AAPM: Lung SBRT spares normal tissue

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PHILADELPHIA—Lung cancer patients treated with stereotactic body radiation therapy (SBRT) experienced few lasting side effects, indicating the technique adequately spares normal tissue, according to a six-year study presented July 19 at the annual meeting of the American Association of Physicists in Medicine (AAPM).

SBRT hits tumors with extremely high—but narrowly focused—radiation doses, typically given in three to five treatments.

Brian Kavanagh, MD, PhD, a professor of radiation oncology at the University of Colorado Denver School of Medicine, and colleagues evaluated the temporal changes of normal lung tissue dose-response in 63 people who received SBRT between 2003 and 2009.

Patients received a hypofractionated treatment with a maximum of five fractions and a median total dose of 54 Gy. Researchers evaluated RT-induced lung density changes after fusion of planning CT scans with a maximum of five post-RT follow-up scans.

After six months, patients had transient density increases of up to 100 percent compared to their pre-treatment lung density. After 12 months, the density changes stabilized to less than 50 percent of pre-treatment levels, and lung morphology was mostly unaffected.

"We saw some changes, but nothing of a catastrophic nature or anything that implies we're going in the wrong direction with this treatment," Kavanagh said. "The first impression is very much a reassuring one."

Understanding how normal lung tissue is affected by the intense radiation will help physicians avoid excess injury to healthy tissue and more aggressively treat tumors, he said.

Using a biological equivalent dose (BED) model that converted SBRT dose schedules into 2 Gy-equivalents, researchers observed different evolutions of the lung density for BED levels of 0-45 Gy and 45-110 Gy.

"For the late response, no significant increase of lung density was visible for doses below 45 Gy. Above this threshold, the dose-density change was linearly increasing, but not as rapidly as after six months," Kavanagh said.

He said this is the "first report, to our knowledge, of SBRT-induced lung density changes showing that the lung density temporal response varied significantly with the received dose."

He concluded, "More than 90 percent of the lung volume experienced an acute response that reversed to nearly no late response, indicating that the lung morphology was mostly unaffected after 12 months. Since the acute response also partially reversed for the remaining 10 percent, SBRT was expected to only marginally affect long-term lung function."