The secondary pulmonary lobule is the smallest visible end unit of the lung, making it a key component to understand when reviewing CT imagery.

“They are these polygonal structures, usually with a little dot in the middle, and there are just thousands and thousands of them throughout the lung,” said David L. Smith, MD, associate professor of radiology, of medicine, and of cell biology and anatomy, and director of the Interdisciplinary Interstitial Lung Disease (ILD) Program, Louisiana State University (LSU) Health Sciences Center New Orleans.

Dr. Smith reviewed the anatomy of these structures in The Secondary Pulmonary Lobule and High-Resolution Patterns of Interstitial Lung Disease. The session is available for viewing on the virtual CHEST 2020 meeting platform through February 1, 2021, for registered attendees.

Veins define the polygon of a secondary pulmonary lobule, which can range from a few millimeters up to 1.5 centimeters, and the center dot indicates the end bronchiole and end arteriole, from which acini branch out. Veins leaving the acini travel to the periphery and then back to the heart in interlobular septa. Lymphatics at the center of the pulmonary lobule travel back with the broncho-arterial bundle, but also exit via the periphery of the secondary pulmonary lobule and travel back along with the veins. This entire structure is supported by a fine parenchymal network of fibers.

“When we see disease at the center of the pulmonary lobule, we know that it’s either an end airway disease, an end arterial disease, or lymphatic disease,” Dr. Smith said. “When we see disease at the center of the pulmonary lobule and along the periphery, we know that’s a lymphatic disease. And when we see reticulation and very fine ground glass, we have an idea that it might be due to thickening of the parenchyma of the lung.”

Case studies

Amita Krishnan, MD, assistant professor of clinical medicine, LSU Health Sciences Center New Orleans, presented three cases that further differentiated the clinical, radiographic, and histopathologic features of centrilobular, perilymphatic, and interstitial patterns of disease related to the secondary pulmonary lobule.

One case involved a 46-year-old woman with follicular bronchiolitis, which Dr. Krishnan described as a lymphoproliferative disorder consisting of airway-centric proliferation of lymphocytes. It is the most frequent airway disease in Sjogren’s syndrome, although many physicians associate lymphoid interstitial pneumonia (LIP) with Sjogren’s.

“Histologically, a continuum does exist between follicular bronchiolitis and LIP, as they are both reactive pulmonary lymphoid diseases,” Dr. Krishnan said. “The main distinguishing feature is the relative sparing of alveolar septa in follicular bronchiolitis.”

Another case, involving a 50-year-old man with Erdheim-Chester disease, highlighted the most common findings in those with the condition, including osseous manifestations, a dense infiltration of perinephric fat, referred to as a “hairy kidney” because of its appearance, and circumferential soft tissue sheathing around the aorta.

“It’s common to see renal artery involvement as well, leading to renovascular hypertension,” Dr. Krishnan said.

The final case looked at a 46-year-old woman with nonspecific interstitial pneumonia (NSIP), which typically shows a histologic pattern of temporally and spatially homogeneous infiltration of the interstitium by reactive lymphocytes due to rheumatoid arthritis.

“All of the areas of the lung seem to be involved,” Dr. Krishnan said. “They’re of uniform appearance and there’s a lack of completely normal lung, unlike [pathology] that you would see with usual interstitial pneumonia (UIP) pattern, where you would have areas of normal lung and areas that are pathologic.”