Cumulative interfraction interval analysis of influence of time and interruptions on radiotherapy results in oral cancers

Abstract

Purpose: To present a new approach to analyze the influence of time and interruptions in fractionated radiotherapy, in terms of cumulative interfraction intervals (CIFIs), that is, the interval between nonsuccessive fractions of radiotherapy. Methods and materials: The subjects were 142 patients with epidermoid cancer of the buccal mucosa-gingiva-palate region treated by 60 Gy in 25 fractions during 5 weeks. The influence of CIFI between the first and sixth, eleventh, sixteenth, and twenty-first fractions (CIFI 1–6, CIFI 1–11, CIFI 1–16, and CIFI 1–21, respectively), as well as overall treatment time on local control, was analyzed by Kaplan-Meier calculation of the primary relapse-free survival and Mann-Whitney U testing of the difference in various CIFIs between the tumors that recurred (recurred group) and those that did not (controlled group). Results: Only CIFI 1–11 influenced local control significantly, with the 5-year primary relapse-free survival rate being 67.9%, 48.4%, and 32.6% in those with CIFI 1–11 of <14, 14, and >14 days ( p = 0.0181). The median CIFI 1–11 in the controlled group was significantly lower than that in the recurred group (14 days vs. 15 days; p = 0.0037). Conclusion: Interruptions during the first 11 fractions, including planned weekend gaps, decrease the effect of radiotherapy, possibly because of successful tumor cell repopulation during the protracted interfraction intervals. The analysis of time in terms of CIFI rather than overall treatment time appears to be a promising area for research.