Introduction: After completion of radioimmunotherapy (RIT) as part of the phase II CheckRad-CD8 trial, lymph nodes that showed increased enhancement on PET-CT were extirpated to rule out existing lymph node metastasis as part of a salvage neck dissection (SND). This study investigates the value of PET-CT diagnostics for the presence of lymph node metastases after RIT. Methods: Retrospective evaluation of all 75 patients who were treated in the CheckRad CD8 study in Erlangen from 10.2018 to 01.2022. Included were 10 patients who had received a completed RIT and SND according to current PET-CT. The collective was divided into group 1 (metastases, n=5) and group 2 (lymph nodes without malignancy, n=5). The test parameters were the maximum standardized uptake value (SUVmax) detectable in the PET-CT before and after RIT and the decrease in SUV, which was correlated with the lymph node histology. Results: The groups did not differ significantly in terms tumor location (p=1.000), T-stage (p=0.524), N-stage (p=1.000), p16 status (p=0.524) and mortality rate (p=0.444). The average SUVmax of the lymph nodes was 6.4±6.3 SD and 9.4±6.2 SD preinterventionally in groups 1 and 2 (p=0.469), and 5.5±4.8 SD and 2.0±1.9 SD respectively postinterventionally (p=0.181). The mean SUV reduction after RIT was 11.4±93.3% SD in group 1 and 51±57.4% SD in group 2 (p=0.245). Conclusion: In this selected monocentric cohort of the CheckRad-CD8 study, the SUVmax of persistent cervical lymph nodes after RIT tended to be higher with tumor involvement. Due to the small number of cases, statistical testing is not meaningful. The diagnostic value of PET for the identification of active lymph node metastases after RIT can not be conclusively assessed and will be evaluated in the entire study population.

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