Inalveolarrhabdomyosarcoma,anetworkoffibrousseptaedividethecellsintoclustersoraggregates,creatingacruderesemblancetopulmonaryalveoli.Thetumorcellsareuniformlyroundwithlittlecytoplasmandtheyareonlyminimallycohesive (Fig.21.48B
Pleomorphicrhabdomyosarcomaischaracterizedbynumerouslarge,sometimesmultinucleated,bizarreeosinophilictumorcellsthatcanresembleotherpleomorphicsarcomas.Immunohistochemicalidentificationofmusclespecificproteinssuchasmyogeninisusuallynecessarytoconfirmrhabdomyoblasticdifferentiation.
Rhabdomyosarcomas are aggressive neoplasms that are usually treated with surgery and chemotherapy, with or without radiation therapy. The botryoid variant of embryonal rhabdomyosarcoma has the best prognosis, whereas the pleomorphic subtype is often fatal.
Fig.21.48Rhabdomyosarcoma.(A)Embryonalsubtypecomposedofmalig-nantcellsrangingfromprimitiveandroundtodenselyeosinophilicwithskeletalmuscledifferentiation.(B)Alveolarrhabdomyosarcomawithnumer-ousspaceslinedbydiscohesive,uniformroundtumorcells.
Leiomyoma, a benign tumor of smooth muscle, is most common in the uterus but can arise in any soft tissue site. Uterine leiomyomas (Chapter 19) are common and may cause a variety of symptoms including infertility and menorrhagia. Leiomyomas also may arise from the erector pili muscles (pilar leiomyomas) in the skin and rarely in the deep somatic soft tissues or gastrointestinal tract. A germline loss-of-function mutation in the fumarate hydratase (FH) gene located on chromosome 1q42.3 leads to multiple cutaneous leiomyomas, uterine leiomyomas, and renal cell carcinoma. FH is an enzyme of the Krebs cycle, and this association constitutes another intriguing example of the link between metabolic abnormalities and neoplasia.
Soft tissue leiomyomas are usually 1 to 2 cm in size and are composed of fascicles of densely eosinophilic spindle cells that tend to intersect each other at right angles. The tumor cells have blunt-ended, elongated nuclei and show minimal atypia and few mitotic figures. Solitary lesions are cured with surgery.
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Soft tissue leiomyosarcoma accounts for 10% to 20% of soft tissue sarcomas. They occur in adults and affect women more frequently than men. Most develop in the deep soft tissues of the extremities and retroperitoneum or arise from the great vessels. Leiomyosarcomas have complex genotypes that stem from acquired defects that lead to profound genomic instability.
MORPHOLOGYLeiomyosarcomaspresentaspainlessfirmmasses.Retroperitonealtumorsmaybelargeandbulkyandcauseabdominalsymptoms.Theyconsistofeosinophilicspindlecellswithblunt-ended,hyperchromaticnucleiarrangedininterweavingfascicles.Theyexpresssmoothmuscleproteins(actin,desmin,caldesmon),whichcanbedetectedbyimmunohistochemistry.Unlikeleiomyomas,mitoticactivityandnecrosisarecommoninleiomyosarcoma.
Treatment depends on tumor size, location, and grade. Superficial leiomyosarcomas are usually small and have a good prognosis, whereas those of the retroperitoneum are difficult to control and cause death by both local extension and metastatic spread, especially to the lungs.