The TG21 protocol introduced theNgascalibration method for parallel plate chambers in high‐energy electron beams. This calibration method was performed for a Markus parallel plate chamber in proton and electron beams of various energies as well as a60Co beam. For an individual chamber, theNgasvalue in proton beams differs from theNgasvalue in cobalt and electron beams by the ratio of (W̄/e) for proton beams to that of a60Co beam. While the replacement correction factor is essential for Markus chambers in low‐energy electron beams, the results of ourNgasppmeasurements in proton beams showed that the Markus chamber does not need a replacement correction factor for therapeutic proton beams of energy 20–170 MeV. These results indicate that the 0.7‐mm guard ring of the Markus chamber is adequate to prevent the in‐scattering of secondary electrons produced by proton irradiation of the chamber wall or medium.