Over-response of tissue-equivalent ionization chambers at low energies: effect of high-Z surface contamination

Inner wall surface contamination of an ionization chamber by high-Z materials can produce an over-response to low-energy photons which is greatly underestimated by the percentage composition of the contaminant in the entire chamber. Experience with a chamber contaminated in such a manner is presented. Examples are included which illustrate the magnitude of possible clinical effects from low-energy units which have been calibrated with a chamber contaminated with a thin layer of relatively high-Z material.