Exercise: Solitary Pulmonary Nodule

EXERCISE 4-6. SOLITARY PULMONARY NODULE

4-11. Characteristics suggesting that a nodule is benign are thatA the  size  of  the  nodule  does  not  change  over 2 months.it contains central calcification. CT attenuation values within the nodule are over 30 Hounsfield units.it is semisolid on CT.

Radiologic Findings

4-11. Frontal chest radiograph (Figure 4-31 A) shows a nodule in the left mid-lung that appears solid but is slightly lobulated. CT (Figure 4-31 B) of the chest demonstrates a popcorn pattern of calcification (arrow) (B is the correct answer to Question 4-11).

Discussion

In attempting to determine whether or not a nodule is be-nign, the characteristics to consider are the age of the pa-tient, any history of previous malignancy, and the nodule’s growth rate, density, shape, and edge characteristics. The most important of these are the growth rate and density. If a solid nodule has had no growth over a 2-year period and has calcification of the types associated with benign causes, then the nodule is almost certainly benign. Because of the importance of time in assessing growth, comparison with old images is the most important test and the least expen-sive method of determining whether a nodule is benign. Doubling times of lung cancers are variable, but an increase in diameter of the tumor would be expected in a 2-year pe-riod. The absence of growth of a solid nodule over a 2-year period is evidence that the nodule is stable is size and must, therefore, be benign. If radiographs demonstrate growth over this 2-year interval, then the nodule should be as-sumed to be malignant.

If a solid nodule is diffusely and completely calcified (Figure 4-32 A), if it is calcified centrally (Figure 4-32 B), or if it has a laminated pattern (Figure 4-32 C), then the nod-ule may be assumed to be benign. A popcorn pattern of cal-cification, also benign (Figures 4-31 B, 4-32 D), can be seen in a hamartoma. Calcification may not be apparent on the initial radiograph because the most commonly used tech-nique for chest radiography obscures subtle calcification. Demonstration of calcification may require fluoroscopy or repeated chest radiography with a lower kVp technique to enhance its depiction. When it is not clear from these studies whether calcification is present, CT should be used to iden-tify it. CT has an extended range of tissue discrimination compared to conventional radiographs. The presence of calcification within a pulmonary nodule can be determined by evaluating the attenuation values within a region of in-terest (ROI) centered over the nodule (Figure 4-33 A–C). Air within the lung measures –800 Hounsfield units, non-calcified nodules measure 30 to 100 HU, and calcified nod-ules measure over 200 HU. Nodules with attenuation values between 0 and 200 are not necessarily malignant; they just do not have enough calcification to be categorized unequiv-ocally as benign.If a nodule is not calcified or if it has shown growth over 2-year period, it should be considered as a possible malig-nancy, and further assessment should be dictated by the clin-ical circumstances. Most patients will need evaluation for possible tissue biopsy and surgical resection to determine the cause.

Nodules that are larger than 1 cm in diameter are gener-ally evaluated with PET-CT. Smaller nodules are generally considered below the threshold of resolution for this tech-nique. Nodules considered hypermetabolic on PET-CT (in-creased radiotracer uptake relative to background) are considered potentially malignant. Generally, these nodules then undergo percutaneous or surgical biopsy. However, whereas most cancers are hypermetabolic, bronchoalveolar cell carcinoma (BAC) and carcinoid may not be hypermeta-bolic (Figure 4-34). If these cancers are suspected on CT, a negative PET-CT examination could be a false-negative and should not preclude biopsy.

Nodules can also be ground glass in appearance (Figure 4-35). However, this appearance is nonspecific and can be seen in multiple etiologies including infection as well as bronchoalveolar cell carcinoma. BAC can present as a ground-glass nodule that may not demonstrate any signifi-cant growth over a 2-year period. Therefore, ground-glass nodules require more extended monitoring than solid nod-ules do.

 Note that the margins of the lesion, whether smooth or spiculated, are of no value in determining the benignity or malignant potential of a lesion. Only uniform or central calcification, absence of growth over a 2-year period, or CT attenuation values over 200 HU throughout the nodule are reliable noninvasive indicators of benignity.