Hello,
I am afraid that from your description of the mass - firm growth attached to the muscle, not painful, no noticeable blood flow - it is difficult to say whether the growth can be malignant or benign.
A very common childhood cancer called Rhabdomyosarcoma can occur in the muscles of the body with similar characteristics - it is the 6th most common cancer in childhood.
Benign tumors attached to the muscle is rare - especially one that has grown so quickly.
Your doctor is right - you need to consult an oncologist as early as possible. CT scan MRI scan, PET scan will be needed for an immediate diagnosis and treatment.
You can read more about Rhabdomyosarcoma here:
- http://www.cancer.org/docroot/CRI/CRI_2_3x.asp?dt=53
I am sorry I could not help you more. I really hope your son's growth is diagnosed as benign and easily treatable.
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Experienced in the management of Pregnancy,Infertility,Menopause and all problems in Pregnancy and Gynecology