The Radiologic Appearance of Lung Cancer

The article by O'Donovan discusses the radiologic appearance of lung cancer, with particular em
phasis on the radiographic appearance and work-up of solitary pulmonary
nodules (SPNs).

The pages devoted to the diagnosis of solitary pulmonary nodules are comprehensive and thorough. They include discussions of rate of growth and doubling times, CT densitometry, CT contrast enhancement, the appearance and significance of calcification within pulmonary nodules, and nodule margins, as well as the presence and diagnostic significance of cavitation. The use of PET scanning for diagnosis of solitary pulmonary nodules is briefly discussed.

In my own clinical practice at a cancer center where the incidence of malignant pulmonary nodules is considerably higher than at other institutions, I find that an emphasis on the patient's history (particularly a history of smoking, or possibly of other occupational exposures) and, most importantly, the availability of prior chest radiographs for comparison, are the most important factors in distinguishing benign from malignant solitary pulmonary nodules, prior to histologic confirmation.

Although I was a contributor to one of the early papers on nodule densitometry, this technology has fallen into disuse. The study is moderately complicated, and, unless performed frequently, it becomes somewhat unfamiliar. In addition, the evaluation of nodular density without resorting to cumbersome phantom technology has become more reliable with modern CT scanners.

Similarly, the margins of the lesion, presence of cavitation and the amount and distribution of calcium (unless calcification within the nodule has clearly benign characteristics) within nodules are, at best, highly relative indicators of benignity or malignancy, necessitating confirmation by other means. It may well be that the percentage of solitary pulmonary nodules in which there is a true diagnostic dilemma is actually quite small—the presence of a new nodule in a smoker or a patient with a history of malignancy necessitates histologic confirmation. A solitary pulmonary nodule in a patient from an area with endemic infectious processes that cause nodules also requires further evaluation. A nodule that has not changed size in 2 or more years is presumed to be benign.

Use of PET Scanning

In the last several years, PET has become available at some centers to evaluate patients in whom there is a significant possibility that the nodule may be benign. In these patients, a negative PET scan would strongly suggest benignity, with simple radiographic follow-up advised. For patients with abnormal increased uptake of FDG on PET scanning, histologic diagnosis is necessary, even though there is a discrete possibility that the histology may ultimately prove to be benign. Unfortunately this technology is not universally available and requires expensive equipment as well as geographic proximity to a cyclotron for production of the short-lived radionuclides used in this procedure.

In summary, my own emphasis would be on the clinical history (most importantly, cigarette smoking), the availability of old chest x-rays for comparison, and, if significant doubt persists, the use of PET scanning, followed by histologic diagnosis.

I found the discussion of the varying radiographic appearances of different histologic varieties of lung cancer enlightening. From a clinical point of view, as a practicing radiologist, I tend to focus on the staging of lung cancer, including both noninvasive imaging (CT, PET scan) and invasive procedures (percutaneous biopsy, mediastinoscopy, bronchoscopy). In addition, the impact of staging procedures on therapeutic choices merits scrutiny. The relative strengths and weaknesses of CT have been extensively investigated, with indications of high accuracy in determining extent of local disease (T-stage), as well as deficiencies (with an overall accuracy of no more than 70%) in correctly diagnosing mediastinal lymphadenopathy.

It appears quite possible that PET scanning may well alleviate some of these deficiencies in the detection of mediastinal nodal disease, with suggested overall accuracy in the 85% to 90% range. In addition, it has been indicated that PET scanning may be useful for locating unsuspected distant metastatic (M1) disease. Although these claims are still unconfirmed by large cooperative clinical trials, there appears to be some reason for cautious optimism concerning the use of PET scanning in the pretherapeutic staging of lung cancer.

The relative merits of various forms of invasive staging, as well as their limitations (for example, the inability of mediastinoscopy to sample all mediastinal lymph nodes) would go beyond the scope of an imaging review. Nevertheless, we can be grateful to this author for his succinct and comprehensive discussion of the presentation of lung cancer, including solitary pulmonary nodules.