Discussion
Because COVID-19 is believed to have an interpersonal human-to-human transmission [2], either home residence or a travel history to Wuhan—coupled with exposure to known or suspected cases—is paramount to raise clinical suspicion. The clinical, laboratory, and imaging characteristics of the four patients described in the current report are consistent with a diagnosis of COVID-19. Herein, we therefore describe for the first time the 18F-FDG PET/CT findings of four patients with COVID-19.
In accordance with previously published observations [5, 6], our cases were characterized by the presence of peripheral GGOs and/or consolidative opacities in more than two pulmonary lobes. Notably, all of these lesions showed a high tracer uptake. Although a bilateral involvement of the lung parenchyma can be observed in several benign and malignant lung diseases [7], tumors presenting as GGOs are unlikely to be FDG-avid [8]. The high tracer uptake that characterized COVID-19 pulmonary infections reflects a significant inflammatory burden, similar to that elicited by the Middle East respiratory syndrome or the H1N1 pandemic influenza virus [9, 10]. Although COVID-19 infections do not seem to be accompanied by lymphadenopathy [6], our 18F-FDG PET/CT findings revealed an increased nodal FDG uptake in three of four cases. Although no obvious nodal enlargement was evident, our imaging data indicate for the first time that COVID-19 may cause lymphadenitis—in line with previous data obtained from non-human primates exposed to MERS-CoV [9]. Another interesting finding is that no disseminated lesions were evident in our patients—suggesting that COVID-19 has pulmonary tropism.
Although 18F-FDG PET/CT cannot be routinely used in an emergency setting and is generally not recommended for infectious diseases, our current findings demonstrate that this imaging modality may play a complementary diagnostic role in COVID-19—especially at early stages when clinical symptoms are not specific and differential diagnosis is challenging.
Our case series is limited by the small sample size and the lack of molecular confirmation of SARS-CoV-2 infection. However, only 30–50% of infected patients had positive test for SARS-CoV-2 nucleic acid on RT-PCR (Chinese Academy of Sciences; unpublished data). High rates of false-negative findings may be explained by several reasons, including (1) the lack of SOPs for SARS-CoV-2 nucleic acid detection, differences in sample handling, storage, and processing, (2) disease stages and different viral loads according to anatomical site (e.g., alveoli versus upper respiratory tract), (3) the lack of independent validation of current testing, and (4) the potential high mutation rates of COVID-19. In light of these limitations, some cases of COVID-19 in China are currently diagnosed on clinical, laboratory, and imaging grounds, without resorting to molecular confirmation.
These caveats notwithstanding, we show for the first time that (1) lung lesions of patients with COVID-19 pneumonia are characterized by high 18F-FDG uptake, (2) this condition is accompanied by nodal involvement detectable on 18F-FDG PET/CT imaging, and (3) there is no evidence of disseminated disease, indicating that COVID-19 may have specific lung tropism.