¹⁸F-FDG PET/CT for the diagnosis of sarcoidosis in a patient with bilateral inflammatory involvement of the parotid and lacrimal glands (panda sign) and bilateral hilar and mediastinal lymphadenopathy (lambda sign)

A 43-year-old man was hospitalized for Sjögren’s syndrome which was suspected to be of paraneoplastic aetiology. ¹⁸F-FDG PET/CT revealed grossly enlarged lacrimal (a) and parotid glands (b) with significantly increased FDG uptake while the submandibular glands (c) were only minimally enlarged with faint FDG uptake. Furthermore, hypermetabolic intrathoracic lymphadenopathy was detected with a typical lambda appearance (h) involving right paratracheal (e), left paratracheal and aortopulmonary window lymph nodes (f) of the mediastinum and bihilar lymph nodes (g). The imaging findings were suggestive of sarcoidosis and bronchoscopic evaluation with biopsy was proposed. Histopathology of a transbronchial biopsy specimen confirmed the diagnosis of sarcoidosis.

In 1990 Sulavik et al. first described the panda sign, i.e. normal accumulation of the radionuclide (⁶⁷Ga-citrate) in the nasopharynx combined with increased symmetric accumulation in the parotid and lacrimal glands, giving the impression of the mottled colouring of the giant panda [1–3]. Intrathoracic lymphadenopathy typically manifests as bilateral hilar adenopathy with predominantly right paratracheal adenopathy, referred to as the lambda appearance [4].

With ¹⁸F-FDG PET/CT the appearance of hypermetabolic mediastinal and bihilar lymphadenopathy in sarcoidosis is comparable to the lambda sign of ⁶⁷Ga-citrate scintigraphy [5]. Bilateral inflammatory involvement of the parotid and lacrimal glands is also present, resulting in an increased FDG uptake, although the typical panda appearance is partially obscured due to the physiologic FDG avidity of the brain (d).

The finding of typical lambda and panda patterns supports the diagnosis of sarcoidosis and reinforces the indication to perform an appropriate biopsy.