Chemobrain refers to a syndrome (a bunch of symptoms) that some people get when they take chemotherapy drugs. A more formal name is Chemotherapy-Related Cognitive Dysfunction. Another informal name is "chemo fog". Chemotherapy is a systematic treatment, meaning it goes all over the body. An unfortunate side effect of many chemotherapy agents is a diminshment in mental energy and acuity. As a stimulant, modafinil has found some interest from doctors for treatment of this condition.
Management of Cognitive Changes in Oncology Patients
.A phase III clinical trial of modafinil on people taking the chemotherapy drug doclataxel found "modest" benefit for the fatigue those patients experience.