EMA+ at periphery of nodules


Plexiform schwannoma

Young adults, extremities, head and neck, gastrointestinal tract


Deep variant mostly in females

Dermal or subcutis, rarely deeper


Variably sized nodules of Schwann cells


Mostly Antoni A pattern


Nuclear pleomorphism can be seen, but necrosis rare


Cellular variant has hypercellular fascicles, mitoses

S100 protein+, EMA+ at periphery of nodule


Plexiform neurofibroma

Associated with neurofibromatosis type 1


Can involve large nerves in deep locations or more superficial ones


Can undergo malignant change


Can extend extraneurally as diffuse neurofibroma

Transitions from normal nerve


Nerve expanded by variable myxoid stroma and increased cellularity


Atypical variant has nuclear crowding, pleomorphism


Diffuse extraneural component in some

S100 protein+ focally


Diffuse neurofibroma

Some associated with neurofibromatosis type 1


Children and young adults


Head and neck, subcutaneous infiltrative plaque


Associated with plexiform neurofibroma—extends outside nerve bundles into soft tissue

Sheets of short spindle cells in loose fibrous stroma, infiltrating between normal structures


Wagner Meissner bodies

S100 protein+ diffusely in nuclei


Plexiform xanthoma

Adult males, solitary or rarely multiple


Knee, elbow


Hypercholesterolemia rare

Irregularly sized nodule in dermis and subcutis


Sheets of vacuolated macrophages


Occasional giant cells

CD68+


Plexiform ossifying fibromyxoid tumor

Young adults, limbs, head and neck

Subcutaneous


Each nodule encapsulated


Glomus-like cells with small rounded nuclei and clear cytoplasm

S100 protein+, GFAP+, desmin+ focally. EP400-PHF1 fusion transcripts


Plexiform leiomyoma

Subcutaneous or female genital tract


No specific clinical features

Nonencapsulated nodules without atypia, mitoses, or necrosis

SMA+, desmin+, h-caldesmon+




Pathologic Features

Most examples arise at the dermal-subcutaneous junction (Fig. 22.1) and can extend superficially or deeply (e-Fig. 22.1) but sometimes are wholly in dermis (e-Fig. 22.2). Two principal histologic patterns are described. Fortythree percent display a fibrohistiocytic picture with nodules of rounded or spindled cells with scattered osteoclast-like multinucleate giant cells, chronic inflammation, microhemorrhages, and hemosiderin (Fig. 22.2, e-Figs. 22.3 and 22.4). A total of 23% are predominantly fibroblastic with plexiform bundles of uniform spindle cells (Fig. 22.3, e-Fig. 22.5), and 34% have a mixed pattern (e-Figs. 22.6 to 22.8). Occasional pleomorphic cells are seen, and mitotic figures are described up to 7 per 10 hpf. Myxoid change, bone formation, and focal cytologic atypia7 are rare findings. In older lesions, the nodules are gradually replaced by fibrous tissue (e-Figs. 22.9 to 22.11).


Ancillary Investigations

Immunostaining for CD68 is positive in rounded cells (e-Fig. 22.12) and in the multinucleate giant cells, which are probably histiocytic and nonneoplastic. The spindle cells have a myofibroblastic immunophenotype with focal reactivity for SMA, and occasionally for calponin, but none for desmin or h-caldesmon. S100 protein, FXIIIa, and CD34 are negative in lesional cells. Ultrastructurally, the few reports variously indicate that the lesional cells comprise a mixture of fibroblasts, myofibroblasts, and undifferentiated mesenchymal cells.2,8 The findings are similar to those in other benign or malignant fibrohistiocytic tumors.

Clonal genetic abnormalities have been reported in a small number of cases, but no consistent changes have been found. One example had a complex karyotype with numerous deletions,9 another demonstrated a simpler karyotype of 46,XY,t(4;15)(q21;q15),10 and a third revealed a 46,X,del(X) (q13)[3]/46,XX[23] karyotype.11 All cases examined have been diploid.3






FIGURE 22.1 Plexiform fibrohistiocytic tumor. The tumor comprises multiple nodules centered on dermal-subcutaneous junction and extending into both dermis and subcutis.







FIGURE 22.2 Plexiform fibrohistiocytic tumor. The fibrohistiocytic nodules are circumscribed and contain a mixture of bland histiocyte-like cells and osteoclast-like giant cells, with a sprinkling of lymphocytes.


CELLULAR NEUROTHEKEOMA

Cellular neurothekeoma has many similarities to PFHT. Indeed, in some cases, the distinction often cannot be made, and it has been proposed that the two entities are closely related if not identical, differing essentially by the plane in which they are located.12 Cellular neurothekeoma was
originally thought to be a cellular variant of nerve sheath myxoma, and the term neurothekeoma has included both entities (see Chapter 20)

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  2. Soft Tissue Lesions with Clear or Granular Cells
  3. Superficial Vascular Lesions and Mimics of Vascular Lesions
  4. Deep Vascular Lesions

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Jun 18, 2016 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Plexiform Soft Tissue Tumors

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