BALTIMORE--A number of factors can suggest whether a focal pulmonary lesion is malignant or benign, but no single test affords a quick answer, and biopsy is usually necessary to make the diagnosis, said Peter White, Jr., MD, assistant professor of medicine at Johns Hopkins University.
At a meeting on nuclear medicine sponsored by Johns Hopkins, two speakers outlined advances in noninvasive imaging of such lesions via PET scan and somatostatin receptor scintigraphy, while Dr. White reviewed the lesion characteristics that may suggest malignancy.
A lung cancer goes through 40 doublings of volume in its life cycle and is not visible until it reaches 1 cm in size, Dr. White said. At 3 to 4 cm, it has gone through seven eighths of its life history.
"By the time it reaches 2 cm, the tumor contains 500 million malignant cells and starts to metastasize," he said. "When there are that many cells, it's not surprising that a few will wander off and set up shop somewhere else."
Statistically, the most common sites for lung cancers are in the upper lobe and in the right lung. "But," Dr. White said, "finding a lesion in the lower left lung doesn't mean it's not malignant."
Age is another suggestive factor. Fewer than 1% of malignancies arise in patients under 30, and fewer than 5% under age 40, so Dr. White places the crossover point to suspect malignancy at age 35. Nonetheless, he said, "you can't assume the lesion is benign on the basis of age alone."
Nor does calcification rule out a tumor, since 2% of lung cancers are calcified. Lesions with a wall thickness less than 4 mm are benign 95% of the time, while those with a thickness greater than 16 mm are 84% likely to be malignant. "Anything in between," Dr. White said, "calls for a biopsy." As for size and shape, malignancy is more likely if the lesion is greater than 3 cm, round or oval, has sharply defined edges, and is surrounded by aerated lung tissue.