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General Mesenchymal Neoplasms of the Skin and Subcutis
Mesenchymal neoplasms resemble mesenchymal or stroma cells and they include many different types including
From a general perspective, many are invasive, and therefore require wide excision and or a fascial plane to cure local disease. Few metastasise, fortunately.
Canine sof tissue tumor/sarcoma is an umbrella term used to describe a group of soft tissue tumors in the skin and subcutis in dogs only. It should not be used as in other locations of species.
In humans Solitary fibrous tumor (SFT) is a mesenchymal tumor with uncertain differentiation and intermediate malignant potential. SFTs can arise at any location (pleural or extrapleural) and display a wide histopathological spectrum, frequently with hemangiopericytoma‐like vessels. Several other histological patterns have also been described, including round cell, giant‐cell‐containing, epithelioid, myxoid, fat‐forming, pleomorphic and dedifferentiated variants.3,8,9,22–30 Many of the histological variants reveal a focal patternless architecture, and variably fibro‐collagenous stroma.1–9,22–30 NAB2‐STAT6
was discovered as a novel fusion gene of SFTs, and STAT6 immunoexpression has been established as a surrogate marker of the NAB2‐STAT6 gene fusion, expression of this protein representing a very sensitive and specific diagnostic tool.
There are benign and malignant types.
Fibromatosis is not well defined in the dog, however the same types that occur in humans are found in dogs.
In humans, desmoid-type fibromatosis is a locally aggressive neoplasm composed of long sweeping fascicles of bland fibroblasts/myofibroblasts. There can be variation in the type of collagen found including dense keloidal collagen, myxoid areas, and areas reminiscent of nodular fasciitis. Human subtypes are
- conventional,
- hyalinized/hypocellular,
- staghorn vessel,
- myxoid,
- keloidal,
- nodular fasciitis-like
- hypercellular.
I have diagnosed these mostly arising in an aponeurosis or fibrous band of the subcutaneous striated muscle. They behave like a fibroma except they infiltrate and can reoccur.
These are sometimes called aponeurotic fibroma in dogs.
Spugnini EP, Di Tosto G, Salemme S, Pecchia L, Fanciulli M, Baldi A. Electrochemotherapy for the treatment of recurring aponeurotic fibromatosis in a dog. Canadian Vet J 2013; 54: 606-609.Welle MM, Sutter E, Malik Y, Konar M, Rüfenacht S, Howard JE. Fibromatosis in a young Bernese Mountain Dog: clinical, imaging, and histopathological findings. J Vet Diagn Invest. 2009; 21: 895-900
In humans these tumors have a consistent gene fusion, MYH9–USP6
Ellies tumour!
C. Knight; E. Fan; R. Riis; S. McDonough (2009) Inflammatory Myofibroblastic Tumors in Two Dogs Vet Pathol 46: 273-276.
Synonyms include inflammatory fibrosarcoma, inflammatory pseudotumour
Fibroblastic cells and lymphocytes and plasma cells. Not histiocytes except dendritic cells around vessels.
Well demarcated
IHC profile includes
100% of spindle cells vimentin diffuse cytoplasmic staining
100% of spindle cells Claponin
20% SMA positive cytoplasmic and surface
20% desmin predominantly surface
Negative – panCK, GFAP, s100, Melan A, CD18,
Both did well with removal
Conventional Fibroma and Fibrosarcoma
Keloidal Fibroma and Fibrosarcoma
Myofibroblastic Fibroma and Fibrosarcoma
Angiolipoma
Fibrolipoma
The name says it all. This is a lipoma with dense collagen bundles within it.
Adenolipoma
The name says it all. This is apocrine gland tissue within a lipoma.
Chondrolipoma
The name says it all. This is cartilage within a lipoma.
Osteolipoma
The name says it all. This is bone within a lipoma.
Chondro-osteoblastic lipoma
This is a lipoma that has mature cartilage and bone within it.
