Joan is concerned because she often feels like she is going to fall over when she is walking. when she visits the doctor she is diagnosed with a tumor in the:

When Joan visits the doctor she is diagnosed with a tumor in the cerebellum.

Explanation:

The part of the brain that is referred to as the cerebellum is associated with the function of muscles and ligaments of the body and that is the reason why it is called the section responsible for 'motor control'. It is located on the lower hind side of the brain as is only as big as the size of a full-size walnut.