Lung Screening

Combination of Technologies Offer Most Accurate Results

Growths on a patient's lung are always a serious concern. While single nodules are often benign, tests must be performed to rule out cancer.

Case History: A 60-year-old male smoker underwent a CT scan and was diagnosed with a 4 mm right upper lobe pulmonary nodule. The patient was referred to Main Street Radiology for 6-month follow-up CT.

Findings: A non-contrast chest CT was performed on a 16-detector spiral CT. A small right upper lobe nodule is seen on the routine 5 mm image (Figure 1), which is better appreciated on the high-resolution 0.75 mm image (Figure 2). A 3D volumetric rendering of the nodule was also performed (Figure 3). The nodule measured 36 mm3 in volume, corresponding to 4.1 mm average diameter. This is unchanged since the prior exam, compatible with a benign process.

Discussion: Solitary pulmonary nodule (SPN) is a common finding in clinical practice. These lesions whether found on conventional X-ray or CT scan are usually benign, but a neoplastic process must be excluded, especially in patients with risk factors for bronchogenic carcinoma. Guidelines for evaluating SPNs are not clearly outlined, and opinions vary greatly (AJR 2001; 176:1363-1369).

When a nodule is seen on X-ray, CT scan of the chest should be performed, to exclude an artifact and to better characterize the lesion. Lung nodules may appear calcified on conventional X-ray, suggesting a benign diagnosis, which may eliminate the need for a CT. However, it has been shown that determination of presence of calcification in nodules on plain films is not reliable (AJR 2001; 176:201-204).

When performing a CT for pulmonary nodules, high-resolution images should be performed, with slice thickness of 1 mm or less, which will enable the Radiologist to determine if the lesion is benign. If the lesion is non-specific in appearance, 3D volumetric analysis produces more accurate measurement of the lesion if follow-up studies are warranted. In a recently published study (Radiology 2004; 231: 453-458), routine two-dimensional measurements were shown to be inaccurate for small nodules, with large inter-observer and intra-observer variability. In a different study, 3D volumetric measurement of nodule size has been shown to be significantly more accurate and reproducible (Radiology 2004; 231:446-452).

Considering the reproducibility of volumetric measurement and growth rate of bronchogenic carcinoma, mathematical analysis was performed to determine the appropriate time interval for follow-up of pulmonary nodules (Radiology 2004; 231:446-452). A one-month follow-up was sufficient to detect change in 10 mm nodules, while a 12-month follow-up was necessary for nodules less than 3 mm. Using two-dimensional measurements only, a 12-month follow-up was necessary for nodules less than 5 mm (Radiology 2004; 231:164-168).

Positron emission tomography (PET) has shown to be highly accurate for assessment of pulmonary nodules (JAMA 2001; 285:914-24), with sensitivity reported at 97%. PET scan should be considered for all SPN greater than 8 mm. Combining the volumetric spiral-CT and PET data, the following table summarizes the general guidelines for follow-up of SPN based on size.