Intro to Soft Tissue Pathology Flashcards
Alveolar soft part sarcoma
DDx is alveolar rhabdomyosarcoma and RCC. Alveolar rhabdomyosarcoma is a round blue cell tumor while alveolar soft part sarcoma is a large, pink cell sarcoma.
Alveolar architecture with intervening fibrous septae and cells with a rhabdoid appearance. Cells are polygonal, pink, and have nuclei with a “bite” taken out of them.
TFE3 rearrangements are characteristic. TFE3+ nuclei is diagnostic once RCC has been ruled out.
Note: Metastatic RCCs may look similar and may also have TFE3 translocations, so watch out!
Alveolar rhabdomyosarcoma
IHC: Desmin positive tumor, classically. Myogenin/Myf4 and MyoD1 with diffuse (>90%) nuclear positivity.
VERY prominent, sweeping fibrous septae and fibrovascular interjections. Elongated strap cells or tadpole cells may interject the small, round, blue background. Multinucleated “wreath-like” cells may also be seen.
Molecular: PAX3-FOXO1 or PAX7-FOXO1 translocations.
Embryonal rhabdomyosarcoma
Loose, hypocellular, spindly and round cell stromal neoplasm with a hypercellular layer just underneath the epithelium (cambium layer) in skin and mucosal surfaces. Cells may “condense” in other loose fascicles elsewhere in the tumor, but the prevailing pattern is loose, widely spaced spindle cells and dense edema. Often does not LOOK like a sarcoma.
Generally, once diagnosed, has a pretty good prognosis. Tends to occur in kids, especially those with RAS or SHH pathway mutations (ie, Noonan syndrome, Gorlin syndrome).
Positive: Desmin, myogenin
Unlike other rhabdomyosarcomas, there is no charcteristic translocation or specific molecular abnormality identified as of yet.
Perineurioma
Storiform pattern of whirling fascicles of spindle cells. Well-circumscribed, benign, should not entrap fat or other structures. Cells are thin and spindly in one dimension, oval-shaped and elongated in another dimension – their job is to wrap. You want to see both of these shapes in cross-section.
DDx is DFSP, SFT, and whirling pattern low-grade fibromyxosarcoma.
Positive for EMA, claudin-1, and GLUT-1, negative for S100, negative for MUC4
FISH can rule out DFSP by showing the absence of PDGFB or PDGFD fusions.
Pleomorphic lipoma
Multinucleated giant cells with Floret-shaped, moderately polymorphic nuclei in a background of ropey collagen and well-differentiated adipocytes. Lipoblasts may also be present – this does not make it liposarcoma on its own. Every here and there you may find a spindle cell adipocyte.
IHC: RB1 negative (lost). MDM2 negative/low (not amplified).
Molecular: RB1 loss, just like spindle cell lipoma – these are the same disease, just different morphologies.
Cellular neurothekeoma
Alveolar tumor with nests of spindled cells with cleared out chromatin (PTC-like) and syncytial, histiocyte-like cytoplasm. Often mistaken for a spitzoid melanoma.
Many have myxoid change, sometimes abundantly. Multinucleated giant cells may also be seen.
Once you rule out melanocytic differentiation, you can try S100A6 (different type of S100) or PGP9.5 or NKIC3. These are helpful, but none are entirely sensitive, so if they are all negative but you have ruled out melanoma, you can still feel ok about calling this.
Thought to be derived from fibrohistiocytic precursors. Not really a neural tumor (as evidenced by S100 and SOX10 negative). The name is historical – there is a thing called neurothekeoma, and it was once thought that this was just a more “cellular” form of it. Now we know that these tumors are completely different entities.
Chondroid lipoma
Low power impression is lipoma/lipoid neoplasm. When you zoom in, you will see plenty of well-differentiated adipocytes, but also pink areas with bubbly lacunae and a mixture of lipoblasts and cells with eosinophilic granules.
This is not to be confused with a lipoma that has cartilagenous metaplasia – which is a real thing that can happen.
“Hemangioendothelioma”
Broad term for tumors with a lot of vasculature that are not benign like hemangiomas, but not aggressively malignant like sarcomas.
ie, intermediate malignant potential
Nerve sheath myxoma
aka, conventional neurothekeoma
Benign, and unrelated to cellular neurothekeoma, though occasionally cellular neurothekeoma can look like this. May recur.
Almost alveolar/nodular fibrous septae that envelop swirling spindle cells that appear to be “chasing one another” in a myxoid background. Some areas can have verrucay-like bodies.
