THE EFFECT OF INCREASING THE TREATMENT TIME BEYOND 3 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.