Tolerance of the mature XXXX central nervous system to photon irradiation.

@entity1 @entity39 @entity1514 @entity1781 @entity35 @entity4685
Tolerance of mature @entity1 brain to photon irradiation is described. Isoeffect curves have been derived for tolerance of large and small volumes of brain, by examination of doses determined empirically and in clinical use. These have been compared with isoeffect curves of @entity1514 , @entity1781 , and brain @entity39 . The results show that the best-fitting Ellis-type equations, when five fractions per week are used, have low exponents for overall treatment time and high exponents for the number of increments (N), and are similar to published data on @entity35 @entity4685 . Of the equations used to test the relationship between total dose and number of increments, the power curve was the best fit. Mean values of exponents for N derived for brain and spinal cord tolerance were 0.4 or more. These were similar to values obtained for @entity1781 , though the data are more limited for this complication. Brain @entity39 is observed at slightly higher doses probably because of a difference in the end-point observed rather than because of any fundamental difference in tissue response. Evidence is presented to suggest that some repopulation may occur in widely spaced schedules. The use of the Ellis equation derived from connective-tissue data is inappropriate for central nervous system tissue, and its use may lead to a substantial risk of overdosage. A plea is made for adequate documentation of treatment details when @entity1 data are reported. The importance of dose per fraction is emphasized.
