This finding in the skull does not suggest the presence of Alzheimers or MS. They do not affect the skull at all, so you should not worry about this.
If your doctors are worried about myelofibrosis, this can be worked up. You need to start with a CBC.
A CBC (complete blood count) usually shows abnormally low levels of red blood cells, a sign of anemia common in people with myelofibrosis.
White blood cell and platelet counts are usually abnormal, too.
Your docs can also do x-rays or an MRI of other large bones (e.g. femur, pelvis) to look for more extensive bone involvement.