Radiation therapy for cancer is much different than stereotactic surgery. Radiation therapy for cancer is not accurately targeted on the tumor, as the surrounding tissues must be radiated as well to contain the spread of cancer. Thus the nasty side effects that your dad is suffering from (similarly chemotherapy does not only attack the cancer cells, but also the healthy cells). Additionally, because it also targets healthy tissue, the risk of further radiation induced malignancies is higher.
Stereotactic radiosurgery intends to deliver precise doses targeting only the tumor, while avoiding the surrounding healthy tissue. This is why it is more suitable for benign tumors, that are well defined in shape and contained. So there is no comparison whatsoever. Many physicians who are not familiar with stereotactic radiosurgery inadvertently scaring patients away from it, because they only know about conventional radiation.
Marianna