A large, surgically proven series of 163 patients found that malignant neoplasms enhanced significantly more (mean, 40 Houndsfield units [HU]) than granulomas or benign neoplasms (mean, 12 HU).
Utilizing a 20-HU enhancement threshold as “indicative of malignancy,” the technique was 100% sensitive and 77% specific, with an overall accuracy of 93%. Several smaller series have reported similar results. False-positive results for malignancy occur occasionally in inflammatory lesions, such as active granulomas or round pneumonias, or in any richly vascular inflammatory lesion. False-negative results for malignancy have been reported rarely, and were thought to have occurred because of central (nonenhancing) necrosis of the tumor, which was evident in retrospect in these cases.
Advances In Computed Tomography HRCT Scans
HRCT has proven invaluable in the characterization of many diseases of lung, particularly for the characterization of diffuse interstitial lung disease. HRCT is performed with a thin-section technique, utilizing 1- to 2-mm thick slices and a high spatial frequency reconstruction algorithm.
HRCT has emerged as an important tool for evaluating patients with suspected diffuse interstitial lung disease prior to performing a biopsy. Among many indications for HRCT is its important role in guiding lung biopsy. CT can suggest the relative efficiency of endobronchial or transbronchial vs open lung biopsy for the diagnosis of acute and chronic infiltrative lung disease. Moreover, HRCT can suggest those areas of the lung in which a biopsy is most likely to provide specific histologic diagnoses or to allow assessment of disease activity. add comment
A number of diseases have sufficiently characteristic appearances that, in the appropriate clinical setting, a diagnosis may be possible by HRCT alone, obviating the need for biopsy. Patients with a reticular pattern and evidence of honeycombing have, by definition, diffuse lung fibrosis and generally do not require further histologic evaluation. While controversial, sarcoid and lymphangitic carcinomatosis, when classic in appearance, may be confidently diagnosed without the need for confirmation with a biopsy. In patients with centrilobular “groundglass” nodules, a diagnosis of subacute hypersensitivity can often be made without a biopsy.
The great majority of cystic lung diseases can be easily distinguished by their appearance and by the pattern of distribution of cysts, together with ancillary findings of nodules and the clinical history. These include Langerhans cell histiocytosis (eosinophilic granuloma) (Fig 5), lymphangioleiomyomato-sis/tuberous sclerosis, emphysema, and bronchiectasis. Thus, biopsy in most of these cases is also unwarranted.
Figure 5. HRCT in a patient with Langerhans cell histiocytosis (eosinophilic granuloma). HRCT demonstrates upper lung zone cystic changes and small nodular lesions that are characteristic of this disease.