Purpose: To evaluate brachytherapy dose-response relationships in adults with glioblastoma undergoing temporary 125I implant boost after external beam radiotherapy.
Methods and materials: Since June 1987, orthogonal radiographs using a fiducial marker box have been used to verify brain implant source positions and generate dose-volume histograms at the University of California, San Francisco. For adults who underwent brachytherapy boost for glioblastoma from June 1987 through December 1992, tumor volumes were reoutlined to ensure consistency and dose-volume histograms were recalculated. Univariate and multivariate analysis of various patient and treatment parameters were performed evaluating for influence of dose on freedom from local failure (FFLF) and actuarial survival.
Results: Of 102 implant boosts, 5 were excluded because computer plans were unavailable. For the remaining 97 patients, analyses with adjustment for known prognostic factors (age, KPS, extent of initial surgical resection) and prognostic factors identified on univariate testing (adjuvant chemotherapy) showed that higher minimum brachytherapy tumor dose was strongly associated with improved FFLF (p = 0.001). A quadratic relationship was found between total biological effective dose and survival, with a trend toward optimal survival probability at 47 Gy minimum brachytherapy tumor dose (corresponding to about 65 Gy to 95% of the tumor volume); survival decreased with lower or higher doses. Two patients expired and one requires hospice care because of brain necrosis after brachytherapy doses > 63 Gy to 95% of the tumor volume with 60 Gy to > 18 cm3 of normal brain.
Conclusion: Although higher minimum tumor dose was strongly associated with better local control, a brachytherapy boost dose > 50-60 Gy may result in life-threatening necrosis. We recommend careful conformation of the prescription isodose line to the contrast enhancing tumor volume, delivery of a minimum brachytherapy boost dose of 45-50 Gy in conjunction with conventional external beam radiotherapy, and reoperation for symptomatic necrosis.