When Robin Hollins was diagnosed with an aggressive type of cancer that begins in the lining of the nose and sinuses, she was eager to begin treatment.

But the skull base tumor, called sinonasal undifferentiated carcinoma (SNUC), was very rare. Only about 2,000 people in the United States develop SNUC each year, so few doctors have much experience treating it.

Robin’s doctor in Virginia referred her to skull base tumor specialist...