Treatment of choice for sternocleido-mastoid tumour is:

The presence of a lateral neck mass in infancy in association with rotation of the head toward the opposite side of the mass results from fibrosis of the sternocleidomastoid muscle. The mass may be palpated in the affected muscle in approximately two thirds of cases. In the overwhelming majority of cases, physical therapy based on passive stretching of the affected muscle is of benefit. Rarely, surgical transection of the sternocleidomastoid muscle may be indicated. Ref: Schwaz's principle of surgery 9th edition, chapter 39.