乌鸦传媒

The lungs may have greater capacity for regeneration in adults than thought, according to a case report of new growth after lung cancer resection.

A woman who had her right lung removed at age 33 because of cancer showed progressive growth in the left lung over the next 15 years, Steven J. Mentzer, MD, of Brigham and Women's Hospital and Harvard, and colleagues found.

Not only did the size of the lung increase, but new alveoli developed. She had a 64% increase in the number of these tiny gas exchange units, the group reported in the July 19 issue of the New England Journal of Medicine.

Compensatory lung growth after pneumonectomy wasn't a surprise, but the mechanism was, they explained.

"The prevailing concept has been that lung regeneration after pneumonectomy occurs in the young through the growth of new acinar units and alveoli, whereas in adult humans, parenchymal hyperexpansion and alveolar dilatation -- not growth -- are thought to occur," Mentzer and colleagues wrote.

Prior studies have shown that lung capacity, not surprisingly, is cut in half after removal of a lung in adults, which was taken as evidence for no true growth.

However, some suggestive experimental evidence for adult lung growth in dogs suggested a longtime scale for growth -- on the order of months or years.

So the researchers looked at lung growth over a period of 15 years in the 33-year-old woman who had a right-sided pneumonectomy in 1995 for adenocarcinoma after a 32-pack-year history of smoking.

After surgery and chemotherapy, her lung capacity had dropped substantially from the baseline of at least 100% forced expiratory volume in 1 second (FEV₁) and forced vital capacity (FVC) down to 35% and 49% of the predicted value, respectively.

But by the end of the 15-year follow-up period, progressive improvements in spirometry brought the FEV1 up to 60% and FVC to 73%.

Those represented 51% and 35% relative improvements, respectively, compared with the expected 11% and 9% declines expected with aging by 15 years.

Annual surveillance CT imaging showed that the remaining lung progressively enlarged, herniating partially into the right hemithorax.

That lung expansion included both gas and total lung volume increases along with a gradual, sustained rise in tissue volume to about double what it had been immediately after the surgical resection.

An experimental MRI scan using hyperpolarized helium-3 gas to measure apparent diffusion of inhaled gas showed dimensions in the part of the airways where the alveoli are that were consistent with an increase in the number of alveoli rather than in the size of existing alveoli.

The alveolar depth in the growing lung, though, was shallower than in normal lungs (70 µm versus 138).

Factors that may have contributed to the growth were the relatively young age of the patient at the time of surgery and stretching of the lung from post-pneumonectomy syndrome and exercise, the researchers suggested.

After getting a prosthetic volume expander in the right lung cavity to ease dyspnea at 18 months, the patient exercised daily with a regimen of walking, cycling, and yoga.

"We hypothesize that, reminiscent of the role of stretch in lung development, cyclic stretch as such may be an important trigger for new lung growth," the group concluded. "Regardless of the specific mechanism, the findings in this patient support the concept that new lung growth can occur in adult humans."