There are three therapies available for prolactinoma - medical, surgical and very rarely radiotherapy. But irrespective of the size of tumour, first line of treatment is medical therapy only and should be tried in all patients including those having visual loss. We have treated giant prolactinoma with cabergoline, which your sister has taken for a brief period. The tumour shrinks in size with medications. If patient does not respond to medical therapy and there is visual loss then only one should go for surgical treatment.Your sister needs only medical therapy. Restart her on Cabergoline 0.5 mg twice a daily and after two weeks increase to 1 mg twice a week with monthly prolactin levels and MRI after three months or so. It will be nice to know what was her response to treatment given in terms of prolactin levels and MRI if repeated.Do not worry and do not subject her to surgery even if suggested by other endocrinologist / neurosurgeon.