Photomicrograph showing nodules of tumor cells separated by hyalinised fibrous septae (50×, HE stain). Inset: Discohesive large tumor cells with hyperchromatic nucleus and scant cytoplasm (200×, HE stain). The diagnosis was postauricular congenital alveolar<unk> . Given the difficulty in diagnosing<unk> , definitive classification of subtypes has proven difficult. As a result, classification systems vary by institute and organization. However,<unk>  in the 2020 WHO classification is listed as four histological subtypes: embryonal, alveolar, pleomorphic, and spindle cell/sclerosing. Embryonal Embryonal<unk>  (ERMS) is the most common histological variant, comprising approximately 60–70% of childhood cases. It is most common in children 0–4 years old, with a maximum reported incidence of 4 cases per 1 million children. ERMS is characterized by spindle-shaped cells with a stromal-rich appearance, and the morphology is similar to the developing muscle cells of a 6– to 8-week-old embryo. Tumors often present in the head and neck as well as the genitourinary tract. Embryonal subtype Botryoid<unk>  is almost always found in mucosal lined organs including the vagina, bladder, and nasopharynx (although presentation in the nasopharynx typically affects older children). It often presents in infants younger than a year old, as a round, grape-like mass on the affected organ. Histologically, cells of the botryoid variant are defined by a dense tumor layer under an epithelium (cambium layer). This subtype has a good prognosis. Botryoid<unk>  is also sometimes present in adult women, found in the cervix or uterus. Alveolar Alveolar<unk>  (ARMS) is the second most common type. ARMS comprises approximately 20–25% of<unk> -related tumors, and it is equally distributed among all age groups with an incidence of about 1 case per 1 million people ages 0 to 19. For this reason, it is the most common form of<unk>  observed in young adults and teenagers, who are less prone to the embryonal variant. This type of<unk>  is characterized by densely-packed, round cells that arrange around spaces similar in shape to pulmonary alveoli, although variants have been discovered without these characteristic alveolar spacings. ARMS tends to form more often in the extremities, trunk, and peritoneum. It is also typically more aggressive than ERMS. Pleomorphic Pleomorphic<unk>  (undifferentiated)<unk> , also known as anaplastic<unk> , is defined by the presence of pleomorphic cells with large, lobate hyperchromatic nuclei and multipolar mitotic figures. These tumors display high heterogeneity and extremely poor differentiation. The pleomorphic cells may be diffuse or localized, with the diffuse variation correlating to a worse prognosis. It occurs most often in adults, rarely in children, and is often discovered in the extremities. Due to the lack of discernible separation among cancers of this type, clinicians will often label undiagnosed sarcomas with little to no discernible features as anaplastic<unk> . It is the most aggressive type of<unk> , and will often require intensive treatment. Spindle cell/sclerosing Spindle cell/sclerosing<unk>  is an added subtype listed in the 2020 WHO classification of soft tissue sarcomas. This subtype is very similar to that of leiomyosarcoma (cancer of the smooth muscle tissue), and it has a fascicular, spindled, and leiomyomatous growth pattern with notable rhabdomyoblastic differentiation . It occurs most commonly in the paratesticular region, and the prognosis for this particular form of<unk>  is excellent with a reported five-year survival rate of 95%. The sclerosing aspect of this subtype has a hyaline sclerosis and pseudovascular development. Multiple classification systems have been proposed for guiding management and treatment, and the most recent and widely used classification system is the "International Classification of<unk> " or ICR. It was created by the IRSG in 1995 after their series of four multi-institutional trials aimed at studying the presentation, histology, epidemiology, and treatment of<unk>  (IRSG I–IV). The ICR system is based on prognostic indicators identified in IRSG I–IV. Pleomorphic<unk>  usually occurs in adults rather than children, and is therefore not included in this system.
Rhabdomyosarcoma
Myosarcoma
Pyomyoma
Rhabdomyoma
Hemangiosarcoma
Liposarcoma
Angiosarcoma
Fibrosarcoma