Web Exclusive Content-Signs in Cardiopulmonary Imaging
Appearance: The Cheerio sign is defined by a nodule with a central lucency seen on CT, akin to the ring-shaped “Cheerios” breakfast cereal.1 (All references cited in this article can be found at http://links.lww.com/JTI/A41). Other terminologies used to describe a similar underlying histology include bubble-like lucencies and the open bronchus sign.2
Explanation: A Cheerio in the lung arises from proliferation of either neoplastic cells such as adenocarcinoma or non-malignant cells such as pulmonary Langerhans cell histiocytosis (PLCH) around a patent airway (Fig. 1A).3,4 Cell proliferation surrounding other types of central radiolucencies, such as pseudocavitation, cavitation, alveoli, or multiple thin-walled cysts can also produce appearances similar to Cheerios.3,4
Discussion: The Cheerio sign was originally described in low-grade adenocarcinomas of the lung, which can be solitary, multifocal, or diffuse (Fig. 1B). Pathologically, the spectrum of adenocarcinomas that may give rise to the Cheerio sign is further classified into adenocarcinoma in situ, minimally invasive adenocarcinoma, invasive lepidic predominant adenocarcinoma, and invasive mucinous adenocarcinoma.5 Lepidic growth pattern of the tumor cells, whether purely, predominantly, or partially present in these lesions, classically maintains the alveolar architecture and bronchial patency, thereby at times creating Cheerios on CT.2 A CT-pathologic correlation study by Gaeta, et al previously identified aerated intratumoral bronchioles in 34% of nodular low-grade adenocarcinomas.4 When adenocarcinoma is included as a diagnostic consideration, a longer period of surveillance may be warranted due to its slow growth. Surgery continues to be the mainstay of treatment. Primary mucinous adenocarcinomas arising elsewhere, such as from the gastrointestinal and genitourinary systems, can also present with the same histology and the Cheerio sign when metastasized to the lungs.
PLCH is a smoking related lung disease characterized by proliferation of MHC class II Langerhans cells. PLCH lesions are multiple and distributed predominantly in the upper and mid lungs with conspicuous sparing of the bases.6,7 CT findings of PLCH include both nodules and cysts (Fig. 1C). The nodules are often stellate and ill-defined. With growth, faint central lucencies develop, resulting in Cheerios. More advanced disease is characterized by coalescence of cysts into bizarre shapes. End-stage disease may be difficult to distinguish from advanced emphysema. Smoking cessation is the primary treatment for pulmonary Langerhans cell histiocytosis.7 The clinical course and prognosis vary, with regression in up to 25% of patients following smoking cessation, clinical and radiographic stability in 50% of patients, and progression to end-stage disease in the remaining 25% despite smoking cessation.8
Minute pulmonary meningothelial-like nodules (MPMNs), a relatively rare entity, can present with micro-Cheerios on CT. These millimeter-sized nodules are typically multiple, bilateral, and randomly distributed.9 They represent interstitial proliferation of benign spindle cells associated with a central radiolucent expanded alveolar airspace.9 Of uncertain clinical significance, MPMNs are sometimes incidental findings in cases of malignancy. Recognition of their features helps avoid misinterpreting these nodules as metastases.9
The differential diagnosis of cavitary nodules that may also manifest with the Cheerio sign includes mycobacterial or fungal infections, other primary lung cancers, metastases, granulomatosis with polyangiitis (formerly Wegener granulomatosis), and rheumatoid nodules.2