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Dermpath-India Pathology of Rhabdomyoma Dr Sampurna Roy MD 2022
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Rhabdomyomas are divided into two major categories - Cardiac and Extracardiac. Extracardiac Rhabdomyoma: Extracardiac rhabdomyomas are rare benign tumours showing striated muscle differentiation. The three commonly recognised subtypes, adult, fetal, and genital, are clinically and morphologically distinct. Adult Rhabdomyoma: It is the most common subtype of rhabdomyoma even though it remains relatively rare. Age: Adult rhabdomyomas occur at a mean age of approximately 50 years although there is a wide age range. Site: They have a predilection for the head and neck region. The most common sites for these lesions are the larynx, pharynx, and the floor of the mouth. Unusual sites include the orbit, stomach, mediastinum, prostate, and heart. Gross features: It is a circumscribed mass (usually less than 10 cm) whose surface may be coarsely bosselated. These lesions have a brick-red colour. Occasionally the mass may be multinodular. Microscopic features: There are large polygonal cells arranged in solid sheets. The most characteristic features is the deeply eosinophilic cytoplasm. The cytoplasm may be finely granular or vacuolated owing to the presence of glycogen. So-called "spider cells" are formed by thin strands of cytoplasm separating peripherally placed vacuoles. In some cases vacuolization is so prominent so as to produce a microcystic pattern. The nuclei tend to be small and round, and located both peripherally and centrally. Nuclear pleomorphism, hyperchromatism and mitotic figures are generally absent. The cytoplasm often contains cross striations. In some cells, striations are separated from each other at regular intervals perpendicular to the long axis of the cell imitating mature skeletal muscle, while in others striations are of irregular thickness and haphazardly placed within the cell. Special stains: Histochemical staining with Masson's trichrome or PTAH (phosphotungstic acid hematoxylin) highlights the cross striations. Cell vacuoles are PAS-positive and diastase sensitive. Immunohistochemical staining: Cells react with myogenic markers, muscle specific actin, desmin, smooth muscle actin, myoglobin, as well as vimentin and occasionally S-100 protein. Reactivity for cytokeratin, epithelial membrane antigen, glial fibrillary acidic protein and CD68 are generally not found. Differential diagnosis of Adult Rhabdomyoma: Granular cell tumour; Alveolar soft part sarcoma;Paraganglioma; Hibernomas; Malignant melanoma ;Oncocytoma. Fetal Rhabdomyoma Presents as a slow growing mass, usually less than 5cm in size. Site: Fetal rhabdomyoma occurs primarily in the subcutaneous tissues of the head and neck region (common behind the ear). Age: Infants usually less than 3 years of age. Microscopic features: They are composed of a mixture of undifferentiated spindle shaped cells with an indistinct cytoplasm and myoblasts at different stages of differentiation. There is a zonal appearance with mature eosinophilic rhabdomyoblasts at the periphery and primitive mesenchymal cells together with immature skeletal muscle cells in a myxoid stroma, present at the centre. Nuclear atypia and mitotic figures are absent. Differential diagnosis: Embryonal Rhabdomyosarcoma Genital Rhabdomyoma Genital rhabdomyomas usually presents as solitary polypoid mass usually less than 3 cm in size. Site: The submucosal mass or cyst may be located in the vulva, vagina or cervix. This tumour only rarely involves the male genital tract (prostate gland, tunica vaginalis of the testis, epididymis). Age: Usually occurs in young or middle aged women. Microscopic features: The lesion is composed of elongated strap-like cells usually with ovoid or tapering nuclei and prominent cross striations. Immunohistochemistry: Desmin and myoglobin are strongly positive. Differential diagnosis: Botryoid rhabdomyosarcoma The recommended treatment for rhabdomyomas is complete excision.
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