If this cyst is actually that old,I would most definitely see your doc about it,really.though there is usually some fluid exchange constantly going on when you have something like that cyst,there are still alot of crappy things in there and the introduction of some certain things into your blood stream really can cause you body system issues not to mention change things in the blood.

I just had whats called a bakers cyst removed from the back of my knee two weeks ago and was told before the removal like last year? that if it should pop on its own,I needed to see my doc right away as the fluid could possibly contain some nasty crap and this being suddenly introduced into my system could cause all kinds of problems,and alot of inflammation too.

you may need some type of systemic anti B like cipro or something that is ment to take care of bacteria.I am actually taking that right now as I am having secondary problems just from my surgery/leakage of the cyst during removal.
and my cyst was only about a year and a half old.

sorry I don't know of any actual 'cleanse' that would be a natural way of accomplishing what may require just a round of anti bs.i would just see your doc.you may be okay doing nothing,but at least seeing a doc would keep the worries away anyway.good luck and I hope this doesn't cause you any actual problems.FB