Intrapulmonary lymph nodes

Intrapulmonary lymph nodes, or pulmonary lymph nodes, are normal lymph nodes found within the lung parenchyma itself.

Terminology

Intrapulmonary lymph nodes may be perifissural (lying along a fissure) or juxtapleural, meaning within 15mm of visceral pleura ². The 2024 Fleischner glossary has replaced the term juxtapleural with pleura-based and subpleural or peripheral ¹⁰.

The term intrapulmonary lymph nodes excludes mediastinal, hilar and peribronchial lymph nodes. The latter occur at bronchial bifurcations up to and including the third branch.

Epidemiology

Some publications suggest that lymph nodes may account for 24–44% of noncalcified nodules in a lung cancer screening setting ⁷ and may represent up to 66% of all nodules on CT scans. They are commonly seen in the mid and lower zones and can be difficult to differentiate from malignant pulmonary nodules with certainty. Nodules superior to the carina have a higher incidence of malignancy.

Radiographic features

CT

Thin sections and evaluation in two or three orthogonal planes underpin accurate evaluation. Intrapulmonary lymph nodes are characterized by:

• location: middle and lower lobe predominant, inferior to the carina ⁴

• pleura-based or juxtapleural (within 15 mm of a visceral pleural surface)

• no effect on pleura

• solid soft-tissue attenuation with no fat or calcification

• sharply marginated with a smooth outline

• 3-15 mm diameter, usually less than 12 mm ⁷

• elongated with an aspect ratio often >1.78

• lenticular, ovoid, semicircular, triangular, trapezoidal, polygonal, faceted

• lymph nodes located at the junction of interlobular septa may show a discrete thin line extending to the pleura which may be due to interlobular septum, venule or lymphatics

• may grow or shrink over time

• no arterial attachment

In contrast, small peripheral cancers such as adenocarcinomas may show:

• arterial attachment

• indistinct or irregular margin, spiculation or microlobulation

• as a round nodule which may touch a pleural surface

• signs of visceral pleural invasion ⁹ such as:

  • large area of contact for pleural attached nodules

  • pleural thickening

  • tags to multiple pleural surfaces

  • jellyfish sign (multiple pleural tags from the margins of a pleura-attached nodule)

85% of cancers misdiagnosed as intrapulmonary lymph nodes were in the upper lobes and this was particularly common if the features were ‘atypical’ for intrapulmonary lymph nodes ⁸. Upper lobe nodules should trigger extra vigilance.