Cumulative interfraction interval analysis of time and fraction size in radiotherapy.
OBJECTIVE
Overall treatment time (OTT) and fraction size influence treatment results with radiotherapy; yet, being inversely related, it is difficult to distinguish their effect independently of each other. This paper proposes a new approach for analysing the interrelationship of time and fraction size in terms of cumulative interfraction intervals (CIFI) (i.e. intervals between non-successive fractions of radiotherapy).


METHODS
We analysed the influence of CIFI1-6, CIFI1-11 and OTTon tumour control by Kaplan-Meier calculation of the primary relapse-free survival (PRFS) and Mann Whitney 'U' test in 242 patients with epidermoid cancer of the buccal mucosa-gingiva-palate region treated by either 2.4 Gy/fraction (60 Gy/5 weeks) or 3.5 Gy/fraction (52.5 Gy/3 weeks).


RESULTS
The results showed that (1) prolongation of CIFI1-11 significantly decreased tumour control in the 2.4 Gy but not 3.5 Gy schedule: (2) 3.5 Gy schedule was superior to 2.4 Gy schedule, particularly when CIFI1-11 was prolonged (5-year PRFS 80.3% vs 30.9%); and (3) OTT did not influence either schedule.


CONCLUSIONS
Prolonging interfraction intervals (by treatment interruptions, weekend gaps, etc.) in the first 2 weeks (when accelerated repopulation attempts are maximal) affects treatment results with low fraction sizes but not high fraction sizes; OTT probably vicariously reflects the effect of prolonging the intervening CIFI.
Clinical Radiobiology Section, Department of Radiotherapy, Regional Cancer Centre, Trivandrum, India. vnbtvm@vsnl.com
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Bhattathiri
D000328:Adult; D019583:Dose Fractionation, Radiation; D004307:Dose-Response Relationship, Radiation; D005260:Female; D006801:Humans; D008297:Male; D009062:Mouth Neoplasms / Q000532:radiotherapy*; D010043:Outcome and Process Assessment, Health Care; D012189:Retrospective Studies; D013997:Time Factors