Lipoleiomyoma
This lesion is found exclusively in the reproductive tract, especially the vagina of dogs. It is a smooth muscle tumor with mature lipocytes amongst the smooth muscle bundles.
Spindle cell lipoma
This is an unusual mass. It is a well circumscribed mass that has adipocytes and myxoid matrix, spindle cells and thin collagen bundles that are called 'ropey'. There are variable numbers of adipocytes from few to many.
Infiltrative lipoma
Infiltrative lipomas are just like regular lipoma but they infiltrate surrounding tissue. This is best seen when it infiltrates muscle tissue. Not to be confused with an intermuscular lipoma - which is not strictly infiltrative.
Infiltrative angiolipoma
An infiltrative angiolipoma is just like an infiltrative lipoma with a vascular component. Infiltrative hemangioma is an infiltrative vascular neoplasm with an adipocytic component.
Intermuscular lipoma
These have the typical features of a lipoma and are located within the muscles of the thoracic or pelvic limb.
Lipomatosis
This is a very rare disease where there are prominent skin folds caused by abundant mature fat.
Hibernoma
These are well circumscribed and demarcated tumors that resemble brown fat. Anisokaryosis is minimal, mitoses are infrequent and they dont metastasise. The cells have either a granular cytoplasm (mitochondria) or a vacuole.
They express UCP-1, and variably myogenin and MYOD1
Liposarcomas have either neoplastic mature lipocytes, cells with lipid globules and or lipoblasts.
Liposarcomas are vimentin +, UCP1+, and variably S100+, SMA+, desmin+, and myogenin+.
Well differentiated liposarcoma
This is essentially a lipoma that is multilobulated and with many lipocytes and lipoblasts. On immunohistochemistry, it should be MDM2 postive
Dedifferentiated liposarcoma
This liposarcoma has regions with many spindle cells with rare lipid vacuoles. It is a rare type. On immunohistochemistry, it should be MDM2 positive.
Myxoid liposarcoma
Alcian blue (pH 2.5) positive material is present and many areas are myxosarcoma like. It is the lLeast common of 3 main types. On immunohistochemistry, it should be MDM2 negative.
Pleomorphic liposarcoma
Marked cellular pleomorphism including uninuclear and multinuclear neoplastic giant cells. Lipoblasts and lipid vacuoles are present but there is lLittle lipid. On immunohistochemistry, it should be MDM2 negative
References
Doria-Torra G, Martínez J, Domingo M, Vidaña B, Isidoro-Ayza M, Casanova MI, Vidal E. Liposarcoma in animals: literature review and case report in a domestic pig (Sus scrofa). J Vet Diagn Invest. 2015; 27: 196-202.
If the bewildering array of different types of vascular tumors causes your eyes to involuntary roll, you are not alone!
Angiomatosis is at the crossroads of hyperplasia and neoplasia. They behave like neoplasms so I have placed them here.
Proliferation of well differentiated vessels is at the heart of angiomatosis. They infiltrate the resident structures rather than forming a distinct mass. They are usually not lobular.
Capillary angiomatosis
This entity is dominated by capillaries with normal endothelium that forms a mass. There should not be many if any arterioles but there may be a feeder vessel.
Cavernous angiomatosis
This is a lesion composed of dilated vascuolar spaces lined by normal endothelium that is within the normal stroma rather than forming an expansile mass.
Arteriovenous angiomatosis
This form of angiomatosis has normal appearing capillaries and normal arterioles and venules, complete with their smooth muscle coats. This is a slowly progressing mass mostly with capillaries but with arterioles and venules. There is a myxomatous or edematous stroma.
The major differential diagnosis is arteriovenous fistula.
Progressive angiomatosis
This entity is a rapidly progressive and aggressive form of angiomatosis. Its features are those of angiomatosis as indicated above.
Hemangiomas are categorised based on location and morphology. The vascular channels are lined by well differentiatted and mature endothelium. There should be no smooth muscle and minimal stroma.
