Comments on 'The effective depth of cylindrical ionization chambers in water for clinical proton beams'

In a recent paper (Bhullar and Watchman 2012 Phys. Med. Biol. 57 273-86), the authors comment on data concerning the effective depth of cylindrical ionization chambers in water for clinical proton beams calculated in a previous paper (Palmans 2006 Phys. Med. Biol. 51 3483-501) and propose different results. They present a closed-format expression for an integral in the analytical model and claim that the series expansion I used in the older paper does not converge to the correct solution. They also claim that this is the reason for the resulting values of the shift of the effective depth with respect to the chamber centre to be different in both papers. Both claims are, however, incorrect as I show in this comment. The values they present are most likely based on a mistake in the scaling coefficients to account for the non-water equivalence of wall, sleeve and central electrode materials. The better agreement which they observe between their values and the recommendation of IAEA TRS-398 is thus a coincidence. My conclusion is that the best available data for the effective depth of cylindrical ionization chambers in water in clinical proton beams are at present the values I published before (Palmans 2006 Phys. Med. Biol. 51 3483-501) and which are supported by Monte Carlo simulations and experimental evidence.