The Solitary Pulmonary Nodule

Benign Patterns of Calcification

Many nodules have one of the benign patterns of calcification. In these cases, the nodule is almost always benign and no further workup or followup is necessary. There are a few caveats, however. First, radiologists are not always as good at identifying calcification vs. non-calcification on plain films as they sometimes think they are, but there are many nodules for which there is no real doubt about the presence of calcification. Second, not everything that is calcified is benign -- it is important to identify one of the benign patterns of calcification (and even then remember that nothing in medicine is 100%). That being said, here are the benign patterns of calcification for pulmonary nodules:

diffuse
central
concentric
popcorn (associated with hamartomas)

Non-Calcified SPNs

It is quite often that a patient is getting an imaging study for one reason or another, and a solitary pulmonary nodule is unexpectedlly discovered. The immediate next step in the evaluation of the nodule depends upon how it was dicovered and the patients clinical situation (age, smoking history, symptomatology). In general, the following protocol will serve as a reasonable guide for non-calcified SPNs needing further workup:

Step 1: Prior Films

It's always good to look at prior studies if they exist and can be located. Nodules that are stable (i.e. unchanged in size) over a two-year period are considered to be benign and no further workup is necessarily indicated (though this may not be true in some particularly-high risk patients).

Step 2: Another View

Especially on plain films, overlapping shadows may give the appearance of a nodule when, in fact, there is none. Also, it is common for nipple shadows to simulate nodules. Therefore it is sometimes to helpful to repeat the study (especially PA chest films), maybe with nipple markers, right away. Almost never will the projection be exactly the same so that overlapping normal structures that simulate a nodule on one view will not be mistaken for a nodule on the second. In these cases, no further evaluation is needed. Likewise, it may be helpful to obtain PA and Lateral chest radiographs to better evaluate nodules initially discovered on either portable chest or abdominal radiographs.

Step 3: CT Scanning

If further evaluation of a pulmonary nodule is indicated, the next appropriate step is usually CT scanning. A typical initial CT scan of a solitary pulmonary nodule would consist of (1) a "routine" (e.g. 3 - 5 mm thick axial slilces) study of the entire chest, generally with contrast (to help evaluate for adenopathy) and thin (1 - 2 mm thick slices) through the nodule itself. The thin sections can help better evaluate the nodule and may reveal the presence of a benign pattern of calcification. The full study of the chest may identify other, unsuspected, nodules. This type of initial evaluation would also be appropriate for nodules initially discovered in the lung bases on an abdominal CT scan that did not include the entire chest.

Step 4: PET Scanning

Nodules that have reached this step may benefit from positron emission tomography scanning. It should be remembered that the purpose of PET scanning is to identify lesions that are likely malignant and to therefore identify lesions to biopsy that would not otherwise be biopsied at this time. Therefore, PET is not appropriate for (1) lesions that will be biopsied anyway (unless there is concern about other lesions or metastases or (2) lesions that will not be biopsied even if the PET scan is positive. A negative PET scan does not indicate a benign lesion -- these lesions will need to be followed. A lesion that is "hot" on PET may or may not be malignant, but it should be biopsied/resected. Additionally, to be appropriate for PET scanning, a lesion must meet a certain size requirement. Unfortunately, there is not universal agreement over what the minimum size is for PET scanning. There is near-universal agreement that PET is valuabe for lesions over 1 cm, and most people agree that it is valuable for lesions that are at lest 6 - 7 mm in diameter. The smaller the lesion, the greater the chance that PET would return a false-negative for a concerous lesion.

Nodules that are "hot" on PET should be biopsies/resected
Nodules that are "cold" on PET should be followed with serial CT scanning

Step 5: Serial Follow Up

When other means of lesion evaluation have been exhausted, the final step is evaluation with serial imaging. The goal here is to try to doculment lesion size stability over a two year period (a reliable indicator of benignancy) and to identify lesions growing at rate which may indicate malignancy. It has been shown that lesions which have a doubling time (time it takes for the nodule to double in volume) of less than 30 days or greater than 18 months are almost always benign. Any lesion that has a doubling time between 30 days and 18 months is potentially malignant and needs further evaluation or biopsy. When using lesion diameters to determine doubling time, it should be remembered that a 25% increase in diameter (or radius) indicates a doubling of the volume (assuming a sphere):

          Volume of a sphere = 4/3(pi*r^3)
          V(b) = 2V(a)
          4/3(pi*r(b)^3) = 2 * 4/3(pi*r(a)^3)
          r(b)^3 = 2r(a)^3
          r(b) = [cube root of 2]*r(a)
          r(b)=1.26*r(a)

It is possible that newer software that can automatically calculate lesion volume from CT images may allow better and more rapid determination of benign from potentially-malignant doubling times, but that question remains unanswerd. Thus, a typical follwoup regimen for a nodule will be to do scans at 6, 12 and 24 months from the date of initial discovery. These followup scans, in most cases, can be limited to the region of the chest that contains the nodule and do not require contrast. Thin sections should be used and, if possible, should be obtained using a slice thickness similar to that used on the initial thin-section study to prevent differences in partial volume averaging.

Step Never: Nodule Enhancement Protocol

It is my personal feeling that CT scans looking at how much a nodule does or does not enhance over 5 minutes following the injection of contrast are so rarely (if ever) helpful that they should not be a part of any SPN workup protocol, though not everyone agrees with this stance.