My cyst was diagnosed by MRI in 1996, and I had surgery less than 3 months later. But I was lucky to find a great Neurosurgeon, who believed my symptoms were connected to, and caused by the cyst in the posterior fossa.
I think you need to get a second opinion, and maybe more opinions, until you find a neurosurgeon who believes you and will help.
My experiences have been a lot different and I was able to find a neurosugeon who knows something about Arachnoid Cyst, and was willing to operate, but I live is Australia, and things are somewhat different regarding healthcare...
~Jaime~