I've been diagnosed with a 15mm pineal cyst after having an mri for headaches & tingling. should i be worried? neurologist advised rescan in 6 months

Seems reasonable: If the neurologist checked you over for other signs of mass effect (i.e. effects of the pineal cyst) and doesn't believe that your headaches or other symptoms are likely being caused by it then, I'd tend to vote in his/her direction with "benign neglect" as we say when things don't particularly impress us. In fact, I'm not sure that it's absolutely necessary to rescan unless symptoms change.

Monitor symptoms: If your neurological symptoms worsen or you experience new neurological deficits, then you should see your neurologist sooner. Keep a log of the frequency, duration, location, intensity, and characteristic of the headaches/tingling and take the log when you see your neurologist. http://radiopaedia.org/articles/pineal-cyst