The transmittance of diagnostic X-ray beams through concrete of two different densities were measured. Broad-beam X-ray transmissions were estimated using a Monte Carlo simulation at tube voltages of 50-150 kV and compared with National Council on Radiation Protection and Measurements (NCRP) data. The observed levels were higher than those predicted by the NCRP data; that is, the NCRP data underestimate the leakage radiation from widely used inverter systems. Some of the issues were resolved and new data were proposed.