Which brand of medicine focuses on the study of tumors and cancers

You probably meant branch, instead of brand. But, the branch of medicine that studies cancer, tumors and their cures/causes, is called oncology.

Which means that, all the treatments we now have against cancer for example, chemotherapy, were, if not developed, used, studied and improved by oncologists.

Another example can be stem cell transplant, which is related to both cancer and tumors (which no, are not the same thing).

Hope it helped,

BioTeacher101