The timing of treatment in breast cancer: gaps and delays in treatment can be harmful.

'Timing' of treatment in breast cancer may refer to intervals within a single management or between different managements. Rates of shrinkage of breast cancers in response to treatment are related to histological grade and may be used as surrogates for growth rates. Histological grade should predict appropriate timing of treatment. Four cases of locally advanced breast cancer that illustrate a number of different types of interval are presented. Two tumours of differing histological grade (II and III) had been managed by historical 'split-course' radiotherapy and two similar grade III tumours were managed by primary medical treatment, followed at different intervals by radiotherapy. In the grade III tumours different radiotherapy fractionation régimes and effects of varying intervals between mangements are compared. The theoretical advantage of shrinkage (leading to reoxygenation) during the gap in 'split-course' radiotherapy is realized only in relatively slowly growing and shrinking tumours. Grade III tumours grow rapidly. They have the potential to shrink rapidly in response to appropriate treatment, namely intensive chemotherapy or radiotherapy but not hormones. Inadequate treatment leads to growth in intervals between individual doses, whether of drugs or radiation, and to failure of local control. The advantage of surgery or primary medical treatment will be lost if the interval between managements is too long in relation to the volume doubling time. Histological grade is a good guide of this parameter; the grade III tumours are particularly vulnerable to gaps in treatment.