Lung Cancer.pdf

Thoracic Imaging Techniques 39
Typically, benign opacities have well-defined margins and a smooth
contour while malignant opacities have poorly defined or spiculated margins
and a lobular or irregular contour. There is, however, considerable
overlap in the typical appearances.
Two aspects of the internal morphology of an opacity—fat (x-ray attenuation
40 to 120 Hounsfield units HU) and calcification—are reliable
in distinguishing malignant from benign opacities. Fat within a nodule is
a characteristic finding of hamartomas and obviates further evaluation.
Calcification of an opacity can be useful in determining benignity, although
the majority of benign opacities are not calcified. Calcification that
has a diffuse solid, central punctate, laminated, or “popcorn-like” appearance
is diagnostic of a benign opacity except in patients with primary extrathoracic
osteoid-forming tumors, such as osteosarcomas. Metastases in
these patients can occasionally manifest as nodules with benign-appearing
calcification. Calcification can be detected histologically in up to 14%
of lung cancers and is occasionally visible on CT (Mahoney et al, 1990).
This calcification is typically amorphous and correlates with a high probability
of malignancy.
CT is considerably more sensitive than radiographic evaluation in the
detection of calcification. In most small, calcified opacities, calcification is
detected visually when thinly collimated slices (1–3 mm) are obtained
through the lesion. With the partial volume averaging that occurs when
more thickly collimated slices (7–10 mm) are obtained, calcification within
a small opacity may not be visible. Measurement of CT attenuation values
(CT densitometry) can be used to infer the presence of calcium within an
opacity. The use of this technique is, however, inappropriate if the opacity
is spiculated or is greater than 3 cm in diameter (Swensen et al, 1991). A CT
attenuation value of 200 HU is usually used to distinguish between calcified
and noncalcified opacities. If the density of the opacity is in the benign
range ( 200 HU), serial radiologic studies (radiographs or CT scans) are
obtained at 3, 6, 12, 18, and 24 months to confirm the absence of growth.
The sensitivity and specificity of this technique in the detection of benign
disease are, however, not optimal, and consequently CT densitometry is
not routinely used at M.D. Anderson.
Nodule growth is evaluated by reviewing pre-existing chest radiographs
or CT scans. The majority of malignant opacities double in volume
within 30 to 400 days. Nodular opacities that double in volume more rapidly
or more slowly are usually infectious or inflammatory. Absence of
growth over a 2-year period is generally a reliable indicator of benignity.
Recently, however, the appropriateness of this standard—particularly in
the setting of small nodules—has been questioned (Yankelevitz and Henschke,
1997). For growth to be detectable on successive radiographs, an
opacity must increase in diameter by 3 to 5 mm. In the case of a small
opacity, doubling in volume may produce a change in diameter of less