The prescribed total radiation dose should be administered within a specific time. In most studies, a detrimental effect from the treatment break on the outcome was evident. This is suggestive of the deleterious effect of accelerated repopulation of tumor clonogens. In particular for the cancers of the head and neck for which the evidence is the strongest for such a consequence, even a 1-day interruption resulted in a decrease in the local control rate by 1.4%. The main recommendation is to exert all efforts to retain the planned irradiation schedule;

Disease-free curves by gap length
Effect of gap length and position on results of treatment of cancer of the larynx in Scotland by radiotherapy: A linear quadratic analysis.

Robertson AG . Radiother Oncol. 1998;48:165–173

Purpose: This paper reports on the analysis of the effect of the length and position of unplanned gaps in radiotherapy treatment schedules.

Data from an audit of the treatment of carcinoma of the larynx are used. They represent all newly diagnosed cases of glottic node-negative carcinoma of the larynx between 1986 and 1990, inclusive, in Scotland the end-points are local control of cancer of the larynx in 5 years and the length of the disease-free period.

Results: Unplanned gaps in treatment are associated with poorer local control rates and an increased hazard of a local recurrence through their effect on extending the treatment time. A gap of 1 day is potentially damaging but the greatest effect is at treatment extensions of 3 or more days, where the hazard of a failure of local control is increased by a factor of 1.75 The treatment extension as a result of the gap is more important than the position of the gap in the schedule. Gaps in the treatment schedule have a detrimental effect on the disease-free period. A gap has a slightly greater effect than an increase in the prescribed treatment time. Any gap in treatment is potentially damaging. The position of the gap in the schedule was shown to be not important.

Loss of local control with prolongation in radiotherapy.
Fowler JF,   Int J Radiat Oncol Biol Phys. 1992;23:457–467. Twelve published clinical results of radical radiotherapy of head and neck cancer have been reviewed, seven of them with fresh multivariate analyses, to determine the magnitude of time factors relating local control to overall time. In all but two of the data sets a significant loss of local control was observed with prolongation. The median rate of loss was 14% in only 1 week, the range 3 to 25%. This corresponds to a median loss of 26% in 2 weeks (5-42%). These results are comparable with other, less detailed information. Whether these significant losses are due to proliferation of tumor cells or to other causes such as physician selection, it is clear that modest prolongation is associated with a lower chance of local control.