Infectious and neoplastic diseases of the thorax are major causes of morbidity and mortality in patients with human immunodeficiency virus (HIV) infection. Multiple pulmonary nodules are among the more common findings in these patients, and differentiation of opportunistic infection, Kaposi's sarcoma and lymphoma is clinically important because further diagnostic evaluation depends on which of these entities is present. Edinburgh and associates evaluated whether the computed tomographic (CT) appearance of pulmonary nodules in patients with acquired immunodeficiency syndrome (AIDS) can help determine the potential infectious and neoplastic causes.

Sixty-five thoracic CT scans of patients with AIDS and multiple pulmonary nodules were reviewed retrospectively by two thoracic radiologists. CT scans were obtained when pulmonary disease was suspected clinically, but the chest radiographs were normal or showed questionable or nonspecific abnormalities. Sixty patients with AIDS-related opportunistic infections, Kaposi's sarcoma or non-Hodgkin's lymphoma had intrathoracic disease proved by a variety of invasive procedures (see the accompanying table). Pulmonary nodules were assessed by CT for size, distribution and morphologic character. The cases were classified into one of two groups depending on whether most nodules were smaller or larger than 1 cm in diameter. Nodules were also evaluated for presence of cavitation and associated lymphadenectomy, pleural effusion and airway disease.

Of the 43 patients with an opportunistic infection, 36 had a predominance of nodules smaller than 1 cm in diameter. Of the 17 patients with a neoplasm, 14 had a predominance of nodules larger than 1 cm. If most nodules were smaller than 1 cm in diameter, opportunistic infection was significantly more likely to be present. A predominance of nodules larger than 1 cm favored a diagnosis of neoplasm. Nodule size was of no use in differentiating between bacterial and mycobacterial infections or between Kaposi's sarcoma and lymphoma.

The nodules were evaluated for centrilobular or peribronchovascular distribution. When peribronchial nodules were identified, Kaposi's sarcoma was significantly more likely to be present. When nodules demonstrated a centrilobular distribution, opportunistic infection was significantly more likely, and neoplasm was unlikely to be present. However, a centrilobular distribution did not help to further differentiate bacterial and mycobacterial infection. Nodule cavitation did not help to further differentiate mycobacterial and bacterial infections.

Lymphadenopathy was significantly more common in patients with mycobacterial rather than bacterial infections. The presence of low-attenuating lymph nodes was found to be significant for further differentiating mycobacterial and bacterial infections. Pleural effusion was more common in patients with mycobacterial infection.

Results of this study suggest that nodule size and distribution are the most useful findings for differentiating among opportunistic infection, Kaposi's sarcoma and lymphoma. A predominance of nodules smaller than 1 cm was suggestive of opportunistic infection, while a neoplasm was more likely to be present when there was a predominance of nodules larger than 1 cm. The presence of a peribronchovascular distribution of nodules was suggestive of Kaposi's sarcoma. When a centrilobular distribution was identified, opportunistic infection was likely, and neoplasm was unlikely. The presence of lymphadenopathy and pleural effusion usefully differentiated mycobacterial infection and bacterial infection.

The authors conclude that when pulmonary nodules are seen in patients with AIDS, the size and distribution of the nodules are the CT features most useful for distinguishing among the potential causes and allow the clinician to focus on the diagnostic approach.