Deep fibromatoses, also called desmoid tumors, are large, infiltrative masses that frequently recur but do not metastasize. They most frequently occur in the teenage years to 30s, predominantly in women. Abdominal fibromatosis generally arises in the musculoaponeurotic structures of the anterior abdominal wall but tumors can arise in the limb girdles or the mesentery. Deep fibromatoses contain mutations in the CTNNB1 (β-catenin) or APC genes, leading to increased Wnt signaling. Most tumors are sporadic, but individuals with familial adenomatous polyposis (Gardner syndrome, Chapter 16) who have germline APC mutations are predisposed to deep fibromatosis.
MORPHOLOGYFibromatosesaregray-white,firm,poorlydemarcatedmassesvaryingfrom1to15cmingreatestdiameter.Theyarerubberyandtough,andhavemarkedinfiltrationofsurroundingmuscle,nerveandfat.Cytologicallyblandfibroblastsarrangedinbroadsweepingfasciclesamiddensecollagenarethecharacteristichistologicpattern(
Fig.21.47 ).Thehistologyresemblesascar.
Because of the extensively infiltrative nature, complete excision is often difficult. Recent efforts have concentrated on medical therapy and radiation as alternatives to surgery.
Fig.21.46Nodularfasciitiswithplump,randomlyorientedspindlecellssurroundedbymyxoidstroma.Notethemitoticactivity(arrowheads).
Fig.21.47Deepfibromatosisinfiltratingbetweenskeletalmusclecells.
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Skeletal muscle neoplasms, in contrast to tumors of other lineages, are almost all malignant. The benign rhabdomyoma is more frequent in individuals with tuberous sclerosis.
Rhabdomyosarcoma is a malignant mesenchymal tumor with skeletal muscle differentiation. Three main subtypes are recognized: alveolar (20%), embryonal (60%), and pleomorphic (20%). Rhabdomyosarcoma (alveolar and embryonal) is the most common soft tissue sarcoma of childhood and adolescence, usually appearing before age 20. Pleomorphic rhabdomyosarcoma is seen predominantly in adults. The pediatric forms often arise in the sinuses, head and neck, and genitourinary tract, locations that do not normally contain much skeletal muscle, underscoring the notion that sarcomas do not arise from mature, terminally differentiated mesenchymal cells. The embryonal and pleomorphic subtypes are genetically heterogeneous.
Alveolar rhabdomyosarcoma frequently contains fusions of the FOXO1 gene to either the PAX3 or the PAX7 gene, rearrangements marked by the presence of (2;13) or (1;13) translocations, respectively. PAX3 is a transcription factor that initiates skeletal muscle differentiation, and it appears that the chimeric PAX3-FOXO1 fusion protein interferes with differentiation, a mechanism similar to many of the transcription factor fusion proteins that are found in acute leukemias.
MORPHOLOGYEmbryonalrhabdomyosarcomapresentsasasoftgrayinfiltrativemass.Thetumorcellsmimicskeletalmuscleatvariousstagesofdifferentiationandconsistofsheetsofbothprimitiveroundandspindledcells(
Fig.21.48A ).Rhabdomyoblastswithstraplikecytoplasmandvisiblecross-striationsmaybepresent.Sarcomabotryoidesisavariantofembryonalrhabdomyosarcomathatdevelopsinthewallsofhollowviscerasuchastheurinarybladderandvagina.