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

The concept of tissue�maximum ratios (TMR) as a basis of dose computations has posed problems when applied to the whole range of clinically used megavoltage beams. Another problem is the definition of scatter�maximum ratio (SMR) which assumes a value of zero at the depth of maximum dose. This paper describes new methods of measuring collimator and phantom scatter correction factors. The definitions of TMR and SMR are modified in order 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 as well as non�isocentric modes of treatment.