(1)
Department of Pathology, University of Manitoba Max Rady College of Medicine, Winnipeg, MB, Canada
Keywords
Alveolar rhabdomyosarcomaEmbryonal rhabdomyosarcomaPleomorphic rhabdomyosarcomaRhabdomyomaSkeletal muscle tumorHistologically, skeletal muscle cells are distinguished by eosinophilic cytoplasm containing light and dark striations that correspond to the alternating arrangement of thin and thick myofilaments (Fig. 8.1). These cells can be seen in varying degrees in soft tissue tumors with skeletal muscle differentiation. However, in tumors that are poorly differentiated, rhabdomyoblastic differentiation must sometimes be confirmed by immunohistochemical stains for skeletal muscle markers (e.g., myogenin) [1].
Fig. 8.1
Normal skeletal muscle cells with striated and eosinophilic cytoplasm
8.1 Rhabdomyoma
Rhabdomyomas are rare and benign neoplasms of skeletal muscle differentiation. These are divided into cardiac, adult, fetal, and genital types. Cardiac rhabdomyomas typically arise as single or multiple tumors in the right or left heart ventricles and are often associated with tuberous sclerosis. Patients can present with symptoms of arrhythmia or cardiac dysfunction [2, 3]. Adult rhabdomyomas arise as slow-growing lesions in the head and neck in adults after the fifth decade of life. Patients can present with problems swallowing or experience vocal cord dysfunction [4–6]. Fetal rhabdomyomas predominantly occur in the head and neck in children and have been reported in patients with nevoid basal cell carcinoma syndrome [7, 8]. Genital rhabdomyoma usually arises as a slowly growing polypoid mass in the vagina or vulva in middle-aged women (average age of 45 years) [9].
Pathology
The histologic appearance of rhabdomyomas depends on the subtype. Cardiac rhabdomyomas contain sheets of polygonal cells that are highly vacuolated, otherwise known as “spider cells” (Fig. 8.2). Adult rhabdomyomas show polygonal cells with eosinophilic and granular cytoplasm. Scattered vacuolated cells can also be seen. Fetal rhabdomyomas demonstrate spindle cells with thin and elongated nuclei (Figs. 8.3 and 8.4). Depending on the morphologic maturity of the tumor, these tumor cells may contain cytoplasmic cross striations. Genital rhabdomyomas demonstrate an aggregate of muscle cells with scattered cross striations (Fig. 8.5) [6].
Fig. 8.2
Cardiac rhabdomyoma with “spider cells”
Fig. 8.3
Fetal rhabdomyoma with spindle cells
Fig. 8.4
Fetal rhabdomyoma with spindle cells with very focal cross striations (arrow). These cells can sometimes be difficult to distinguish from a rhabdomyosarcoma
Fig. 8.5
Genital rhabdomyoma with skeletal muscle cells containing cross striations
Ancillary Studies
Consistent with their myogenic nature, rhabdomyomas stain with desmin.
Differential Diagnosis
Hibernoma
Embryonal rhabdomyosarcoma
Comment
- 1.
While hibernomas can have cells with eosinophilic cytoplasm, they usually contain scattered adipocytes that indicate their lipogenic differentiation. Unlike rhabdomyomas, hibernomas are negative for muscle markers such as desmin.
- 2.
Rhabdomyomas with particularly immature cells, such as fetal rhabdomyoma, can be difficult to discern from embryonal rhabdomyosarcoma.
- 3.
Embryonal rhabdomyosarcoma is typically more infiltrative and has increased mitosis and necrosis compared to a rhabdomyoma [6].
- 4.
Adult rhabdomyomas are treated by simple excision. Up to 40% of lesions can recur, but these are typically treated with re-excision [4].
- 5.
Given that cardiac rhabdomyomas often spontaneously regress, surgical excision is only performed in patients who are experiencing arrhythmias or hemodynamic compromise.
- 6.
- 7.
Fetal rhabdomyomas are typically cured by simple excision. Less than 10% recur [7].
8.2 Embryonal Rhabdomyosarcoma
Embryonal rhabdomyosarcoma is a malignant soft tissue sarcoma of skeletal muscle differentiation that predominantly occurs in young children, often under 10 years of age [11]. Frequent sites of occurrence include the head and neck and hollow organs, such as the nasopharynx, bladder, and vagina [6, 12].
Pathology
Grossly these tumors have a white and friable appearance [9]. Microscopically, they are composed of highly immature cells in a variably myxoid background (Fig. 8.6). The neoplastic cells have scant cytoplasm, and only occasionally contain skeletal muscle type striations, otherwise known as rhabdomyoblasts (Fig. 8.7). These rhabdomyoblasts are increased in tumors that have been previously treated with chemotherapy (Fig. 8.8) [13]. In the botryoid variant of embryonal rhabdomyosarcoma, the tumor cells approach but do not overrun the overlying squamous or urothelial mucosa (Fig. 8.9).
Fig. 8.6
Embryonal rhabdomyosarcoma with highly immature tumor cells with limited cytoplasm
Fig. 8.7
Rare rhabdomyoblast demonstrating eosinophilic cytoplasm and cross striations (arrow) in an embryonal rhabdomyosarcoma
Fig. 8.8
Rhabdomyosarcoma previously treated with chemotherapy showing multiple rhabdomyoblasts
Fig. 8.9
The tumor cells in this botryoid variant of embryonal rhabdomyosarcoma condense around the adjacent urothelium
