<unk>  (FMC), also termed sternocleidomastoid tumor of infancy, pseudotumor of infancy, and infancy sternocleidomastoid pseudotumor, is an uncommon (incidence: 0.4%-1.3% of live births), congenital tumor in one of the two sternocleidomastoid neck muscles although rare cases have presented with a FMC tumor in both sternocleidomastoid muscles. A tumor is here defined as a growth of tissue that is not coordinated with the normal surrounding tissue and persists in growing even if the original trigger for its growth is removed. FMC tumors are benign growths that may cause disfigurements but are not cancers and do not metastasize (i.e. spread} to distant tissues. As judged by microscopic cytology analyses,<unk>  tumors consist of spindle-shaped fibroblasts (i.e. the most common cell type in connective tissue) located in a background of<unk> gen fibers, decomposing skeletal muscle fibers, and, in some cases, regenerating skeletal muscle fibers. The fibroblasts have a completely normal appearance with no evidence suggesting that they are malignant. The World Health Organization, 2020, classified<unk>  as in the category of benign fibroblastic and myofibroblastic tumors. In the majority of cases, FMC tumors decrease in size and completely resolve by the newborn's second year. If left untreated, however, a significant percentage of cases progress to, and are the most frequent cause of, congenital muscular torticollis, i.e. an abnormal, asymmetrical head or neck position commonly called wry neck. Untreated FMC tumors may also progress to facial asymmetry, plagiocephaly (i.e. flattened head), permanent loss of neck mobility, scoliosis (i.e. sidewise curvature of the spine), or other structural disfigurements that result from compensatory mechanisms.
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