I am sorry about the cancer. I am assuming your brother was treated with chemotherapy and or radiation. Treated successfully means remission. The painful rt foot could be related to cancer or not. Any painful right foot needs to be evaluated thoroughly. we need to rule out any local infection. And then we need to rule out clot in the leg. Usually cancer patients are very prone for blood clots. to rule out blood clot need to do duplex of lower extremity. Other possibility is if the cancer(lymphoma) comes back it could obstruct blood flow, lymphatic return in the leg. For that we need to do ct scan of the abdomen and pelvis. If all of these done we can find an answer. I wish a speedy recovery for your brother.