Kaposi sarcoma (KS) is widely recognized as the most common malignancy associated with acquired immunodeficiency syndrome (AIDS). Originally, the disease was believed to be a form of primary skin cancer; however, a viral etiology now has been firmly established. [1, 2, 3, 4]
A chest radiograph is usually the initial examination in patients in whom KS is suggested, because radiographic appearances of pulmonary KS are among the most distinctive seen in patients with AIDS; even subtle abnormalities should be viewed as suggestive of pulmonary involvement in a patient with known mucocutaneous disease.
An accurate diagnosis of pulmonary KS can be established by using computed tomography (CT) scanning in 90% of patients. The role of magnetic resonance imaging (MRI) in the diagnosis of KS has not been defined. Ultrasonography of the thorax is useful in the evaluation of pleural disease and for guiding therapeutic thoracentesis. Radionuclide imaging is a useful adjunct to radiography and CT scanning in selected patients; it is not always possible to differentiate KS from opportunistic infections by using anatomic imaging. [5, 6, 4, 7, 8]
The radiologic characteristics of Kaposi sarcoma are depicted in the images below.
Usually, mucocutaneous lesions are the first clinical sign of KS. Lesions usually measure 1-2 cm in diameter and are raised, violaceous, and plaquelike. They may appear brown or black in patients with dark skin. Lesions increase in size and number as the disease progresses. Involvement of the GI tract, lymph nodes, lungs, and other viscera is common. Pulmonary involvement occurs in one third of patients and is recognized clinically in 10-15% of patients.
Symptoms of KS, such as breathlessness, cough, fever, and wheezing, are nonspecific. Hemoptysis may occur but is unusual. Although lung involvement invariably follows mucocutaneous disease, this may not be clinically apparent.
Limitations of techniques
A wide variety of pulmonary complications may occur in patients who are immunocompromised, including opportunistic infections, drug-induced lung disease, malignancy, and unrelated pathologic processes such as pulmonary edema and pulmonary embolism. These disorders may have similar radiographic appearances.
Radiographic findings in diffuse KS consist of 2 major patterns: linear interstitial nodules and fluffy ill-defined nodules. Coexistence of the 2 patterns is not uncommon. Radiographic appearances of pulmonary KS are among the most distinctive seen in patients with AIDS; even subtle abnormalities should be viewed as suggestive of pulmonary involvement in a patient with known mucocutaneous disease.
There is a tendency toward a perihilar distribution. This reflects the bronchovascular-centric nature of the disease, with characteristic thickening along bronchovascular bundles. The thickening becomes more nodular with tumor progression, with eventual confluence of poorly marginated nodules leading to dense airspace consolidation.
The middle and lower portions of the lungs are affected more frequently than the upper zones.
Classically, nodules in KS are described as flame shaped or spiculated. They typically measure 1-2 cm in diameter and frequently number more than 10. Septal lines may be seen as a result of lymphatic obstruction or tumor invasion (see image below).
The less common, focal form of KS appears as segmental, lobar, or masslike consolidation.
Airway involvement is common, occurring in as many as 75% of patients and occasionally leading to distal atelectasis. Effusions are common (30-90% of patients) and may be unilateral or bilateral. Hilar or mediastinal lymphadenopathy is reported in 10-16% of patients, although it is rarely bulky and is often not appreciated radiographically.
In patients with KS, characteristic CT scan findings correlate with findings on chest radiography, with bronchial wall thickening, ill-defined nodules, and areas of consolidation in a perihilar bronchocentric distribution appearing on CT scans (see the images below). Among AIDS-related thoracic diseases, KS is one of the most reliable diagnoses made on the basis of imaging findings. An accurate diagnosis of pulmonary KS can be established by CT scans in 90% of patients. [8, 9, 10, 11]
Frequently, the nodules are seen to be surrounded by a halo of ground-glass attenuation, representing localized hemorrhage. Interlobular septal thickening is a common feature seen on high-resolution CT (HRCT).
Pleural effusions and, less commonly, adenopathy may be identified.
Occasionally, patients with pulmonary KS with hemoptysis may present with patchy ground-glass attenuation caused by hemorrhage.
Magnetic Resonance Imaging
There is little evidence that MRI is as useful as other methods in diagnosing pulmonary KS. Although MRI is an excellent modality for detection of thoracic wall involvement, it is not useful in intrathoracic lesions apart from, perhaps, imaging large-vessel involvement.
In a study by Khalil et al of a series of 10 patients with AIDS-related KS, T1-weighted spin-echo images showed focally increased signal intensity in the pulmonary parenchyma.  Enhancement was observed in parenchymal lesions and along the bronchovascular bundles after the intravenous administration of a gadolinium-based contrast agent. The second echo of T2-weighted spin-echo images resulted in markedly reduced signal intensity in affected areas. The findings were not observed in a control group of patients with AIDS-related PCP. The signal intensity of KS lesions has been postulated to be related to the angiomatous and fibrous components of the tumor.
Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Systemic Fibrosis. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans.
NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see Medscape.
Ultrasonography of the thorax is useful in the evaluation of pleural disease and in the guidance of therapeutic thoracentesis.
Approximately two thirds of patients with KS presenting with new pulmonary abnormalities have coexistent opportunistic infections. Differentiating KS from opportunistic infections is not always possible by using anatomic imaging.
Gallium-67/thallium-201 (67Ga/201Tl) radionuclide imaging has been used to distinguish AIDS-related KS from other pulmonary disease processes. KS is 201Tl avid, but it does not take up 67Ga. Abnormal chest radiographic findings in association with negative 67Ga findings suggest the presence of pulmonary KS.
Indium-111–labeled polyclonal immunoglobulin is taken up by infection but not by KS or lymphoma. Indium-labeled liposomes have been shown to accumulate in KS, but uptake has also been reported with lymphoma. 
Radionuclide scans are a useful adjunct to radiographs and CT scans, particularly when radiographic findings are complex and when the exclusion of opportunistic infections that complicate KS is important. Indium-11-labeled polyclonal immunoglobulin uptake has a sensitivity of 97% in the diagnosis of AIDS-related pulmonary infections; by contrast, chest radiography has a sensitivity of 62%. Although HRCT is superior to chest radiography, some have suggested that HRCT may be less sensitive than 67Ga scintigraphy in the assessment of suspected Pneumocystis jiroveci pneumonia (PCP).
A false-positive rate of 15% has been reported for gallium scanning in the assessment for Pneumocystis jiroveci pneumonia (PCP). Other infections and lymphomas may also take up gallium. It has been suggested that thallium uptake may occur in lymphoma and infections such as PCP.