Cancer-rates in inter-exam-time defined batches of screening DBT images

Abstract

There is currently substantial interest in understanding batch reading to improve performance in breast-cancer screening, and to identify mechanisms of performance. We evaluated batch reading of digital breast tomosynthesis (DBT) images for breast cancer screening using observational data acquired at the University of Pittsburgh Medical Center (UPMC). We studied batches of screen exams that were defined by completion-time differences between sequentially interpreted cases, in which a completion time exceeding a threshold led to defining a new batch. After exclusions the data consisted of 121,652 exams from 15 readers, with a total of 1,081 cancers.We found that the inter-exam time threshold used for batch definition introduces selection bias that had a large impact on the cancer rate in the first case of a batch. For the smallest threshold (< 1 minute), all cases are defined as the first case of a new batch, and the cancer rate was the overall cancer rate of the data, 8.9/1000. As the threshold increased to 4-5 minutes, the cancer rate of the first case in the batch increased to nearly double the overall rate, 16.0/1000. This threshold excluded many non-cancer cases, which are typically read in 2-3 minutes for DBT, while still capturing most cancer cases, which take longer to complete. At a 10-minute completion-time difference, the first-case cancer rate decreased to 12.6/1000, and stabilized. We argue that this increase in cancer rate is likely due to readers terminating batch reading upon encountering a difficult case.Our results demonstrate a clear selection bias in batches defined by inter-exam time, and suggest using cancer rate for adjustment to reduce the effect of this bias.