Impact of COVID-19 pandemic on 2-[18F]FDG PET/CT imaging work-flow in a single medical institution: comparison among the three Italian waves

Purpose To compare the impact of COVID-19 pandemic on 2-[18F]FDG PET/CT imaging work-flow during the three waves in a medical institution of southern of Italy. Methods We retrospectively reviewed the numbers and results of 2-[18F]FDG PET/CT studies acquired during the following three periods of the COVID-19 waves: 1) February 3-April 30, 2020; 2) October 15, 2020–January 15, 2021; and 3) January 18-April 16, 2021. Results A total of 861 PET/CT studies in 725 patients (388 men, mean age 64 ± 4 years) was acquired during the three waves of COVID-19 pandemic. The majority (94%) was performed for diagnosis/staging (n = 300) or follow-up (n = 512) of neoplastic diseases. The remaining 49 studies (6%) were acquired for non-oncological patients. The distribution of number and type of clinical indications for PET/CT studies in the three waves were comparable (p = 0.06). Conversely, the occurrence of patients positive for COVID-19 infection progressively increased (p < 0.0001) from the first to third wave; in particular, patients with COVID-19 had active infection before PET/CT study as confirmed by molecular oro/nasopharyngeal swab. Conclusion Despite the restrictive medical measures for the emergency, the number of 2-[18F]FDG PET/CT studies was unchanged during the three waves guaranteeing the diagnostic performance of PET/CT imaging for oncological patients.


Introduction
A fundamental role of medical imaging techniques has been showed for the clinical management of patients with COVID-19 infection mainly to assess the pulmonary involvement, but also to evaluate different organs besides the lungs since the disease is characterized by multi-organ damage [1,2]. The main clinical recommendation consists of performing computed tomography (CT) in case of severe respiratory symptoms considering the local prevalence of the disease; however, chest X-ray seems to be sufficient to diagnose the presence and extent of pulmonary opacifications even in patients with moderate symptoms [3]. Several previous studies reported the potential role of integrated positron emission tomography (PET) and CT with 2-deoxy-2-[ 18 [4,5,6]. Of note, these PET/CT imaging preliminary reports highlighted the clinical importance of an early detection of COVID-19 infection to guide subsequent patient management. In fact, oncological patients for which the wide majority of 2-[ 18 F]FDG PET/CT studies is requested are at high-risk of COVID-19 infection [7]. Indeed, since incidental COVID-19 cases may occur in 2-[ 18 F]FDG PET/CT scans, nuclear medicine physicians should be able to recognize typical patterns of the disease on the co-registered CT images; additionally, nuclear medicine staff should adopt all safety measures for preventing COVID-19 diffusion [8]. Moreover, the restrictive medical measures for COVID-19 emergency have implied subsequent limitations in imaging workflow, even though the diagnostic assistance for oncological patients should be guaranteed [9,10]. We recently reported our experience regarding the diagnostic work-flow of 2-[ 18 F]FDG PET/CT imaging during the first wave of the COVID-19 pandemic from February to April 2020 demonstrating an unchanged number of 2-[ 18 F]FDG PET/CT oncological studies compared to the same period of the 2019 [11]. The present study was performed to compare the impact of COVID-19 pandemic on 2-[ 18 F]FDG PET/CT imaging work-flow during the three waves in our medical institution of southern of Italy.

Patients
We retrospectively reviewed the results of 2-[ 18 F]FDG PET/CT imaging studies acquired during the COVID-19 pandemic. In particular, the three periods representing the three COVID-19 waves were evaluated, each of three months, as follow: from February 3 to April 30, 2020 (first wave), from October 15, 2020, to January 15, 2021 (second wave) and from January 18 to April 16, 2021 (third wave). The number, the clinical indications and the imaging findings of 2-[ 18 F]FDG PET/CT studies were directly compared. Before imaging study, patients were screened for COVID-19 infection using clinical assessment and/or laboratory evaluation by rapid serological tests and/or molecular oro/nasopharyngeal swabs (reverse transcription -Real Time PCR) according to the official guidance corresponding to the three different COVID-19 waves. In particular, in the first wave molecular oro/nasopharyngeal swabs were not available for all patients, but clinical screening for COVID-19 was performed in all cases; when COVID-19 infection was suspected according to clinical and/or imaging findings molecular oro/nasopharyngeal swab was performed.

PET/CT imaging
2-[ 18 F]FDG PET/CT studies were acquired using a Gemini TF 64 scanner (Philips Healthcare, Best, The Netherlands). All patients fasted for at least 6 h prior to imaging, and blood glucose levels were <180 mg/ dL at the time of tracer injection. PET scans were acquired in 3-D mode starting 60 min after 2-[ 18 F]FDG administration (activity range 200-300 MBq, according to body weight). A low-(70 mA) and high-dose (230 mA) CT scans (rotation time 1.5 s, collimation 16 Â 0.625) were acquired for attenuation correction of emission data. The sinogram of emission data was reconstructed using the 3-D row action maximum likelihood algorithm, considering attenuation, detector efficiency, scatter and random coincidence corrections. Attenuation correction was performed using CT images. CT and 2-[ 18 F]FDG PET images were matched and fused into transverse, coronal and sagittal total-body images.

