In many respects, Dr. Siebel represents a new type of retiree, the professional who, late in life, deploys his or her training in some new way. In doing so, this person is blurring the lines between work and leisure, and redefining traditional ideas about the nature of retirement.

For up to three months of every year — the limitation is Dr. Siebel’s choice — a medical employment agency books him for short stints in remote parts of Alaska, California or Idaho. He will only accept assignments near wilderness areas.

Weekdays, he sees patients. On weekends, he heads to the mountains and explores.

“I work for doctors on vacation or maternity leave,” Dr. Siebel said. “It’s easy for people with my particular training to get these jobs. What oncologists do is more or less universal, so you can easily integrate into an ongoing practice.”

What’s different now is how Dr. Siebel practices. “I have more time to find out about the patients’ lives, which helps me clinically,” he said. “I don’t have to worry about ‘the office,’ which can be consuming. Now all I do is sit there and be with the patients, which is one of the beauties of the new situation.”

This type of work flexibility isn’t possible for everyone. “You need in-demand skills, good health and some financial security,” Dr. Siebel said.