Ok, first step is to breathe! Yes, lumps are always a concern on boxers, but not every lump, abd bump is something serious. I have had several cysts taken off my dogs, but only one mct, (mass cell tumor). Try not to out too much energy into "what if's". That is just wasted energy.
Next step is just what you are doing educating yourself and gathering info. Your vet may want to do a needle aspiration. This is sticking a needle into the bump and pulling some cells out to look at. This is a good first step, tho' not always totally accurate. Since your lump is marble size they should be able to get a sample. If they find a cyst they may just drain it then and there and give you some antibiotics.
If your vet recommends surgery to remove the lump, make sure it is sent out for a pathology. Also with surgery many vets use Acepromazine or ACE as a pre-surgery sedative. Tell your vet this is not to be given to your dog under any circumstances! (There is sticky thread at the top of this forum
http://www.boxerworld.com/forums/do...nfo-before-your-dog-has-surgery-sedative.html That you should read.) Some vets will argue that it is safe, but this dog is your and your BF's responibility and you are the ones paying the vet, so stand your ground if need be. I have only had to do this with one vet, all the others have always follwed my wishes or just didn't use ACE anyway.
Also if they do surgery, the incision will be bigger that you think. They take extra tissue from around the lump to make sure they get everything. So be prepared and try not to freak out. No one warned me and it took a lot for me to keep my composure when they brought my boy out to me. Then comes the hard part the waiting for the results. Your next step from there depends on the results.
I hope it is nothing but a cyst. Like I have said, my dog's have lots more cysts than anthing else. Let us know how your appointment goes! Sending positive haelthy ((Vibes)) to Lucy!