The addendum to TG‐51, which is currently under review, will provide an expanded new set of beam quality conversion factors, kQ. The calculated values of kQ in the TG‐51 protocol are based on analytic calculations which make use of various tabulated values of measured and calculated factors uch as the water to air stopping‐power ratio, Pwall, Prepl and Pcel (for details see Ch 9 of the 2009 AAPM Summer School book, Med. Phys. Publishing, Madison, WI). However, with the development of the EGSnrc Monte Carlo system (Kawrakow, Med Phys 27(2000) 499) it is possible to accurately calculate the response of ion chambers. Also computers have became faster per unit cost and clever variance reduction techniques have been implemented for doing ion chamber calculations in complex geometries (Wulff et al, Med Phys 35(2008)1328). Hence ab initio Monte Carlo calculations of kQ have become both accurate and feasible using large clusters of computers. Values of kQ have been calculated this way for a total of 33 commonly used cylindrical ion chambers (Muir and Rogers, Med Phys 37(2010)5939) and these kQ values are accurately fit as a quadratic function of the beam quality specifier, %dd(10)x. Detailed estimates of the systematic uncertainty in these calculations have been made and range between 0.6% and 1.0%, depending on the assumptions made. The largest component is the uncertainty (previously 0.5%) in the assumed constancy with beam quality of (W/e)air, which relates the energy deposited in the cavity to the charge released in the air.In a detailed comparison of the calculated kQ values to the extensive high‐quality measurements by McEwen (Med Phys 37(2010)2179), Muir et al (Med Phys 38 (2011) 4600) found the mean percentage differences between the calculations and the experiments are 0.08(0.17), 0.07(0.32) and 0.23(0.31) in 6, 10 and 25 MV beams respectively (bracketed values are rms deviations). These discrepancies are well within the stated uncertainties of the measurements (about 0.3% to 0.4%) and the calculations (about 0.3% to 0.4% ignoring W/e uncertainties and assuming correlated uncertainties in photon cross section). These and similar comparisons for plane‐parallel chambers (Muir et al, Med Phys 39(2012) 1618) can be used to set an upper limit of 0.36% on the variation of (W/e)air with beam quality between 60Co and 25 MV beams (95% confidence). More importantly, the close agreement with experiment gives confidence in the accuracy of the Monte Carlo calculated values of kQ with a 68% confidence uncertainty of between 0.4% and 0.5%.The addendum to TG‐51 recommends use of these calculated values of kQ for the 21 ion chambers which meet the requirements for a reference class ion chamber. Learning Objectives 1. To become aware of the new addendum to TG‐51 regarding reference dosimetry in photon beams. 2. Understand how ab initio calculations of the TG‐51's beam quality conversion factor, kQ, are done 3. Understand the uncertainties involved in ab initio calculations of kQ.