The International Atomic Energy Agency (IAEA) released a protocol named Technical Report Series (TRS-398) to measure the absorbed dose in water for external radiotherapy beams. It provides a unified approach for using calibrated ionization chambers that are traceable to standard laboratories in determining the absorbed dose to water. The objective of the study was to compare the reference dosimetry [dose at 10 cm depth ( \({D}_{10}\) )] between TRS-398 and its revised version. Reference dosimetry was performed for flattened photon beam with nominal energies of 6, 10, and 15 MV as well as flattening filter free (FFF) beam of energies 6 FFF and 10 FFF based on the guidelines of both TRS-398 and its revised version using two different ionization chambers of varying sensitive volumes such as 0.65 cm3 (FC65-G) & 0.13 cm3 (CC13) IBA ionization chambers with calibration coefficient traceable to absorbed dose to water (Dw) standards. The comparison of absorbed dose at 10 cm ( \({D}_{10}\) ) depth using both protocols with FC65-G chamber shows a difference ranging from 0.40 to 0.63% in FF beams and 0.03 to 0.05% in FFF beams. For CC13 chamber, the difference ranged from -0.60 to -0.77% in FF beams and -0.40 to -0.50% in FFF beams. The differences in absorbed dose between TRS-398 old and revised protocols were evaluated and a variation of up to 0.63% in \({D}_{10}\) was observed for FC65-G and -0.77% in \({D}_{10}\) was observed for CC13 chamber. The use of old \({k}_{Q,{Q}_{O}}\) value affects the reference dose measurements, which overestimates the results by an average of 0.5%. The use of cross-calibrated chamber following the old protocol in determining the \({D}_{10}\) underestimates the results by maximum of -0.77% in FF beams and -0.5% in FFF beams.
