Respected Sir, There Is A Lump (about 4-5 Cm Diameter)

Respected sir, there is a lump (about 4-5 cm diameter) on my right buttock since very long (about 15-18 years). It might has increased a little within last 5 years. It pains a little when it is pressed otherwise it doesn't create much problem for me so I was not worried but now suddenly a fear came to my mind that can it be anything serious like cancer? When I sit for some time then it compresses in size and then it regains it's original size. Now it feels firm upon palpation. I consulted in OPD of a local hospital's surgery department then they said to do FNAC.I asked them whether it's a cancer then they said no, but I couldn't discuss in detail because they were busy.Sir, I am worried about cancer only. If it is not cancer then can I leave it untreated?