Air Kerma Based Dosimetry versus Absorbed Dose to Water Based Dosimetry for High-energy Photon

Introduction: In the last 5 years the American Association of Physicists in Medicine Task Group 51 (AAPM TG-51) and the International Atomic Energy Agency (IAEA) published a new high-energy photon and electron dosimetry protocol. These protocols are based on the use of an ion chamber having an absorbed-dose to water calibration factor. These are different from the previous NCS report-2 and IAEA TRS-277 protocols, which require air kerma calibration factor. Aim of the Study: Is to present the dose comparison between various dosimetry protocols and the IAEA TRS-398 protocol for clinical reference dosimetry of high energy photon beams. The absorbed-dose to water measured according to the NCS Report-2, International Atomic Energy Agency technical Report Series No. 277 (IAEA TRS-277) and, TG51 are compared to that measured using the TRS-398 protocol. Results and Discussion: This study shows that the absorbed dose which is measured with The IAEA TRS-398 formalisms is higher than that calculated with NCS Report-2 and IAEA TRS-277 formalisms within range from 0.4 to 1.3% and from 0.7 to 2.1%, respectively, for different higher energy photon beams of Co-60, 6, 8 and 18 MV. as sensed by different ionization chambers, The chambers used are PTW 30001, 30004, and NE-2571; which have calibration factors NK and ND,W traceable to the Bureau International des Poids et Mesures (BIPM). In contrast, the absorbed-dose to water measured according to TG-51 is in good agreement with TRS-398 within about 0.3% for photon beams.