Juvenile or congenital hemangioma
Neonatal or young animals may develop a vascular tumor of the skin. They have a variety of morphological appearances. They tend to be lobular and have vessels of varying types.
Dermal hemangioma
These are located in the skin and are usually locally infiltrative.
Subcutaneous hemangioma
These are well circumscribed and expansile
Cavernous hemangioma
As the name indicates, the vascular structures are cavernous and distended.
Capillary hemangioma
Asa the name indicates, these have capillary like vascular channels. They are often in lobules, and may be raised but not truely exophytic.
Inflammed lobular capillary hemangioma
Exophytic tumor
often ulcerated and the inflammation is from ulceration and thus is on the surface.
lobular arrangement of capillary like vascular channels
Usually has a feeder vessel for each lobule.
Overlying epidernal hyperplasia and hyperkeratosis is common.
Mixed capillary - cavernous hemangioma
Spindle cell hemangioma
These have a distinct appearance of being lobular, having a cavernous center and a peripheral capillary arrangement with slit like spaces and intervening stroma. They are just like karposiform hemangioendothelioma but are mature endothelium and no epithelioid or glomeruloid arrangements.
Epithelioid hemangioma
These are multinodular tumors with no atypia or mitoses and have polygonal or round cells.
Angiokeratoma
These are unique and characteristic lesions of the superficial dermis and epidermis. The distended vessels are superficial and exophytic, and the epidermis overlying and surrounding some of the channels is very hyperplastic.
Infiltrative hemangioma
These invariably have adipocytes with the vascular channels. The lining cells are well differentiated and mature. These often have a central cavernous region and a peripheral capillary arrangement.
Hemangioendothelioma is a well recognised entity in humans. It is a vascular neoplasm with relatively low grade features - mild pleomorphism, lacking in anisokaryosis and and few mitoses. It has an abundant stromal component. It is invasive and can metastasise to the local lymph node and systemically. They often have a hobnail appearance.
Epithelioid hemangioendothelioma
Retiform hemangioendothelioma
Retiform tumors of this group have low grade features, and are often well circumscribed with prominent stroma and dilated vascular spaces that arborise and which have endocapillary papillae
Kaposiform hemangioendothelioma
Karposi's sarcoma came into infamy with its association with HIV/AIDS and HHV-8. The typical appearance of Karposiform vascular tumors is slit like vascular spaces. Epithelioid areas and glomeruloid arrangements are characteristic.
Hemangiosarcoma is an aggressive infiltrative neoplasma of irregular vascular channels that are lined by cells with the typical criteria of malignancy - mitoses, anisokaryosis and hyperchromasia. There are usually solid areas in addition to the spongy channels.
Dermal hemangiosarcoma may be solar induced and more differentiated with a low metastatic potential. Subcutaneous hemangiosarcoma is more aggressive, more likely to metastasise and more difficult to affect local control.
Conventional hemangiosarcoma
Epithelioid hemangioendothelioma
These are typically solid tumors with minimal channel formation. The neoplastic cells are polygonal to round and with intermixed spindle cells in solid sheets.
Kaposiform hemangioendothelioma
The Kaposiform pattern is one of slit like blood filled spaces, and with aggregates of spindle shaped cells. There are spindle cells between the vascular channels.
Separation of the various entities in the group of neoplasms and lesions in the lymphatic group is difficult because of overlapping features. I will highlight those features that I find useful
Lymphangioma is a well circumscribed expansile lesion that is not aggressive or progressive. It is composed of lymphatic channels with a single layer of endothelium of lymphatic type (PROX1 positive). The endothelium is well differentiated and mature. There should not be any blood within the channels, unless there was surgical hemorrhage, ulceration or trauma.
This is an aggressive progressive disease that may metastasise. It is usually extensive before diagnosis and clinically will be edematous and the lesion often oozes fluid through the skin. Some are very well differeniated, others not so much. The classical example is feline ventral abdominal lymphangiosarcoma.
The vascular channels of this mass are irregular infiltrative and may have densely cellular areas.There is usually edema of the stroma and lymphocytes in nodules are common.
