Bostons can be prone to tumors. One of my Bostons had one on his brow as a pup. The vet gave him some cream noted that it may recede and may not. It only got larger so I had a biopsy done and it was benign. Legs, face, and head are common areas for them to appear. The biopsy was very inexpensive and fast. Good luck...
Mine is 6 yrs. 2 yrs ago had a lump size of a nickel on back leg knee. Vet said cancer wanted to remove leg. Vet said take it off. I said no way. Just took tumor off. She did 2 yrs now and it came back again. I love this dog and she is very sensitive and I would not have her go through that and chemo. She has no problem at all with it. I took her off all shots except heart worm. I am hoping she lives another couple of years at least but will not take off leg to hip bone.