Rhabdomyosarcoma
Differential Diagnosis
Embryonal rhabdomyosarcoma
- DDx is generally that of small round blue cell tumors
- Usually resolved by immunohistochemistry
| Nblast | ALL | Ewing Sarc (PNET) | Rhabdo | MNTI | DSCT | Wilms Tumor | |
|---|---|---|---|---|---|---|---|
| Synapto | pos | neg | neg | neg | pos | neg | neg |
| NB84 | pos | neg | neg | neg | neg | neg | neg |
| CD45RB (LCA) | neg | pos | neg | neg | neg | neg | neg |
| CD99 | neg | pos | pos | neg | rare | neg | neg |
| Myogenin | neg | neg | neg | pos | rare | neg | neg |
| MyoD1 | neg | neg | neg | pos | neg | neg | neg |
| Keratin | neg | neg | rare | rare | pos | pos | neg |
| Desmin | neg | neg | neg | pos | rare | pos | neg |
| WT1(C) | neg | neg | neg | neg | neg | pos | pos |
| S100 | neg | neg | rare | neg | rare | neg | neg |
| HMB45 | neg | neg | neg | neg | pos | neg | neg |
The small cell pattern of DSCT may be confused with rhabdomyosarcoma. Both are desmin positive.
| Embryonal Rhabdomyosarcoma | Desmoplastic Small Cell Tumor |
|---|---|
| No desmoplastic stroma | Prominent desmoplastic stroma |
| Keratin variable, faint and focal | Keratin 90% positive, prominent |
| EMA negative | EMA 90% positive |
| WT1 rare, focal | WT1 positive |
| Muscle actin positive 95% | Muscle actin rare |
| Median age 7 years | Mean age 25 years |
| No consistent genetic abnormality | t(11;22)(p13;q12) |
| Rhabdomyosarcoma | Extrarenal Rhabdoid Tumor |
|---|---|
| Desmin, actin, myoglobin or MyoD1 positive | Muscle markers negative (definitional) |
| Keratin 5-50% positive, focal, weak | Keratin 100% (definitional) |
| INI1 expression retained 100% | INI1 loss 85% nearly definitional |
| May have cross striations | No cross striations |
| May have strap cells | Strap cells rare |
| No consistent abnormalities of 22q | Various deletions, translocations involving 22q11 may be seen |
| Alveolar rhabdomyosarcoma | Alveolar Soft Part Sarcoma |
|---|---|
| Variably shaped alveolar spaces | Regular alveolar spaces |
| Fibrous septa around nests | Thin walled vascular septa |
| Cells generally 10-30 microns diameter | Cells larger |
| Pleomorphic nuclei | Only mild pleomorphism |
| Dense nuclei | Vesicular nuclei |
| Small nucleoli | Prominent nucleoli |
| No crystals | Crystals in up to 80% of cases |
| Uniformly desmin, actin positive | Occasionally desmin, actin positive |
| Frequently MyoD1 positive | MyoD1 rare or negative |
| Translocations t(2;13) or t(1;13) | der(17)t(X;17)(p11.2;q25) |
| Sclerosing Rhabdomyosarcoma | Desmoplastic Small Cell Tumor |
|---|---|
| Keratin 5-50% positive, focal, weak | Keratin 90% positive, prominent |
| EMA negative | EMA 90% positive |
| WT1(C) negative | WT1(C) positive |
| Any age group | Mean age 25 years |
| No consistent genetic abnormality | t(11;22)(p13;q12) |
- Other sarcomas with a prominent sclerotic/desmoplastic background may also mimic sclerosing rhabdomyosarcoma
- These include, but are not limited to, osteosarcoma, extraskeletal myxoid chondrosarcoma, epithelioid fibrosarcoma and angiosarcoma
- These sarcomas can be readily differentiated from sclerosing rhabdomyosarcoma by the characteristic histologic features (i.e. malignant osteoid formation, etc…) and the immunohistochemical profile (i.e. myogenin/myoD1 expression in rhabdomyosarcoma)
| Pleomorphic Rhabdomyosarcoma | High Grade Pleomorphic Sarcoma (MFH) |
| Myogenin and/or MyoD1 positive | Myogenin and MyoD1 negative |
| Desmin positive | Desmin reactivity infrequent |
| Pleomorphic Rhabdomyosarcoma | Embryonal Rhabdomyosarcoma with Numerous Round Differentiated Myoblasts |
| Rare in children, peak incidence in 5th decade | Most occur in children, typically children < 10 years of age |
| Lacks uniform background of immature cells | Has uniform background of immature cells |
| Nuclei of large cells are very pleomorphic | Nuclei of large cells typically uniform |
| Pleomorphic Rhabdomyosarcoma | Alveolar Rhabdomyosarcoma with Multinucleated Giant Cells |
| Rare in children, peak incidence in 5th decade | Most occur in children |
| Lacks uniform background of immature cells | Has uniform background of immature cells |
| Nuclei of large cells are very pleomorphic | Nuclei of giant cells typically uniform and located in a peripheral ring |
| Lacks mutations involving PAX8-FOXO1 | Mutations involving PAX8-FOXO1 in 80% |
