Room: Exhibit Hall
Purpose: Functional Gamma Knife radiosurgery (GKRS) procedures have been increasingly used for treating patients with trigeminal neuralgia (TN). Although its rates of toxicity are low, GKRS has been associated with some, if low, risks for serious sequelae, including hemiparesis. Dose rate changes over the course of the useful lifespan of cobalt-60 are not routinely considered. The purpose of this study was to assess the efficacy, complications and to investigate the potential radiobiological impact of cobalt-60 source age on BED in functional GKRS procedures.
Methods: A retrospective review was performed of 16 patients (12 females, 4 male, average age 65.3 years) who underwent GKRS for a diagnosis of TN. Patients were typically treated using a single 4-mm isocenter placed at the trigeminal nerve dorsal root entry zone. Patients were treatment doses: 38-43 Gy/50% isodose. BED in functional GKRS with a dose-protraction factor to correct for intrafraction DNA-damage repair and used standard single-fraction doses for trigeminal nerve ablation for TN (80 Gy).
Results: After replacement of the cobalt-60 source, the functional GKRS dose rate rose from 1.54 to 3.7 Gy/min, treatment time fell, and estimated BED increased. Assuming the most biologically plausible parameters, source replacement resulted in an immediate relative BED increase of 11.7% for GKRS-based TN management with 80 Gy. Sensitivity analyses with a/b values ranging from 2 to 10 Gy were also performed. For these sensitivity analyses, we again used the aforementioned biologically most plausible estimates of DNA repair halftimes.
Conclusion: Use of a new cobalt-60 source after replacement of an old source substantially increases the predicted BED for functional GKRS treatments for the same physical dose prescription. Source age, dose rate, and treatment time should be considered in the study of outcomes after high-dose functional GKRS treatments.
Not Applicable / None Entered.