The effect of increasing the treatment time beyond three weeks on the control of T2 and T3 laryngeal cancer using radiotherapy.

Local control of cancer by radiotherapy may be prejudiced by accelerated tumour clonogen repopulation particularly during protracted treatment schedules. A series of 496 cases of T2 and T3 larynx cancer treated here by radiotherapy has been studied to examine the impact on local control of treatment durations ranging from 9 to 41 days. Data were analysed using a linear-quadratic formulation describing the fractionation sensitivity, with the incorporation of a parameter relating to treatment time. Using combined T2 and T3 data, the increase in dose required to maintain a constant local control (the time factor) was between 0.5 and 0.6 Gy per day. These values are similar to those reported for 4 weeks or more in the literature. Also, the calculated dose to control 50% of tumours, given over the standard Christie duration of 21 days, was on the line projected back from literature data over 28-66 days. The present data are consistent with the presence of such a time factor following a lag phase of not more than 3 weeks after starting radiotherapy. Hence, further consideration should be given to using shorter overall treatment times in radiotherapy for head and neck cancer.