Comments on 'Calculation of water equivalent thickness of materials of arbitrary density, elemental composition and thickness in proton beam irradiation'.

We comment on a previous article by Zhang and Newhauser (2009 Phys. Med. Biol. 54 1383-95) which presents several approximate ways of computing the water equivalent of an arbitrary degrader. First, we present a simple exact method which depends only on the range-energy relation of water and of the degrader material. Second, we point out that any theoretical method, approximate or exact, ultimately depends on the range-energy relation, that is to say, the correct value of the mean excitation energy I for the materials in question. Unfortunately I is particularly problematic for water. Therefore, at the present state of knowledge, we should measure water equivalent, rather than computing it, whenever an accurate value is needed.