When i was at 18 yrs, a cyst appeared at my butt-cheek. it was left untreated. nw i'm 23, the cyst became ping pong ball sized lump. is a tumor(cncer)?

I do not know: At your age, cancer would be unlikely, but the definitive diagnosis is made with a biopsy. Sounds more like a sebaceous cyst or a lipoma, but as stated, see the Dr. and get a therapeutic excisional biopsy. Best wishes.

Possibly: The most likely type of tumor is a benign tumor known as a lipoma, which is essentially fat that grows a bit oddly in one location. these can wax and wane in size depending upon how much total body fat we have. These do not become cancer. Had this been a malignant tumor you would have not gone 4 years without that becoming obvious. Get it checked out anyway, a family doc will be able to check you.