Image analysis
CT chest images were evaluated by two experienced radiologists who worked in consensus and reviewed each set-in random order to evaluate the presence and the location of abnormal findings in lung parenchyma according to COVID-19 reporting and data system (CO-RADS) classification [12]. In case of disagreement, a third senior radiologist was consulted to reach a final consensus for CT imaging interpretation. Successively, 2-[ 18 F]FDG distribution in the lungs was qualitatively evaluated using PET-CT fusion images and maximum standardized uptake value (SUV max) was measured on areas of increased 2-[ 18 F]FDG uptake corresponding to abnormal CT findings. SUV max value of the most 2-[ 18 F]FDG -avid lung abnormality was recorded. In particular, CO-RADS classification of CT findings [12] represented the level of suspicion of COVID-19 infection graded as illustrated and listed in Table 1. Finally, the presence of 2-[ 18 F]FDG abnormal uptake was also assessed on PET/CT total-body images in extra-pulmonary locations.

Statistical analysis
Continuous data are expressed as mean AE standard deviation and categorical data as percentage. Two-tailed t test and chi-square test were used to compare the differences in continuous and categorical variables, respectively. A p value <0.05 was used to define statistical significance. Dummy variables were created to indicate diagnostic admissions that occurred during the three waves. Linear regression analysis was used to evaluate trends in the number of imaging studies over the periods. The Cochran-Armitage test was used to test for trend in the proportion of patients positive for COVID-19 infection referred for 2-[ 18 F]FDG PET/CT evaluation over the three waves. Statistical analysis was performed with Stata 17 software (StataCorp, College Station, Texas USA).

Ethical approval
All procedures performed were in accordance with the ethical standards of the institutional research committee and with the principles of the 1964 Declaration of Helsinki and its later amendments.

Informed consent
Informed consent was obtained from all individual participants included in the study.

Results
A total of 861 2-[ 18 F]FDG PET/CT imaging studies in 725 patients (388 men, mean age 64 AE 4 years) was acquired during the three waves of the COVID-19 pandemic. The majority (94%) of imaging studies (n ¼ 812) was performed for diagnosis/staging (n ¼ 300) or follow-up (n ¼ 512) of neoplastic diseases. The remaining 49 (6%) studies were acquired for the evaluation of non-oncological diseases of which vasculitis (n ¼ 10), endocarditis (n ¼ 5), fever of unknown origin (n ¼ 5), histiocytosis x (n ¼ 2) or other inflammatory diseases (n ¼ 27). In detail, the distribution of the number and the type of clinical indications for 2-[ 18 F]FDG PET/CT studies in the three COVID-19 waves were comparable (Table 2). Conversely, the occurrence of patients positive for COVID-19 infection progressively increased (p < 0.0001) from the first (n ¼ 0) to the second (n ¼ 10) and the third (n ¼ 21) waves (Table 2). In patients with COVID-19, the diagnosis of infection occurred before PET/CT study: 1.9 AE 0.9 months in the 10 patients of the second wave and 2.2 AE 0.9 months in the 21 patients of the third wave. All these patients had negative molecular oro/nasopharyngeal swab before imaging and in all cases PET/CT was performed for evaluation of oncological diseases. High suspicion: unilateral peri-broncho vascular ground-glass CT opacities without any other typical findings The clinical characteristics and PET/CT findings of the 10 patients positive for COVID-19 during the second wave are illustrated in Table 3. In this group no abnormal lung CT findings according to CO-RADS classification were observed in the majority (n ¼ 8, 80%) of patients, while in the remaining two patients CO-RADS abnormalities were found of which only in one case the lung pattern was highly suspicious of COVID-19 infection (CO-RADS 5) with significant increase of 2-[ 18 F]FDG uptake (SUV max 5.1); in the last patient a CO-RADS 2 pattern was observed with slightly increased 2-[ 18 F]FDG uptake (SUV max 2.6). Figure 1 shows an example of a patient (#7 in Table 3) with respiratory symptomatic COVID-19 infection consisting of interstitial pneumoniae on CT with fever, cough, dyspnea and asthenia that was treated with medications and continuous positive airway pressure (CPAP). Two months later, after a negative molecular oro/nasopharyngeal swab, the patient underwent PET/CT to search a primary cancer of unknown origin ( Figure 2).
The clinical characteristics and PET/CT findings of the 21 patients positive for COVID-19 during the third wave are reported in Table 4. Also, in this group of patients no abnormal lung CT findings according to CO-RADS classification were observed in the majority (n ¼ 16, 75%) of patients, while in the other five patients CO-RADS abnormalities were found of which in only one case the lung pattern was highly suspicious of COVID-19 infection (CO-RADS 5) with a significant increase of 2-[ 18 F] FDG uptake (SUV max 5.1). In the remaining four patients CO-RADS lung patterns (3 or 2) were not highly suspicious for COVID-19 infection. Figure 3 shows an example of a patient (#21 in Table 4) with respiratory symptomatic COVID-19 infection consisting of interstitial pneumoniae on CT with fever, cough and dyspnea that was treated with medication and CPAP. Two months later the patient had a negative molecular oro/ nasopharyngeal swab and underwent PET/CT to search a primary cancer of unknown origin (Figure 4).