This is a progressive disease with infiltrative lymphatic vessels that are mature and well differentiated. The spread peripherally is progressive, usually slow.
Glomus tumors are well-demarcated expansile masses composed of tightly packed round cells with some spindle cells and marked vascularity. Their cytoplasm is pale eosinophilic and the nuclei are round. Anisocytosis and anisokaryosis are mild, and the mitotic count are usually low. The tumor has vascular channels that are compressed and indistinct. Clusters of cells may bulge into the lumen of blood vessels but the endothelial lining is intact. Glomus cells express vimentin and α-smooth muscle actin. Desmin expression can be seen in scattered cells. PAS positive basal lamina surrounds single neoplastic cells. This is also laminin and collagen IV positive on immunohistochemistry.
In human medicine, PEComa is a mesenchymal tumours with distinct epithelioid cells, that have smooth muscle and melanoma markers so they are α-SMA and vimentin, and PNL2, HMB45, and Melanin A positive. S100 and desmin immunoreactivity is only occasionally detected. Some may be CK and CD117 positive.
PWT are reported in dogs prodominantly and will be discussed in the section on canine PWT. They are a heterogeneous bunch with many different patterns and arrangements of cells. What is typical is one or more of the following patterns somewhere in the neoplasm.
Perivascular whorls.
These are whorls around a central vessel lumen that is often collapsed and the vascular lumen may no longer be visible.
Placentoid pattern
This is multiple neoplastic lobules separated by an empty space and often containing central capillaries.
Staghorn (antler-like) vessels
These are thin-walled, branching and dilated vascular structures.
Bundles of spindle cells arising directly from the vascular wall,
The subtypes of PWT include
Angiofibroma
This tumor resembles a fibroma but has one of the perivascular patterns. It is positive with vimentin-only.
Angiomyofibroblastoma
This is similar to a myopericytoma but on electron microscopy has a fibronexus-like junction.
Hemangiopericytoma
This is a rare tumor has staghorn vasculature and spindle cells arranged in bundles admixed with collagenous to myxoid matrix.
The cells express vimentin and only rarely smooth muscle actin. It expresses the pericytic marker 3G5, which is only available for frozen sections.
Myopericytoma
This neoplasm has perivascular whorls and placentoid areas. Neoplastic cells express smooth muscle actin, desmin and occasionally heavy caldesmon, but are smoothelin and myosin negative.
Perivascular myoma (angioleiomyoma)
This is similar to a myopericytoma and expresses smoothelin and myosin.
Smooth muscle tumors, like elsewhere in the body, are separated into leiomyoma and leiomyosarcoma on vague and imprecise criteria as indicated below. I do not, for a minute, believe we should separate them without justification. They are smooth muscle actin positive and well differentiated types may be vimentin negative.
Leiomyoma
This is a well differentiated neoplasm that has cells resembling smooth muscle. Most have no mitoses, and are well circumscribed. In humans there are infiltrative, high mitotic count and metastasising varieties.
Piloleiomyoma, angioleiomyoma and deep seated leiomyoma are the three types. Piloleiomyoma is located in the dermis and is well circumscribed and well differentiated tumor. Angioleiomyoma have the vascular arrangement of perivascular wall tumors
They are smooth muscle actin positive and vimentin negative.
Leiomyosarcoma
This is an infiltrative neoplasm with pleomorphism, but with some resemblance to smooth muscle. In humans, marked pleomorphism, invasion and a mitotic count above 20 p10HPF together is sufficient to make this diagnosis.
Nerve sheath tumors arise in peripheral nerves, nerves outside the central nervous system. These include cranial nerves (except cranial nerve I and II), spinal nerves and autonomic nerves. It is relatively easy to make a diagnosis of nerve sheath tumor (NST) when the mass is within the nerve. It presents a conumdrum in diagnosis when it is not growing in a recognised nerve. NST have features that, in dogs at least, are shared with fibrous tumors and perivascular wall tumors especially. Studies comparing features and immunohistochemistry with commonly available reagents mean separation of the subtypes of soft tissue tumors is impossible to do with accuracy on routine formalin fixed paraffin embedded tissues.
