Rationale: We aimed to predict the presence of vaccine-induced hypermetabolic lymph nodes (v-HLN) on 18 F-FDG PET/CT after Coronavirus disease 2019 (COVID-19) vaccination and determine their association with lymphocyte counts. Methods: In this retrospective single-center study, we included consecutive patients who underwent 18 F-FDG PET/CT imaging after mRNA-or viral vector-based COVID-19 vaccination between early March and late April 2021. Demographic, clinical parameters and absolute lymphocyte count (ALC) were collected and their association with the presence of v-HLN in the draining territory was studied by logistic regression.Results: Two hundred and sixty patients were eligible, including 209 (80%) women and 145 (56%) with breast cancer. The median age was 50 years (range, 23-96). Two hundred thirty-three patients (90%) received the mRNA vaccine. Ninety (35%) patients had v-HLN with a median SUVmax of 3.7 [range, 2.0-26.3] and 74 (44%) displayed lymphopenia with a median ALC of 1.4 G/L [range, 0.3-18.3]. Age ≤ 50 years (odds ratio [OR] 2.2, 95%CI 1.0-4.5), the absence of lymphopenia (OR 2.2, 95%CI 1.1-4.3) and the delay from the last vaccine injection to the date of 18 F-FDG PET/CT, if < 30 days (OR 2.6, 95%CI 1.3-5.6), were independent factors for v-HLN in multivariate analysis. In breast cancer patients, the absence of lymphopenia was the only independent factor significantly associated with v-HLN (OR 2.9, 95%CI 1.2-7.4).Conclusions: Patients with normal values of ALC after COVID-19 vaccine were more likely to have v-HLN on 18 F-FDG PET/CT, which might both be associated to a stronger immune response to vaccination. KEYWORDS 18 F-FDG PET/CT; COVID-19 vaccination; absolute lymphocyte count; hypermetabolic lymph nodes; immune response. KEY-POINTSQUESTION: Could we use lymphocyte count lymphocyte for predicting the presence of vaccineinduced hypermetabolic lymph node(s) in the drainage territory on 18 F-FDG PET/CT following COVID-19 vaccination?PERTINENT FINDINGS: This retrospective and monocentric study included 260 vaccinated patients who underwent 18 F-FDG PET/CT following mRNA based-or viral vector based-COVID-19 vaccination. Patients' absolute lymphocyte count (>lower limit of normal), along with patients' age (≤ 50 years) and the timing of last injection dose (< 30 days), significantly correlated with vaccine-induced hypermetabolic lymph node(s).IMPLICATIONS FOR PATIENT CARE: Patients displaying normal count of lymphocytes after COVID-19 vaccination are more likely to present vaccine-induced hypermetabolic lymph node(s) on 18 F-FDG PET/CT and could subsequently have higher seropositivity likelihood and antibody titers.

Malignant struma ovarii (MSO) is a rare malignant ovarian tumor, histologically identical to differentiated thyroid cancers. Given the rarity of this disease, there are no treatment guidelines, and the place of imaging for response assessment remains controversial. We report a metabolic response assessed by 18F-FDG PET/CT in a 71-year-old woman with radioiodine-refractory metastatic MSO treated by targeted therapies (first line with lenvatinib and second line with pazopanib). This case of exceptional response also highlights the usefulness of 18F-FDG PET/CT for therapeutic assessment of targeted drugs in such a rare clinical entity of malignant MSO.