LAUSR

OBJECTIVES How variations of treatment time affect the safety/efficacy of Gamma Knife (GK) radiosurgery is a matter of considerable debate. Due to the relative simplicity of treatment planning for trigeminal neuralgia (TN) this question has been addressed in a group of these patients. Using the concept of biologically effective dose (BED) the effect of the two key variables, dose and treatment time have been taken into account. METHODS A retrospective analysis was carried out on 408 TN cases, treated between 1997 and 2010. Treatment involved the use of a single 4 mm iso-center. If conditions allowed, the iso-center was placed at a median distance of 7.5 mm from the emergence of the trigeminal nerve from the brain stem. Effects were assessed in terms of the incidence of the complication, hypoesthesia, or in terms of efficacy, the incidence of 'pain free' after 30 days and at 1 and 2 years. These responses were evaluated with respect to both the physical dose and the biological effective dose (BED), the latter using a bi-exponential repair model. RESULTS Re-evaluation showed that the prescription doses, at the 100% iso-dose, varied between 75 and 97.9 Gy, delivered over 25 - 135 min. The relationship between physical dose and the incidence of hypoesthesia was not significant; the overall incidence being approximately 20%. However, there was a clear relationship between BED and the incidence of hypoesthesia, the incidence increasing from < 5% after a BED of ∼1800 Gy2.47 to 42% after ∼2600 Gy2.47. Efficacy, in terms of freedom from pain, was approximately 90%, irrespective of the BED (1550-2600 Gy2.47) at 1 and 2 years. There was a suggestion from the data that 'pain free' status developed more slowly at lower BED values. CONCLUSION These results strongly suggest that safety/efficacy may be better achieved by prescribing a specific BED instead of a physical dose. A dose/time to BED conversion table has been prepared to enable iso-BED prescriptions. This finding may dramatically change dose-planning strategies in the future. This concept needs to be validated in other indications where more complex dose-planning is required.