We just found that my wife, age 58, has a "golf ball" sized arachnoid cyst in her left frontal area. could this create personality changes?

Possible: Yes, it is possible if there is significant involvement of the cortex (grey matter) of certain brain lobes (frontal, temporal, for example).

Unlikely: A cyst this size, in this location, is most likely congenital, and has been present for a long time. Unless there has been a significant change in size in a short time, it typically does not cause symptoms.

Yes: There is a famous case of a man who threw his wife out the window of their building and his pet scan showed clinical effects of an arachnoid cyst pressing on her right (?) orbital frontal and anterior temporal lobes. He got his sentence greatly reduced because of the neurobehavioral effects of the cyst. Does your wife have personify changes?

Yes: Any space-occupying lesion of that size impinging upon the frontal lobes could affect behavior in any number of ways.

Yes: Frontal lobe pathology commonly presents with head-aches and personality changes. Arachnoid cysts are an uncommon cause of frontal lobe pathology at your wife's age, but certainly possible. At a minimum, she needs a head MRI with and without contrast, neurosurgical evaluation and, if surgery is not felt to be appropriate, a repeat MRI in 6-12 months to confirm stability would be appropriate.