Optimal Levels of Isoeffective Doses for Two Fractionation Modes in Glioblastoma

Abstract

Purpose: Determination of the optimal level of a/b for the calculation of isoeffective doses in glioblastoma. Material and methods: Two fractionation modes were studied in 141 patients with primary glioblastoma (GBM). Fractionation with the prescribed dose of 2 and 3 Gy was used alternately (a study involving a pairwise selection strategy). In addition, 34 GBM patients treated in the pilot study of the National Medical Research Radiological Centre earlier were added to the database, the total number of patients was 175 people. Results: As of December 2022, 134 (76.6 %) of 175 treated patients had a fatal outcome. In the range of values of the coefficient a/b from 5.0 to 10.0, the indicator a/b=7.5 turned out to be the most informative. According to the criterion of overall survival, a favorable level of isoeffective dose exceeds 59.5 Gy. The median overall survival in the group with an isoeffective dose of >=59.5 Gy was 22.3 months (95 % Cl: 17.7–26.9), in the group with a lower dose level – 10.3 months (p<0.0001). According to monovariate analysis at an isoeffective dose of >=59.5 Gy for fractionation regimens with a prescribed dose of 2 and 3 Gy, survival rates were almost identical (p=0.745). There were no differences in overall survival in the groups with prescribed doses of 2 and 3 Gy, taking into account the use of temozolomide on the background of radiation therapy (OR=1,087; p=0.677). In the older age group, a/b= 7.5 should be used to determine the level of isoeffective dose. The median overall survival with an isoeffective dose of >=59.5 Gy was 20.57 months (n=98; 95 % Cl: 17.7–23.4), with a lower dose level – 6.35 months (n=24; p<0.0001). In persons younger than 50 years of age, a decrease in the a/b coefficient should be expected. Conclusion: From our point of view, the obtained data on the equivalence of regimens with a prescribed dose of 2 and 3 Gy indicate the correct choice of the level of a/b in glioblastoma and the selection of the minimum level of isoeffective dose (59.5 Gy) according to the criterion of overall survival.