Most clinical trials employ fixed-sample designs in which the data of all patients are collected and first examined at the end of the study. Planning, conduct, analysis, and report of clinical trials require comprehensive resources. Group-sequential testing may have a place in accuracy and agreement studies. The testing strategy must, though, be defined at the planning stage, and sample sizes must be reasonably large at interim analysis to ensure robustness against single outliers. Group-sequential testing may enable early stopping of a trial, allowing for potential time and resource savings. Other partitions did not suggest early stopping after adjustment for multiple testing due to one influential outlier and our small sample size. When performing interim analyses with one third and two thirds of patients, sufficient agreement could be concluded after the first interim analysis and the final analysis. The fixed-sample analysis ( N = 45) was compared with the group-sequential analysis strategies comprising one (at N = 23), two (at N = 15, 30), or three interim analyses (at N = 11, 23, 34), respectively, which were defined post hoc. Differences in SUVmax were assumed to be normally distributed, and sequential one-sided hypothesis tests on the population standard deviation of the differences against a hypothesised value of 1.5 were performed, employing an alpha spending function. Primary lesion maximum standardised uptake value (SUVmax) was determined twice from preoperative FDG-PET/CTs in 45 ovarian cancer patients. The purpose of this paper was to demonstrate a group-sequential analysis strategy in an intra-observer study on quantitative FDG-PET/CT measurements, illuminating the possibility of early trial termination which implicates significant potential time and resource savings. Group-sequential testing is widely used in pivotal therapeutic, but rarely in diagnostic research, although it may save studies, time, and costs.
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