1-2-3-4 Sign FDG PET Sign in Sarcoidosis

Sarcoidosis is a multisystem granulomatous disease that commonly affects the lungs and intrathoracic lymph nodes. On imaging, characteristic patterns of lymphadenopathy, such as the lambda sign on fluorodeoxyglucose positron emission tomography (FDG PET), are considered suggestive of the disease and can assist in differentiating sarcoidosis from differential diagnoses. FDG PET/CT is increasingly used to assess disease extent, identify extrapulmonary involvement, guide biopsy and monitor inflammatory activity. The prevalence and imaging characteristics of the lambda sign and periportal/portocaval lymphadenopathy, termed the 1-2-3-4 sign on FDG PET/CT in sarcoidosis, remain poorly defined.

A retrospective cohort study was performed of patients with sarcoidosis who underwent FDG PET/CT imaging at a tertiary centre in 2019. PET studies were assessed for the presence of the lambda sign and for FDG-avid periportal and/or portocaval lymph nodes. The frequency of the lambda and 1-2-3-4 sign, nodal distribution, and metabolic activity were analysed, including maximum standardised uptake values (SUVmax).

The lambda sign was identified in a substantial proportion of patients. FDG-avid periportal and/or portocaval lymph nodes were observed in a subset of cases, although only one patient demonstrated the complete 1-2-3-4 sign. Periportal and portocaval nodes showed FDG uptake comparable to intrathoracic nodal disease.

FDG-avid periportal and portocaval lymphadenopathy may occur alongside classic intrathoracic nodal patterns in sarcoidosis, although the complete 1-2-3-4 sign appears uncommon. Recognition of this imaging pattern on FDG PET/CT may support diagnostic confidence in appropriate clinical contexts and help differentiate sarcoidosis from alternative causes of mediastinal and abdominal lymphadenopathy.