Histological feature of NST include:
- Spindle cell morphology
- Residual axons
- Antoni type A (densely cellular) and B (poorly cellular) pattern
- Nuclear rowing or pallisading (this is often very distinctive)
- Verocay bodies
- Ancient change (mega and poly polykarya)
- Myxoid stroma with descrete nodular arrangement.
- Wavy nuclei
- Sclerosis around blood vessels
- Nuclei tend to arrange linearly in the direction in the adjacent axons
- 'onion bulb' arrangement of tissue around a nerve (perineurinoma).
Storiform or herringbone arrangement is too ubiquitous in soft tissue tumors to be useful.
Immunohistochemisty staining includes
- S100 (glial cells, neurons, Schwann cells, chondrocytes, adipocytes, dendritic cells and melanocytes)
- Olig-2 (transcription factor in oligodendrocytes and rarely in other cells)
- GFAP (astrocytes and ependymal cells but also found in some myoepithelial cells)
- NFP - neurofilament protein (normal axons)
- MBP - myelin basic protein
- Collagen IV (basement membranes of endothelium, smooth muscle, Schwann cells)
- Laminin (basement membranes of endothelium, smooth muscle, Schwann cells)
- NGFR - Nerve growth factor receptor (Schwann cells, dendritic cells, basal epithelial cells, and myoepithelial cells)
- Claudin-1 (perineural cells, perivascular wall cells, epithelial cells)
- SOX10 neural crest origin, transcription factor. Rules out mimics like PWT and meningioma
There is a subdivision of NST into benign (neurofibroma, neurolemmoma and perineurinoma) and malignant types. There are no features that clearly and accurately separate benign and malignant behaviour. Some use invasion and a mitotic count above 4 p 2.37sq mm (10HPF).
A confident diagnosis of NST means that it is not graded as a soft tissue tumor/sarcoma.
These are rare in veterinary medicine. They were once thought to be common in dogs, as soft tissue sarcomas of dogs were considered to be benign or malignant nerve sheath tumors. That is no longer believed to be the case.
They are well circumscribed and noninvasive tumors within a nerve. They tend to expand and compress residual normal axons and their sheaths.Growth patterns that are typical of schwanoma include Antonio A and Antoni B, nuclear palisading, Verocay bodies, and nuclei that are wavy. It may be hyalinization of blood vessels. The nuclei are small and uniform (and wavy), and mitoses are usually difficult to find. There may be osseous, chondroid, rhabdoid and epithelioid differentiation. Some are pigmented. Nuclei may show "ancient" change which is essentially polykaria and pleomorphism.
On immunohistochemistry they should be S100 and SOX10 positive, and cells may stain for GFAP and Olig-2. Residual nerve fibers stain with neurofilament protein (NFP).
Conventional
These have some or many of the histological features listed above.
Cellular
These are mostly or exclusively Antoni A types.
Plexiform
distinct multiple nodules with typical appearance.
Melanocytic
This is a mixture of Schwann cells and fibroblasts, usually with the latter predominating. In a nerve, it tends to be throughout the nerve, not expansile like a Schwannoma.
The cellularity is low and cells are invested in collagen or myxoid matrix.
Residual axons may be present as these tend infiltrate within a nerve rather than expand like a schwanoma.
May have many mast cells within the tumor, just like in normal nerves.
The Schwann cell component will stain positively for S100, NFGR, and Olig-2. The fibroblast component will be negative for these.
There should not be distinct nuclear palisading, Verocay bodies or Antoni A and Antoni B regions.
These are distinct and produce 'pseudo-onion bulb' structures that surround axons. They try to recapitulate a neurolemmal (Schwann) cell encirlcing an axon. They have a distinct whorling around an axon resulting in multiple circular patterns. This pattern may be seen in a neurolemmoma (Schwannoma), but is the main pattern in a perineurioma.
