When David Wolfe's hips began hurting in 2013, he dialed back his fitness routine.

But the Wyncote resident's five-mile runs weren't the problem. More than a decade after radiation helped cure his anal cancer, toxic side effects of the treatment were progressively damaging muscles and nerves in his lower spine, legs, and hips.

Medication and physical therapy gave him little relief. A specialist warned that a wheelchair might be in his future.

Instead, the tall, trim 55-year-old has made a dramatic recovery thanks to hyperbaric oxygen, a century-old technology that remains somewhat controversial because of a lack of rigorous medical studies supporting its use.

For 60 days in December and January, Wolfe spent at least two hours a day breathing pure oxygen inside a giant pressurized chamber at the Hospital of the University of Pennsylvania. Supersaturating the blood with oxygen is believed to promote healing.

"Now, six months after treatment, each week I'm feeling better," said Wolfe, a cancer clinical trials administrator for Merck. "I'm not back to running, but I'm off pain medications and back to work full time."

Patients such as Wolfe are a triumph — and a mystery – for doctors. Penn radiation oncologist James Metz, who treated Wolfe's anal cancer in 2002, recommended trying hyperbaric oxygen, even though Metz and the hyperbaric medicine team had never used it to treat such a long-delayed radiation injury.

"This is outside the box. I've never used it this far out," Metz said. "My radar is up if I see [such an injury] again. As we get more follow-up from David, it will be important to publish this case so people will be aware."

Diving into novel medicine

Developed in Europe, hyperbaric therapy was introduced in the U.S. in the early 1900s. University of Kansas anesthesiologist Orville Cunningham used it to save gravely ill victims of the Spanish flu pandemic. But later, he touted his treatment for everything from cancer to syphilis and was investigated by the American Medical Association, leading some to dismiss him as a fraud, according to historical accounts.

In the 1940s through the 1960s, hyperbaric oxygen was proven to help deep-sea divers suffering decompression sickness, and to combat carbon monoxide poisoning. Essentially, the increased pressure in the chamber – double normal atmospheric pressure — reduces the size of harmful gas bubbles in the patient's bloodstream so they dissipate.

Penn's hyperbaric medicine division grew out of that era of diving medicine research. The unit was established in 1968 by Christian J. Lambertsen, a primary inventor of scuba and the "father of the U.S. Navy Frogmen."

Penn's hyperbaric chamber is a 50-year-old, submarine-shaped behemoth, complete with portholes, that can accommodate up to eight patients and a technician. Its nickname is "the beast."

"It speaks well of the design that it has held up so well," said Kevin Hardy, a Penn hyperbaric and emergency medicine specialist.

Besides blood gas-related trauma, hyperbaric therapy has been approved to treat conditions including burns, crush injuries, stubborn diabetic wounds, certain infections of the skin or bone, and radiation injury. There is evidence that it fights infection, stimulates growth factors and stem cells, and triggers the development of blood vessels, a process called angiogenesis.

Nearly 1,400 U.S. hospitals have installed hyperbaric therapy units — either one-person units that look like long plastic caskets, or multi-person rooms, according to the Undersea and Hyperbaric Medical Society.

As with many medical technologies, there are concerns about overuse. Hyperbaric therapy has boomed since 2002. That's when Medicare began covering it for certain diabetic wounds, paying $450 for a two-hour session — even though the evidence of effectiveness is sketchy, Kaiser Health News reported.

>>READ MORE: Persistent wounds fuel $5 billion business, but does it work?

"It's expensive, and some practitioners overuse it, which gives a black eye to the field," said Matthew Kelly, chief of Penn's hyperbaric medicine division.

For Wolfe, the therapy was a lifeline.

Chasing a diagnosis

Radiation-induced injury is common. After all, half of cancer patients undergo radiation therapy. But the severity of damage varies, and the symptoms can be mistaken for common problems such as osteoarthritis — especially when those problems show up long after the cancer treatment.

That's why Wolfe chased a diagnosis for more than three years after his hips began hurting in 2013.

As his leg numbness, weakness, cramps, and pain grew steadily worse, orthopedic and physical medicine specialists at Penn diagnosed him with arthritis, bursitis, and a series of muscle tears – in his hips, his hamstrings, and a thigh.

"The leg pain worsened and the weakness became chronic," he recalled. "My legs would sometimes buckle underneath me. Sitting and standing even for short periods became increasingly challenging."

His treating physicians said the problems were unrelated to his 2002 chemotherapy and radiation, but he wasn't convinced.

"I just couldn't reconcile that the nature of the pain and symptoms could be normal age-related disease," he said.

Wolfe searched the medical literature, which led him to Michael D. Stubblefield, an international expert in radiation fibrosis syndrome at the Kessler Institute for Rehabilitation in West Orange, N.J.

Stubblefield explained that radiation to Wolfe's abdomen and pelvis had damaged the DNA in his healthy cells, triggering them to produce fibrin, which is normally involved in clotting. This fibrous protein damages blood vessels, hampering the blood supply to ligaments, tendons, muscles, and nerves.

"The process never stops or goes away, although there may be no symptoms for years," Stubblefield said.

Stubblefield advised Wolfe to continue the supportive treatments that had barely helped: physical therapy combined with drugs to relieve spasms, inflammation, and pain.

About a year ago, Wolfe took a medical leave from work. In desperation, he went back to Metz. The radiation oncologist proposed hyperbaric therapy.

It's a cumbersome, time-consuming procedure. Six days a week, for up to five hours a day, Wolfe sat in the chamber, wearing a bulky oxygen mask. For safety reasons, electronics, cosmetics, glass, watches, synthetic fabrics, and even pens and newspapers are forbidden. Mostly, Wolfe said, he would doze or watch the movie that technicians piped in.

About three weeks after he began the sessions, his physical therapist noticed subtle improvements.

"A few weeks later, I was able to begin weaning off the pain medication," Wolfe said. "In late January, I returned to work on a part-time basis, progressing to full time."

Stubblefield is thrilled for Wolfe – but can't say his success could be widely repeated.

"After David's experience, I did a medical literature search," Stubblefield said. "The data on effectiveness is still grim. In my experience with patients with chronic late radiation effects, hyperbaric therapy is usually not helpful. There are exceptions, and clearly, he's one. I don't know why he responded where others haven't. There's no guarantee his response is durable."

Wolfe can live with that.

"The quality of life I've gotten back is wonderful," he said. "It's really profound to me to see the difference it has made."