Revision of tissue‐maximum ratio and scatter‐maximum ratio concepts for cobalt 60 and higher energy x‐ray beams

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Abstract
The concept of tissue-maximum ratios (TMR) as a basis of dose computations has posed problems when applied to the whole range of megavoltage beams used in human radiotherapy. Another problem is the definition of scatter-maximum ratio (SMR) which assumes a value of zero at the depth of maximum dose. New methods of measuring collimator and phantom scatter correction factors were described. The definitions of TMR and SMR are modified to compute phantom scatter at any depth, including depth of maximum dose. The revised concept is basic and general enough that it can be applied to X-ray beams of any energy, fields of any shape and isocentric and non-isocentric modes of treatment.