Sternomastoid tumour (cyst). Close up of the swollen neck of a three week old baby with a sternomastoid tumour (centre). The prominent sternocleidomastoid muscle in the neck of the baby has been accentuated by turning its head. The sternomastoid tumour is a cyst in the embryonic tract which allows migrating nerves to descend into the chest through the belly of the sternocleidomastoid muscle. It is nonmalignant and is painless to the child. Surgery is performed for cosmetic reasons or if the cyst becomes infected.