Time factor in postoperative radiotherapy: a multivariate locoregional control analysis in 868 patients.

PURPOSE: To study locoregional tumor control in postoperative radiotherapy (PRT) for head-and-neck cancer in relation to the position and duration of treatment gaps, duration of the interval surgery-radiotherapy, and to the other potentially prognostic variables. METHODS AND MATERIALS: The retrospective study included 868 patients with cancer of the larynx, oral cavity, oropharynx, and hypopharynx treated in Gliwice between 1980 and 1997. Mean total radiation dose, dose per fraction, overall radiation treatment time (OTT), and the interval surgery-PRT were 62.8 Gy, 2.1 Gy, 45 days, and 63 days, respectively. No interruptions during PRT (except for weekend breaks) appeared in 30% of patients, whereas 19% had more than 5 days of gap. The actuarial locoregional recurrence-free survival (RFS) has been examined using multivariate Cox regression model, and the ultimate locoregional tumor control probability using a logistic model.
RESULTS: Increased duration of treatment gaps, positive resection margins, pathologically proven metastases to the neck nodes, and extralaryngeal site of cancer were significantly related to a decrease in RFS. The duration of time interval surgery-PRT appeared, by contrast, only borderline significant for RFS. The relative risk of locoregional recurrence was approximately the same for the gaps in days 1-21 as for the "late" gaps (days >21), except in a subgroup of patients with positive clinical margins. The logistic analysis revealed a significant time-related displacement of the dose-response curves for PRT. The detriment from the protraction of OTT appeared to be larger than the benefit from the equivalent shortening of OTT.
CONCLUSIONS: Although the conclusions from this study must be regarded as only hypothesis-generating, we assume that a highly significant adverse influence of radiation treatment gaps on the rate of tumor control is consistent with rapid repopulation of cancer clonogenes during PRT. Lack of significant effect of the position of gaps on locoregional tumor control after radical surgery may suggest that a lag time for the onset of repopulation in PRT is short. A less likely explanation is that the total amount of regeneration during OTT is the same, regardless of the timing of the gap, even if all the repopulation occurred late. The magnitude of the detriment in tumor control from prolonged interval surgery-PRT indicates that repopulation of cancer cells between surgery and radiotherapy is not as fast as between the fractions of radiotherapy.