Retrospective study of dynamic imaging acquisition length optimisation for sentinel lymph node scans

Current guidance recommends dynamic imaging immediately post radiopharmaceutical injection for sentinel lymph node (SLN) scans. However, there is limited information on the recommended acquisition duration. We aimed to determine the time during a 30 min dynamic acquisition at which no additional SLNs were detected, and evaluate how applying a 20 min cut-off influenced clinical interpretation when considering all retrospectively acquired standard-of-care imaging. We retrospectively reviewed 106 SLN dynamic acquisitions for five primary sites: head and neck (19), limb (21), trunk (24), penis (21), and vulva (21). A qualified reporter identified studies in which lymph nodes appeared beyond 15 min. These datasets were independently reviewed by two expert reporters to find the time-point when no further SLNs were observed, and to evaluate differences in the number and location of identified SLNs using a 20-min protocol. We also conducted a survey of centres providing an SLN service to document the current practice of dynamic scan durations for contextual background. Across all studies, no new SLNs were detected after 25 min. When considering all acquired images, a 20 min dynamic identified the same SLNs. Interobserver agreement between expert reporters was 0.82 (95% confidence interval: 0.61-0.93). The eight survey responses revealed a median dynamic duration of 15 min (interquartile range: 13-28 min). While a 25 min dynamic captured all SLNs for all studies, incorporating reporting with all retrospectively acquired standard-of-care imaging indicates a 20 min dynamic duration appears unlikely to alter image reporting. Adopting a 20 min dynamic would reduce clinical workload and improve patient comfort without detriment to image interpretation.