PET provides clues to improving lung cancer radiotherapy

LOS ANGELES-Positron emission tomography (PET) can be used to better characterize patterns of local failure after definitive radiation therapy for non-small-cell lung cancer, which may help to improve that therapy, investigators said at ASTRO 2007 (abstract 102).

"Local failure remains a common problem in NSCLC treated with radiation. More importantly, the characteristics of local failure have implications for treatment-planning strategies," said lead author Sonal Sura, MD, formerly of Memorial Sloan-Kettering Cancer Center and currently at George Washington University. "However, evaluating failures with CT scans is difficult because it is hard to differentiate between tumor recurrence and radiation changes."

Of 230 NSCLC patients who underwent involved-field radiation therapy, the researchers identified 26 patients who had both a local recurrence and a postrecurrence PET scan that could be registered with the treatment planning CT scan.

Patterns of failure

Local failures were evaluated independently by three radiation oncologists. They categorized each failure according to where it arose (primary tumor vs nodal area) and assigned it to one of four patterns of failure:

• Completely within the gross tumor volume (GTV).

• Within the GTV but extending outward.

• Marginal miss arising from the planning target volume (PTV) and extending outward.

• Geographic miss (outside the PTV but within 1 cm of the margin).

The patients had a total of 34 local failures; 26 of them arose from the primary tumor, whereas 8 arose from previously uninvolved nodal areas, Dr. Sura reported.

Nodal failures

Of the nodal failures, 63% had a marginal or geographic miss pattern, compared with only 4% of the primary tumor failures (P = .001).

"From this we conclude that nodal recurrences are mostly marginal or geographic misses. This suggests that improving the lymph node delineation, such as registering PET scans with CT scans by software or hardware methods, may improve outcomes," Dr. Sura said.

Primary tumor failures

Among the primary tumor failures, when the dose received was less than 60 Gy, 75% of the failures were within the GTV. In contrast, when the dose received was 60 Gy or greater, 33% of failures were within the GTV (P < .05).

"Dose affected the pattern of local failure, so when the dose was low, the failure was within the tumor, suggesting inadequate dose," Dr. Sura said. "When the dose was high, the failures were at the margin. This suggests that improving target delineation and/or accounting better for tumor motion may improve outcomes."