Hi Kathleen,
As you know by now, lipomas are considered benign and not a concern unless unsightly or interferring with something. I have removed some large ones, and getting the whole mass out is sometimes impossible. They wrap around structures, and displace things like muscle, but otherwise don't do damage.
If enough of this mass has been removed, I would leave it alone unless it reappears, as there has already been some unpleasantry associated with radiation. It may not reappear, if left alone post-op.
A fatty mass is 90%+ fat, and very few cells for radiation to act upon.
I see no age difference associated with a treatment plan, or anything untoward in finding the problem in a younger dog. The results would be the same. I know of no prediliction for lipomas, as many dogs have them. Granted, they appear more commonly in older dogs, but it is not a requirement. I am not aware of any defined cause for their appearance...lots of supposition, however.
My advice....do nothing more unless it reappears. Then plan accordingly. I hope this is of help.
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