My Roxi has had three in the last three years and is still a very healthy (almost) nine year old. Her last one was on her lower front leg, where there isn't any extra skin, so getting a good margin was difficult. I have read in threads here that injections can be done directly into the tumor to shrink them.
Don't panic - *most* MCT's in Boxers are mild - grade 1 or 2.
Mom to Harley (white, 03/2006), Lizzie (adopted from AABR 08/2013, age 3?)
and Turbo (fawn, adopted from AABR 12/2008, born 2004)