Relatively little is known about the extent and effects of horizontal mergers among physician specialists. We developed and implemented a methodology to document changes in physician practice structure resulting from horizontal integration among urology groups. We merged cancer registry records from four large states with Medicare Part B claims to identify all urologists who treated men with prostate cancer. We added information from SK & A surveys and extensive internet searches to assign a practice structure to each urologist-year (2005-2014). Horizontal integration among small urology groups led to a sharp increase in the proportion of urologists who belong to large urology practices with ownership in intensity modulated radiation therapy and/or anatomical pathology services. By 2014, more than half of New Jersey urologists and about 43% of urologists in Florida and Texas were members of such large practices, whereas small percentages (7%-16%) were employed by a health system. In contrast, more than 27% of California urologists were employed but only 17.5% had ownership in intensity modulated radiation therapy and/or pathology services. Importantly, we found our indicators of market share of urologists associated with each practice structure type were highly concordant with indicators of market share based on number of prostate cancer episodes treated by each practice structure type.
Keywords: changes in urologist practice structure; horizontal integration.