In early October of 2009, my 19 yr old son was complaining that he was having very bad headaches, so bad that he could not gt out of bed and didn't want to lights on in his room. I took him to his PCP and he suggested a CAT scan of the head and the result were normal. He prescribed sinus meds just in case he was having sinus problems. The headaches were almost gone, until December when they came back, again took him to the PCP and this time he ordered a MRI which showed an arachniod cyst in the posterior fossa of the brain. The PCP suggested that we leave it alone and for my son to take 600 ml of Motrin everytime he gets a headache. My question is should we do what was suggested by the dr.or will he need surgery or will if go away on it's own? I am very worried, this child has been through so much, he also has chronic ITP since the age of 11 yrs old and recently had a splenectomy.