This is a rare condition where there are similtaneous multiple nerve sheath tumors in a nerves in the body.
The exact separation between benign NST like neurolemmoma (Schwannoma) and neurofibroma and MNST is not reported in domestic animals. Some use the human criteria of anaplasia/pleomorphism of nuclei and a mitotic count of >4 p 2.37sq mm (10HPF) and invasion.
Necrosis and hemorrhage is common in MNST. Myxoid matrix is common too, but it is Alcian blue negative.
These may have chondroid, osseous, epithelioid or rhabdoid differentiation. Can metastasise, but there are no published details of liklihood.
Lanigan LG, Russell DS, Woolard KD, et al. Comparative Pathology of the Peripheral Nervous System. Vet Pathol. 2021; 58: 10-33.
A granular cell tumor is not a specific diagnosis but rather it is an umbrella diagnosis, because the cells of this neoplasm may be of different types but have a similar appearance. They are round with an abundant granular cytoplasm. Although not always the case, the granules stain positive with PAS and they are diastase resistant. Many of the cells are S100 positive suggesting they have a neuroectodermal phenotype.
This is not a neoplasm! It is a cluster of axons and their accompaning myelin sheath - Schwann cells that occur at sites of amputation or neurectomy. When a nerve is injured the Schwann cells proliferate in this can be followed by extensions of the axons into the Schwann cells. Traumatic neuromas typically have accompanying scar tissue.
The embryology of muscle begins with a myoblasts which fuse to form myotubes. In rhabdomyosarcoma, round immature myoblasts may have very little cytoplasm with variable amounts up to multinucleate myotubular cells that resemble skeletal muscle. Embryonal muscle tumours include embryonal rhabdomyosarcoma, alveolar rhabdomyosarcoma and botryoid rhabdomyosarcoma.
In humans the classification scheme alveolar rhabdomyosarcoma is given its own category because of the poor prognosis. They all occur in young children. Pleomorphic rhabdomyosarcoma is a disease of adults.Thus the classification scheme is as follows
Embryonal
Rhabdomyoblastic - round to polygonal cells with abundant eosinophilic cytoplasm and only rare cross striations
myotubular - dominated by strap cells forming myotubes with frequent cross striations
spindloid – this is composed of interlacing fasicles of plump spindle cells.
Botryoid - these have a classical gross appearance and anatomical location. They recapitulate skeletal muscle embryogenesis and contain undifferentiated myoblasts as well is multinucleate admired tube cells within a myxomatous stroma.
Alveolar (classic and solid alveolar pattern) these have packets of small round cells with a scant cytoplasm and separated by fibrous stroma in an alveolar pattern. They resemble a neuroendocrine pattern.
Pleomorphic - these typically occur in adults within skeletal muscle and are haphazardly arranged spindle cells with rare multinucleated cells present. They often have bizarre mitotic figures, a high degree of cellular and nuclear pleomorphism. They have no embryonal or alveolar morphology
DiagnosisThis depends on detecting muscle specific markers and absence of smooth-muscle markers.
They are vimentin and Desmin positive. Desmin staining can be patchy and sparse and other cells express Desmin. Myogenin, myo D1 and nuclear transcription factors are required.
Muscle specific actin is found in skeletal, cardiac and smooth-muscle, and myofibroblasts, myointimal cells and reactive mesothelial cells.
They should not immune stain for smooth-muscle actin.
Sarcomeric actin should be positive.
