Planned and unplanned gaps in radiotherapy: the importance of gap position and gap duration.

Local tumour control in 971 patients with squamous carcinoma of the supraglottic larynx has been examined in relation to the occurrence of gaps in radiation therapy. The minimum follow-up time was 3 years. The reasons for a gap in radiotherapy fell into four categories: independent of the patient (national holidays, machine break-down, etc.), planned gaps (split-course therapy), severe normal-tissue reactions, and intercurrent disease. Only 11.7% of patients had no gap at all, 75.5% a single gap, and 2.1% had more than four gaps. The probability of tumour control increased with dose in all patient sub-groups; the average percentage increase for a 1% increase in dose was 4.3. The data were subjected to multivariate analysis, leading to the following conclusions. Patients in whom there was a single gap showed a remarkable trend of local control: if the gap began before day 19 after the start of therapy, the local tumour control was considerably below that in patients who did not suffer a gap in treatment. The local tumour control in patients whose gap began at day 20-29 was indistinguishable from that in patients who had no gap in treatment. A gap further towards the end of treatment was again associated with a severe drop in local control. This trend was independent of the recorded cause of the gap. The mechanism of this phenomenon is not clear. The effect of the timing of a treatment gap appears in this data set to have had a considerable impact on outcome and our observations should stimulate further study of this phenomenon in other clinical settings.