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Background
Lymphocytic interstitial pneumonia (LIP) is a benign lymphoproliferative disorder characterized by lymphocytic infiltration of the pulmonary interstitial space. Clinically, it is frequently associated with gradual onset of dyspnea and cough over months to years, although systemic symptoms like fever, night sweats, and weight loss can be seen. LIP has no firm diagnostic criteria, but radiologic and pathologic assessment are typically used to help pinpoint a diagnosis. The purpose of this exhibit is to review the typical CT findings and pathologic characteristics of LIP in comparison with other interstitial pneumonias.

Educational Goals / Teaching Points
The combination of multiple characteristic imaging findings and the clinical presentation is used to help pinpoint a diagnosis of LIP, but there are many other interstitial pneumonias on the differential that must be ruled out. Desquamative interstitial pneumonia can have a similar appearance on CT imaging with lower lobe predominant ground-glass opacities and possible cysts, but typically lacks the centrilobular nodules and thickened bronchovascular bundles. Additionally, there is a significant association with cigarette smoking and male sex with frequent symptom resolution with smoking cessation and oral corticosteroids. Nonspecific interstitial pneumonia also presents with bilateral ground-glass opacities with lower lobe predominance, but it frequently coincides with traction bronchiectasis and subpleural sparing of the lower lobes. Usual interstitial pneumonia and interstitial pulmonary fibrosis also warrant consideration, but they are classically associated with honeycombing, which is rare in LIP.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
On CT imaging, LIP is typically associated with ground-glass opacities, centrilobular nodules, and perivascular cysts. The perivascular cysts are one of the more distinctive imaging findings in this disorder and are usually thin-walled, measuring less than 3 cm. There is typically a bilateral, diffuse distribution of these features with a basilar predominance. In addition, thickening of bronchovascular bundles and interlobular septa are frequently seen in these patients. Of note, there is an increased prevalence of LIP associated with Sjögren syndrome, AIDS, and less commonly systemic lupus erythematosus (SLE), and other autoimmune conditions. Histologic examination of LIP typically shows significant lymphocytic and plasma cell infiltrate present in the interstitium along with the presence of reactive lymphoid follicles along the peribronchiolar regions. Immunohistochemical analysis is required to differentiate LIP from a malignant lymphoproliferative disease, with polyclonal lymphocyte proliferation more indicative of LIP.

Conclusion
In summary, LIP is a rare interstitial pneumonia with no defined diagnostic criteria that relies heavily on CT imaging and histologic examination to make a diagnosis. Understanding the typical findings in both mediums is critical for differentiating it from other lung pathology and ensuring patients receive adequate treatment.