Myoglobin may be positive, especially in embryonal and botryoid rhabdomyosarcoma.Caserto BG. (2014) A comparative review of canine and human rhabdomyosarcoma with emphasis on classification and pathogenesis. Vet Pathol. 2013; 50: 806-826
Rhabdomyoblastic
Round cells with rhabdomyoblasts
Myotubular
Many myotubes and many crossstriations
embryonal type that forms bunches of grapes
Many giant cells including multinucleated cells
Many round cells in nodules that separate
Caserto BG. (2014) A comparative review of canine and human rhabdomyosarcoma with emphasis on classification and pathogenesis. Vet Pathol. 2013; 50: 806-826
The embryology of muscle begins with a myoblasts which fuse to form myotubes. In rhabdomyosarcoma, round immature myoblasts may have very little cytoplasm with variable amounts up to multinucleate myotubular cells that resemble skeletal muscle. Embryonal muscle tumours include embryonal rhabdomyosarcoma, alveolar rhabdomyosarcoma and botryoid rhabdomyosarcoma.
In humans the classification scheme alveolar rhabdomyosarcoma is given its own category because of the poor prognosis. They all occur in young children. Pleomorphic rhabdomyosarcoma is a disease of adults. Thus the classification scheme is as follows
Embryonal
Rhabdomyoblastic - round to polygonal cells with abundant eosinophilic cytoplasm and only rare cross striations
myotubular - dominated by strap cells forming myotubes with frequent cross striations
spindloid – this is composed of interlacing fasicles of plump spindle cells.
Botryoid - these have a classical gross appearance and anatomical location. They recapitulate skeletal muscle embryogenesis and contain undifferentiated myoblasts as well is multinucleate admired tube cells within a myxomatous stroma.
Alveolar (classic and solid alveolar pattern) these have packets of small round cells with a scant cytoplasm and separated by fibrous stroma in an alveolar pattern. They resemble a neuroendocrine pattern.
Pleomorphic - these typically occur in adults within skeletal muscle and are haphazardly arranged spindle cells with rare multinucleated cells present. They often have bizarre mitotic figures, a high degree of cellular and nuclear pleomorphism. They have no embryonal or alveolar morphology
Diagnosis
this depends on detecting muscle specific markers and absence of smooth-muscle markers.
They are vimentin and Desmin positive. Desmin staining can be patchy and sparse and other cells express Desmin. Myogenin, myo D1 and nuclear transcription factors are required.
Muscle specific actin is found in skeletal, cardiac and smooth-muscle, and myofibroblasts, myointimal cells and reactive mesothelial cells.
They should not immune stain for smooth-muscle actin.
Sarcomeric actin should be positive.
Myoglobin may be positive, especially in embryonal and botryoid rhabdomyosarcoma.
The presence and proliferation of mature striated muscle cells in a location outside a normal muscle is a hamartoma. While called skeletal muscle hamartoma, it is more accurate to call it syncytial striated muscle hamartoma....
Histology
Undifferentiated pleomorphic sarcoma is a sarcoma with many multinucleated or uninuclear neoplastic giant cells. Some have an inflammatory component with histiocytes, multinucleated giant cells (to make it tough for interpretation) and lymphocytes. There should be no perivascular patterns - perivascular whorls, staghorn, placentoid or bundles from a vascular wall.
Subtypes
There are 3 main subtypes - storiform, giant cell and inflammatory. Many have all of these features but one predominates (if you like to split phenotypes).
Immunohistochemistry
The neoplastic cells are positive for vimentin and often positive for smooth muscle actin and desmin, suggesting they are myofibroblastic in type. They should be negative for markers of striated muscle (myoglobin, MYOD1, myogenin), and lipoblasts (for liposarcoma)
Prognosis
These are poorly studied and as such, do not have good followup data. Until this is the case, I use the grading system for generic soft tissue tumors/sarcomas.
Histology
Malignant mesenchymoma is a sarcoma with different lineages of soft tissue, and a minimum of 2 different lineages are required for the diagnosis. These could be osteosarcoma, liposarcoma, fibrosarcoma, rhabdomyosarcoma, chondrosarcoma or any other type. Care should be taken to ensure the sarcoma is not a malignant nerve sheath tumor, dedifferentiated liposarcoma, or soft tissue fibrous osteosarcoma.
Prognosis
These are poorly studied and as such, do not have good followup data. Until this is the case, I am use the prognosis for the element that has the worst prognosis which is usually that of osteosarcoma if present.