There are various kind of cysts that guinea pigs suffer from but the most common is the Sebaceous Cyst, with the Ovarian cyst coming in at second place.
In the main they are harmless fluid, or semi fluid filled pockets that can form in tissue or organs, and only a danger when they grow large enough to disrupt some function of tissue or organ.
I shall be dealing with the sebaceous cyst because A, it is the most common and B, it is easy to treat and providing it is treated early on is not threat whatsoever to the continued health of the guinea pigs.
These cysts are most common on the back and down towards the rump but they can occur elsewhere. It is a good idea to have a good palpate all over the surface of the guinea pig regularly when the skin checks are being made, see Skin problems. If you come across a soft pea sized lump which is not attached muscle or bone then it is almost certain to be a cyst.
There are some similar types of lump which for want of a better description, I call jelly beans because that's just what they feel like. They are a little larger and flatter and are usually found anywhere on the under side of the guinea pig. I haven't got the slightest idea about what they are, only that they are completely harmless and sometimes completely disappear but more often remain, getting no larger for the rest of the guinea pig's life.
Providing the soft cysts remain small, check a week later, then leave alone but if they continue to grow it is advisable to get them lanced and the waxy material expressed out then the cavity has to be flushed out with either Demisol or Aserbine fluid. Anaesthesia is totally unnecessary for this. In all the years I have been treating my own animals for cysts I have not lost one through stress or post operative infections. I can also speak for many other owners I know who have undertaken this simple work. It is, after all no more than our grand mothers used to do for us as children, long before governments and professions decided we were not bright enough to care for ourselves!.
By treating early, only a small wound is made so is less liable to become infected. If cysts are left to grow, eventually they will burst open and there will be a huge crater which obviously is more likely to become infected. There is no other option than to have all the debris that is commonly found in large cysts, removed under local anaesthetic and the wound sutured.
Ovarian Cysts can grow up to the size of a golf ball and still cause the animal no discomfort. One of the side effects of them, however, is that it makes the sows hyper active when they are in season. A more steady eye should be kept on sows with these cysts and sometimes surgery is necessary to remove them, but in the main they manage to live a normal life despite what would be for a woman a cyst the size of a football in her!.