Is an mri definite? i did an mri incidental finding of small arachnoid cyst, wondering if mri without contrast could mistake this for like cancer or tumor?

Very likely : Arachnoid cysts are faiily common congenital lesion. We see them all the time. In general lesions outside the brain are benign, and lesions inside the brain can be more serious. When lesions are small and in a typical location, it is not usually a problem to recognize them. IV contrast usually not necessary. But it could help to detect and characterize other causes of symptoms.

Common cysts: An MRI is almost perfect, but not 100% definite. However, incidental arachnoid cysts are not uncommon and most people do not need a contrast MRI.