Lambda sign | Radiology Signs

Answer:

The lambda sign is the pattern of gallium-67 citrate uptake in the right paratracheal nodes together with both hilar nodal groups, which together trace the outline of the Greek letter lambda on the anterior planar image. Gallium accumulates in activated macrophages and lymphocytes through transferrin receptor mediated uptake, so the sign reflects active, cellular granulomatous inflammation rather than fibrosis, and it becomes negative once the process burns out or is suppressed by corticosteroids. Alone it is not specific, because lymphoma, tuberculosis and metastatic nodal disease can produce paratracheal and hilar uptake; its value is in combination with the panda sign of symmetrical lacrimal, parotid and nasopharyngeal uptake. The two together are considered highly specific for sarcoidosis and were historically used to make the diagnosis without biopsy in a patient with a compatible clinical picture. Gallium imaging requires a 48 to 72 hour delay after injection, delivers a comparatively high radiation dose and has poor spatial resolution, so in current practice it has largely been replaced by FDG PET/CT, which shows the same nodal distribution with better resolution, identifies extrathoracic and cardiac involvement, and is used to select biopsy targets and to monitor treatment response. The equivalent finding on chest radiography is bilateral hilar and right paratracheal adenopathy, the 1-2-3 sign.

Why is it called so?

Uptake in the right paratracheal chain forms the upright limb and the two hilar collections form the diverging arms, so the combined outline resembles the Greek capital letter lambda.

Pathophysiology

In sarcoidosis, CD4 T lymphocytes and activated macrophages accumulate in lymph nodes and form non-caseating granulomas. Activated macrophages express transferrin receptors and secrete lactoferrin, and gallium-67 behaves as an iron analogue bound to transferrin, so it is concentrated in these cells in proportion to disease activity. The nodal distribution follows the lymphatic drainage of the lung, giving symmetrical hilar involvement with right-sided paratracheal extension, while the same granulomatous infiltration of exocrine glands produces the parotid and lacrimal uptake of the panda sign.

Alternative names: Lambda pattern; part of the panda-lambda combination

Other associated named signs: Panda sign of lacrimal and parotid uptake, the 1-2-3 sign or Garland triad on chest radiography, and the galaxy and cluster of grapes signs on HRCT

References

1. Hisata Y, Tago M, Fujiwara M, Yamashita SI. The lambda sign in gallium-67 scintigraphy is a useful clue to the early diagnosis of sarcoidosis. Clin Case Rep. 2020;8(12):3602-3603. PMID: 33363998.