The effect of treatment time and treatment interruption on tumour control following radical radiotherapy of laryngeal cancer.

A significant effect of overall treatment time on local control was found in a retrospective review of 1012 radically irradiated squamous cell carcinomas of the larynx. The actuarial local relapse free rate (LRFR) at 5 years for the whole group was 59%. The effect of treatment time on local control was modelled to the linear-quadratic equation. Using logistic regression analysis treatment time and dose were significant (p = 0.008 and p = 0.04, respectively). When the analysis was adjusted for the influence of stage and laryngeal subsite treatment time remained a significant prognostic factor (p = 0.02). The derived value of gamma/alpha was 0.7 Gy/day and when adjusted for stage and sub-site 0.8 Gy/day. This equates to a dose increment to maintain iso-effective local control of 0.64 Gy/day and 0.73 Gy/day respectively for daily fractions of 2.5 Gy and an assumed alpha/beta for tumour of 25 Gy. To provide an estimate of the clinical impact of treatment interruptions not compensated for by dose escalation a Cox regression was performed. Significant variables were T stage, N stage, sex, total dose and total length of treatment interruption. Using the proportional hazard model it was calculated that each day of treatment interruption resulted in an increase in the hazard of local relapse by 4.8% (p = 0.006). Based on our data it was calculated that this would result in a decrease in local control of 1.4% for each day of uncompensated treatment interruption.