Congenital muscular torticollis is frequently associated with the development of a mass in the sternocleidomastoid muscle which resembles a malignant tumor. The mass is not a tumor in the specific sense of a true neoplasm but rather a localized swelling, the result of a reparative process.
The torticollis develops shortly after birth followed by a progressive development of a fusiform swelling of the involved muscle. The mass, which becomes apparent between the tenth and the fourteenth day, reaches its maximum size within three to five weeks and then slowly regresses, occasionally persisting until the fourth month.1 The "tumor" frequently involves the entire sternocleidomastoid muscle, or it may involve one or both poles. At times it is present in the absence of torticollis. If not excised, the mass gradually disappears and the muscle remains as a fixed fibrous cord. The torticollis becomes more marked with the growth of the child.
BRECKENRIDGE RL. STERNOCLEIDOMASTOID "TUMORS" IN CONGENITAL MUSCULAR TORTICOLLIS. AMA Arch Surg. 1951;62(1):134-139. doi:10.1001/archsurg.1951.01250030137016