These comments about when to remove a suspicious lump came from Dr. Dennis Macy, DVM, is a noted veterinary oncologist affiliated with the University of Colorado and a member of the Feline VAS Task Force, via a member of the VAS Support Group.
Dr. Macy's Rule of Threes for lumps
Since we've had several folks recently with cats that had lumps trying to decide if they should be removed or not.... I asked Dr. Macy about them for the group. He said that he always goes by the "Rule of 3's." He said that's 3, 2, 1; three rules, the rule of 3s.
3) = if the lump is still there three months later regardless of biopsy results, REMOVE it. He said lumpectomy is fine for those (e.g., not radical removal).
2) = if the lump is EVER 2cm or larger in any dimension (width, length, height), at any time, REMOVE it.
1) = If the lump is GROWING AT ALL after one month, remove it.
He added that the lump should be wedge or punch biopsied first, before removal (NOT needle biopsy). That's because if it turns out to be a granuloma or just inflammatory cells then a lumpectomy is ok, but if its VAS, well then we all know it needs MUCH bigger and deeper margins, and apparently its much easier to get that accurately if most of the lump is intact for the actual removal.