Intensity Modulated Radiotherapy (IMRT) is extensively used for cancer patients treatment in modern Radiotherapy Clinics. Patient-related Quality Control (QC) is essential part of IMRT clinical implementation, as fluencies of Intensity Modulated (IM) fields has to be independently verified prior to delivery of the IMRT plan to the patient. In most cases, combination or ionometric and film dosimetry is in use for patient-related QC. The aim of this study was to evaluate volume-averaging effect of different ionization chambers used for ionometric dosimetry of the IMRT fields.