Discussion
Previous studies showed the potential role of PET/CT with 2-[ 18 F] FDG to incidentally identify COVID-19 lung abnormalities in patients undergoing imaging for conventional, oncological or non-oncological, clinical indications [4,5,6]. In the present study, we compared the impact of COVID-19 pandemic on 2-[ 18 F]FDG PET/CT work-flow during the three waves in our medical institution in the South of Italy. Our results indicate that, despite the restrictive medical measures for the emergency, the number of 2-[ 18 F]FDG PET/CT studies was unchanged during the three waves guaranteeing the diagnostic performance of PET/CT imaging mainly (94%) for oncological patients; in detail, PET/CT studies during the three waves were performed not only for diagnosis/staging (n ¼ 300) of tumor diseases, but mainly during the follow-up (n ¼ 512). Furthermore, a progressive diffusion increase of COVID-19 infection was observed in the three waves, suggesting a major spread of pandemic in the South of Italy in the second and third wave.
In our experience, the majority of 2-[ 18 F]FDG PET/CT studies acquired during each of the three waves was performed for oncological clinical indications and the number of imaging studies was comparable between the three waves. These findings reflect that during the emergency for pandemic the diagnostic assistance for neoplastic patients was guaranteed. While this pandemic triggered an immense pressure on the health-care system, the efforts to avoid any postponing in cancer diagnostics are of pivotal importance to avoid poorer outcomes in oncologic patients [7]. Moreover, we recently reported that the number of 2-[ 18 F]FDG PET/CT oncological studies was unchanged during the first COVID-19 wave compared also to the same period of the previous year (2019) when there were not restrictive medical measures, furtherly supporting the diagnostic assistance for cancer patients in our institution [11]. A similar multi-center study was performed by Wong et al. [13] in England during the first COVID-19 wave comparing the impact of pandemic on oncological and non-oncological 2-[ 18 F]FDG PET/CT  [11,13], an overall reduction in nuclear medicine procedures has been reported in worldwide and national surveys [18,19,20]. Of note, the decrease and/or the postponement of diagnostic examinations due to the pandemic might impact on the disease's clinical course, potentially affecting the quality of life and patient survival [21]. Different factors might concur in the reduction of nuclear medicine departments activity, including patients' fears and preferences as well as safety measures adopted [20]. Therefore, efforts should be made towards balancing security measures and the need to ensure medical assistance without sacrificing patients' healthcare [21]. In our series, the 2-[ 18 F]FDG PET/CT imaging evaluation of the 10 patients who had previous COVID-19 infection during the second wave demonstrated no significant lung abnormalities in the majority of patients of which 3 were asymptomatic, 3 had slight clinical symptoms, 3 had moderate respiratory symptoms requiring conventional O2-therapy, while only the last patient had severe respiratory symptoms needing CPAP treatment. Of note, only in this latter patient who had a severe respiratory failure a significant CO-RADS pattern suggestive of COVID-19 interstitial pneumoniae (CO-RADS 5) was observed as well as showing   Table 3) showing bilateral lung abnormalities (CO-RADS 6) suggestive of interstitial pneumoniae. patient with previous severe respiratory syndrome a significant CO-RADS pattern suggestive of COVID-19 interstitial pneumoniae (CO-RADS 5) was observed as well as showing increased 2-[ 18 F]FDG activity (SUV max 5.1). In this regard, similar SUV values in lung abnormalities as CO-RADS 5 or 6 have been recently reported in a study by Wakfie-Corieh et al. [22] in which the correlation between metabolic and structural lung COVID-19 changes has been investigated. Our findings suggest that in patients with previous COVID-19 infection the presence of persistent    [24] in single patient with previous COVID-19 infection. In this regard, a recent pathology report described interstitial mononuclear inflammatory infiltrates in both lungs of a patient with COVID-19 suggesting that significant inflammation may persist in the lungs during convalescence after the infection [25]. Thus, the increased 2-[ 18 F]FDG uptake could be interpreted as reflecting increased glycolytic activity due to infiltration and inflammation of lung tissue.
In conclusion, this study shows that despite the restrictive medical measures for the COVID-19 emergency, the number of 2-[ 18 F]FDG PET/ CT imaging studies was unchanged during the three waves guaranteeing the diagnostic performance of PET/CT scanning mainly for oncological patients.

Author contribution statement
Simone Maurea and Alberto Cuocolo: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Wrote the paper.
Claudia Bombace and Arnaldo Stanzione: Performed the experiments; Analyzed and interpreted the data; Wrote the paper.
Silvana Del Vecchio: Contributed reagents, materials, analysis tools or data; Wrote the paper.