EFFECT OF RADIATION FRACTION SIZE ON LOCAL-CONTROL RATES FOR EARLY GLOTTIC CARCINOMA - A MODEL ANALYSIS FOR IN-VIVO TUMOR-GROWTH AND RADIORESPONSE PARAMETERS

Background: Two recent studies have suggested that the probability of local control for early glottic carcinoma (stages T1 and T2) following radiation therapy is lower when daily fractions of 1.8 Gy are given i nstead of 2.0 Gy when the total dose is kept constant at approximately 66 Gy. Methods: We reviewed our experience with a total of 42 patient s with T1, NO, MO squamous cell carcinoma of the glottic larynx who we re treated with radiation alone during the period 1979 to 1989. All pa tients received conventional fractionation radiation therapy (one trea tment per day, 5 days per week). Eight patients received 2.0 Gy per fr action for an average to tal dose of 64.8 Gy, and 34 patients received 1.8 Gy per fraction for an average total dose of 66.7 Gy. We also cal culated overall survival statistics. Combining the clinical data from our series and those of patients from other series (171 total patients ), we analyzed the tumor response curve and doubling time for early gl ottic carcinoma by using a time-dependent, linear quadratic equation m odel. The analysis is valid for the typical range of radioresponsivene ss for most tumors in healthy tissue. Results: Using statistical z ana lysis, we noted a significant difference in the local control rates at 5 years, with better local control in the group that received 2.0 Gy per fraction (100% vs 70%). Our results for the calculated doubling ti me and tumor response curves correspond with experimental values for i n vitro squamous cell carcinoma systems. Conclusion: It appears from t he literature and from our data that the control rates for T1, NO, MO glottic carcinoma may be signifiantly improved by using 2.0 Gy per fra ction dose vs 1.8 Gy per fraction dose. Our analysis has provided a mo del of in vivo tumor response of early glottic carcinomas based on rep orted local control rates using these two different treatment regimens .