Sclerosing rhabdomyosarcoma¹ (sclerosing pseudovascular rhabdomyosarcoma²)

Definition

This is a proposed rare subtype of rhabdomyosarcoma with an abundant matrix. It may represent a variant of embryonal rhabdomyosarcoma (evidenced by the presence of strap cells and the preferential expression of MyoD1 over myogenin).

Clinical features

The few cases reported occurred across a wide age range in adults and at a range of anatomical sites (forearm¹, hand¹ orbit¹, nasopharynx¹, jaw², lower leg², sacrum²), including sites unusual for other types of rhabdomyosarcoma.

Macroscopic appearances

These tumours are tan to yellow, rubbery and grossly infiltrative.

Histopathology

• Margins are infiltrative. Coagulative necrosis may occur.

• The tumour cells are spindled with varying amounts of eosinophilic cytoplasm. The cells have little cytoplasm and irregular nuclei with coarse chromatin and small nucleoli.

• Abundant hyaline matrix resembles osteoid or chondroid, but lacks calcification or lacunae.

• Within the matrix, the neoplastic cells form small nests, lobules or trabeculae (as seen in sclerosing osteosarcoma or sclerosing epithelioid fibrosarcoma).

• Anastomosing chords may resemble angiosarcoma². Irregular spaces r lined by atypical cells forming small papillae and tufts.

• There may be a microalveolar pattern but not the large alveoli of alveolar rhabdomyosarcoma. The wreath-like giant cells of alveolar rhabdomyosarcoma are also lacking.

• Focally "strap cells" may be seen.

• The mitotic rate is typically high (> 25 mitoses per 20 HPF).

Immunohistochemistry

Molecular genetics by PCR

Differential diagnosis

• sclerosing osteosarcoma: the matrix calcifies. Osteoclasts are present. The malignant osteoblasts commonly have an epithelioid morphology. Other patterns of osteosarcoma (chondroblastic, telangectatic) commonly co-exist.

• chondrosarcoma: the appearances are bland and there is a lack of highly malignant small round cells. There is true cartilage with lacunae. The vasculature has a branching pattern.

• alveolar rhabdomyosarcoma: patients are younger. There are well-formed pseudo-alveoli but they lack osteoid- or chondroid-like matrix and strap cells are very unusual. Staining for myogenin is almost always very strong. There is almost universally either a t(2;13)(q35;q14) translocation producing a Pax3/FXHR fusion gene, or a t(1;13)(q36;q14) producing a Pax7/FXHR fusion gene.

• leiomyosarcoma

• myofibroblastic sarcoma: also show stromal hyalinisation but the cytopalsm is paler and skeletal muscle markers are lacking.

• triton tumour (MPNST with rhabdomyoblastic differentiation)

• sclerosing epithelioid fibrosarcoma: there are foci of typical fibrosarcoma. Musclee markers are negative.

• angiosarcoma: the surrounding tissues are diffusely infiltrated by anastomosing vascular channels.

• PNET may show anomalous dot-like desmin positivity. Sclerosing rhabdomyosarcoma may also be CD99 positive.

• acantholytic / pseudovascular squamous cell carcinoma.

References

¹Folpe, A. L., J. K. McKenney, et al. (2002). "Sclerosing rhabdomyosarcoma in adults: report of four cases of a hyalinizing, matrix-rich variant of rhabdomyosarcoma that may be confused with osteosarcoma, chondrosarcoma, or angiosarcoma." Am J Surg Pathol 26(9): 1175-83.

²Mentzel, T. and D. Katenkamp (2000). "Sclerosing, pseudovascular rhabdomyosarcoma in adults. Clinicopathological and immunohistochemical analysis of three cases." Virchows Arch 436(4): 305-11.

This page last revised 26.12.2003.

©SMUHT/PW Bishop