Cysts are a frequent finding in LIP and DIP and are occasionally found in HP. In comparison with LAM or LCH, cysts in these conditions are not as profuse and so the distribution of disease does not help in refining the differential diagnosis. Instead, careful scrutiny for ancillary features, such as nodules and ground-glass opacities, is required if a diagnosis is to be suggested on radiological grounds. While nodules are a well recognised feature of LCH, ground glass opacification is not typically associated with either LCH or LAM.
LIP
LIP is an uncommon, benign, lymphoproliferative disorder characterised pathologically by a diffuse lymphocytic infiltrate. It is often associated with collagen vascular disorders, in particular Sjogren’s syndrome, but is also seen in patients with HIV infection [14]. Non-infection associated LIP typically presents in women in the 5th decade, with shortness of breath or cough [3]. The condition usually responds to treatment, but is known on rare occasions to be complicated by pulmonary fibrosis or to transform into lymphoma [15].
Ground-glass opacities and nodules are almost universal features in LIP, with cysts seen in about two-thirds of patients [16]. Interlobular sepal thickening and reticular opacities have also been reported [7, 16]. The lung cysts are usually small (less than 3 cm), thin walled and distributed in a scattered, random distribution (Fig. 8), though much larger cysts are also recognised [17].
Ultimately, most patients with suspected LIP on clinico-radiological grounds will require lung biopsy to confirm the diagnosis and in some cases to exclude low-grade lymphoma. At the same time, clinico-radiological integration with histological findings is essential, because it can sometimes be difficult for the pathologist to distinguish LIP from hypersensitivity pneumonitis and NSIP [15].
Sub-acute HP
HP is a diffuse granulomatous inflammatory disease of the lungs caused by exposure to a wide variety of agents ranging from organic particles to chemotherapy agents. Patients often present with gradually worsening dyspnoea, but because the presence of a precipitating agent is not always obvious from the clinical history, radiology has a role to play in suggesting the diagnosis. The key histological findings are bronchocentric lymphocytic infiltration and poorly formed granulomas, but these abnormalities are not always identified on lung or transbronchial biopsy, adding sometimes to diagnostic uncertainty [18].
Cysts on HRCT are seen in approximately 10% of patients with subacute HP and are usually few in number and random in distribution (Fig. 9) [19]. Despite their low frequency, the presence of cysts can be a helpful clue in making a radiological diagnosis of HP, when identified in conjunction with the more classical signs of the disease: these are centrilobular ground glass nodules and a mosaic attenuation pattern. Furthermore, all of these signs of sub-acute HP can be seen in the more chronic fibrotic form of hypersensitivity pneumonitis [19].
Contrasting imaging features and clinical history mean that differentiating between HP and PCP (Pneumocystis jirovecii) is never usually a diagnostic dilemma. PCP in patients with AIDS commonly presents with diffuse ground glass opacification, sepal thickening and occasional cysts, but with no small airways disease [20].
DIP
DIP is an uncommon disorder characterised by macrophage accumulation within the alveoli [21]. It causes dyspnoea or cough and occurs almost exclusively in individuals exposed to cigarette smoke. Consequently, knowledge of the smoking history is mandatory before suggesting the diagnosis on radiological grounds. On HRCT, diffuse ground glass opacification is an expected finding in all patients [22]. By itself, diffuse ground-glass opacification has a wide differential diagnosis, but the identification of small cysts admixed within the ground glass opacity is a unique feature to DIP, seen in about a third of patients (Fig. 10) [23, 24].