Penn State football coach Joe Paterno's swift decline after his lung cancer diagnosis may not be as surprising as the type of cancer that killed him, according to an oncologist who specializes in treating the disease.

The storied coach, who was 85, died Sunday, just 65 days after his son Scott said he had been diagnosed with a "treatable" lung cancer.

Mount Nittany Medical Center said Paterno, a nonsmoker, died of "metastatic small-cell carcinoma of the lung," an aggressive cancer that had spread beyond the lung.

Small-cell lung cancer accounts for 13 percent of all lung cancers, but usually occurs in smokers or former heavy smokers, said Barbara Campling, an oncologist at Thomas Jefferson University's Kimmel Cancer Center.

"It's extremely rare to have small-cell cancer in a nonsmoker," she said.

Fifteen percent of all lung cancers develop in non-smokers, but usually these are a type of non-small-cell cancer called adenocarcinoma, Campling said.

While the small-cell form responds well to chemotherapy, it grows rapidly and is virtually uncurable after it spreads to other organs. The chemotherapy drugs are "rigorous," especially for an older person, Campling said.

In addition to dealing with the effects of chemo and radiation, Paterno fell and broke his pelvis a few weeks after his diagnosis, according to news reports.

The emotional and psychological toll of being fired over the Penn State child sexual abuse scandal may also have hastened Paterno's death, observers have speculated.