We investigate the occurrence of radio minihalos --- diffuse radio sources of unknown origin observed in the cores of some galaxy clusters --- in a statistical sample of 58 clusters drawn from the Planck Sunyaev-Zeldovich cluster catalog using a mass cut ($M_{500}>6times 10^{14} M_{odot}$). We supplement our statistical sample with a similarly-sized non-statistical sample mostly consisting of clusters in the ACCEPT X-ray catalog with suitable X-ray and radio data, which includes lower-mass clusters. Where necessary (for 9 clusters), we reanalyzed the Very Large Array archival radio data to determine if a mihinalo is present. Our total sample includes all 28 currently known and recently discovered radio minihalos, including 6 candidates. We classify clusters as cool-core or non-cool core according to the value of the specific entropy floor in the cluster center, rederived or newly derived from the Chandra X-ray density and temperature profiles where necessary (for 27 clusters). Contrary to the common wisdom that minihalos are rare, we find that almost all cool cores - at least 12 out of 15 (80%) - in our complete sample of massive clusters exhibit minihalos. The supplementary sample shows that the occurrence of minihalos may be lower in lower-mass cool-core clusters. No minihalos are found in non-cool-cores or warm cores. These findings will help test theories of the origin of minihalos and provide information on the physical processes and energetics of the cluster cores.
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