I had a Persian cat for well over 20 years and as a veterinary assistant I've seen many long haired cats come in with "large and unusual tumors."
Actually they weren't tumors at all, they were clumps of matted fur.
Mats if left in place will cause sores on the skin that will lead to infection and or parasite (maggot) infestation. They will also make it difficult for the cat to move, and their claws will get stuck in the mats when they scratch themselves.
So they must be removed, but never, EVER cut them out. I'll explain that procedure later.
First let's talk prevention. Mats occur on long haired cats for three reasons.
1) The cat got wet or damp in (usually from being outdoors) and the drying hair clumped together. If this happens, try blow drying then combing. If the cat won't allow it, let the hair dry naturally then comb as soon as possible. Don't comb wet as it will tangle more.
2) An irritant such as a burr, twigs or even tape tangled in the hair and enlarged over time. Do daily fur checks for mats and take care of them immediately.
3) Not grooming (brushing or combing) the cat at least twice weekly. Again daily checks and grooming are the best solution.
Remember: Never Cut Out A Mat. Sometimes skin gets into the mat and I don't have to tell what will happen if you cut it.
If you locate a mat remove it by doing the following:
Hold the mat ball and carefully pull it apart from its center. If it's large pull away from the edges working toward the center, and breaking it into smaller pieces.
While some hair may come out with the mat it's usually part of the ball so you're not ripping out good hair, so continue working the mat until it's gone.
When you're done check the area. Sometimes it will be bald with redness. This means you got to it in time.
If there is bleeding, pus, or raised rash, keep the area clean, and seek veterinary advice as the cat may need antibiotic ointment or treatment.
Worse yet, if maggots are inside remove them with tweezers if possible, keep area clean and seek vet advice.
The best way to keep your long haired cat healthy is through proper grooming.
Shaving is also allowed if you don't show the cat.. You can learn this yourself with clippers and an instruction video.