Skeletal Muscle Tumours Notes

Skeletal Muscle Tumours

Rhabdomyoma and rhabdomyosarcoma are benign and malignant soft tissue tumours respectively of striated muscle.

Rhabdomyoma:

Rhabdomyoma is a rare benign soft tissue tumour. It is of two types: cardiac and extra-cardiac. Cardiac rhabdomyoma is probably a hamartomatous lesion and not a true tumour. Soft tissue rhabdomyoma is predominantly located in the head and neck, most often in the upper neck, tongue, larynx and pharynx.

Histologically, the tumour is composed of large, round to oval cells, having abundant, granular, eosinophilic cytoplasm which is frequently vacuolated and contains glycogen. Cross striations are generally demonstrable in some cells with phosphotungstic acid-haematoxylin (PTAH) stain. The tumour is divided into adult and foetal types, depending upon the degree of resemblance of tumour cells to normal muscle cells.

Rhabdomyosarcoma:

Rhabdomyosarcoma is a much more common soft tissue tumour than rhabdomyoma and is the commonest soft tissue sarcoma in children and young adults. It is a highly malignant tumour arising from rhabdomyoblasts in varying stages of differentiation with or without demonstrable cross-striations. Depending upon the growth pattern and histology, 4 types are distinguished: embryonal (including sarcoma botryoides), alveolar, pleomorphic, and spindle cell/sclerosing type.

1. Embryonal Rhabdomyosarcoma: The embryonal form is the most common of the rhabdomyosarcomas. It occurs predominantly in children under 12 years of age. The common locations are in the head and neck region, most frequently in the orbit, urogenital tract and retroperitoneum.

Grossly, the tumour forms a gelatinous mass growing between muscles or in the deep subcutaneous tissues but generally has no direct relationship to the skeletal muscle.

Histologically, the tumour cells have a resemblance to an embryonal stage of development of muscle fibres. There is considerable variation in cell types. Generally, the tumour consists of a mixture of small, round to oval cells and spindle-shaped strap cells having tapering bipolar cytoplasmic processes in which cross-striations may be evident. The tumour cells form broad fascicles or bands. Mitoses are frequent.

Sarcoma Botryoides: Botryoid sarcoma is regarded as a variant of embryonal rhabdomyosarcoma occurring in children under 10 years of age. It is seen most frequently in the vagina, urinary bladder and nose.

Grossly, the tumour forms a distinctive grape-like gelatinous mass protruding into the hollow cavity.

Histologically, the tumour grows underneath the mucosal layer, forming the characteristic cambium layer of tumour cells. The tumour is hypocellular and myxoid with a predominance of small, round to oval tumour cells.

2. Alveolar Rhabdomyosarcoma: Alveolar type of rhabdomyosarcoma is more common in older children and young adults under the age of 20 years. The most common locations, unlike the embryonal variety, are the extremities.

Grossly, the tumour differs from the embryonal type in arising directly from skeletal muscle and growing rapidly as a soft and gelatinous mass.

Histologically, the tumour shows a characteristic alveolar pattern resembling pulmonary alveolar spaces. These spaces are formed by fine fibro collagenous septa. The tumour cells lying in these spaces and lining the fibrous trabeculae are generally small, lymphocyte-like with frequent mitoses and some multinucleate tumour giant cells. Cross-striation can be demonstrated in about a quarter of cases.

3. Pleomorphic Rhabdomyosarcoma: This less frequent variety of rhabdomyosarcoma occurs predominantly in older adults above the age of 40 years. They are most common in the extremities, most frequently in the lower limbs.

Grossly, the tumour forms a well-circumscribed, soft, whitish mass with areas of haemorrhages and necrosis.

Histologically, the tumour cells show considerable variation in size and shape. The tumour is generally composed of highly anaplastic cells having bizarre appearances and numerous multinucleate giant cells. Various shapes include racquet shapes, tadpole appearance, large strap cells, and ribbon shapes containing several nuclei in a row.

4. Spindle Cell/Sclerosing Rhabdomyosarc-Oma: This variant has been separated from embryonal rhabdomyosarcoma due to different morphology and genetic changes. It occurs in children as well as in adults but the prognosis is worse in adults.

Histologically, the tumour is composed of dyscohesive round cells with scanty cytoplasm lying in the densely sclerotic stroma.

Conventionally, the cross-striations can be demonstrated with PTAH stain in a few rhabdomyosarcomas. Immunohistochemical stains include myogenin, Myo-D1, desmin, actin, myosin, myoglobin, and vimentin. Based on cytogenetic and molecular abnormalities, alveolar rhabdomyosarcoma with t(1;13)(q36;q14) and PAX3-FOXO1A fusion gene have a better prognosis, compared to cases having translocation t(2;13)(p35;q14) andPAX7-FOXO1A fusion gene which has a worse prognosis.

Skeletal Muscle Tumours:

• Rhabdomyoma is an uncommon benign tumour which may be cardiac type and a soft tissue tumour.
• Rhabdomyosarcoma is a much more common soft tissue sarcoma, arising from rhabdomyoblasts. It is the commonest soft tissue sarcoma in children and young adults.
• Four histologic types are distinguished: embryonal (including sarcoma botryoides), alveolar, pleomorphic, and spindle cell/sclerotic type.
• Pleomorphic rhabdomyosarcoma is more common in adults while others occur in children.