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(PP182) Calculation of repair half-time of the trigeminal nerve after gamma knife radiosurgery (GKRS) using clinical data from 256 procedures for trigeminal neuralgia.
Stieber, Volker*,1, Robbins, Michael 1, Balamucki, Christopher3, de Guzman, Alan1, Tatter, Stepehn2, Ekstrand, Kenneth1, Munley, Michael1, Branch, Charles2, Shaw, Edward1, Bourland, J.2, Lovato, James4, Ellis, Thomas2, 1 Wake Forest University, Winston-Salem, NC, United States3 Wake Forest University, Winston-Salem, NC, United States2 Wake Forest University, Winston-Salem, NC, United States4 Wake Forest University, Winston-Salem, NC, United States
ABSTRACT- Purpose/Objective: A biological model taking repair and dose rate into account can calculate a biologically effective dose (BED) for varying treatment times and prescription doses. It suggests that over the range of typical treatment times of 25 60 min, BED may vary from 2-31% depending upon / and T . We analyzed the outcomes of 256 GKRS procedures to calculate a clinical value for T . Materials/Methods: From 9/99-3/04, 326 GKRS procedures (256 evaluable) for patients with facial pain were performed. Typically the 50% isodose line was placed tangential to the brainstem, with the shot isocenter targeted at the proximal trigeminal nerve root. Radiation dose was prescribed at the 100% isodose line. The self-reported degree of pain relief was recorded in four categories: excellent; good; fair; poor. The biological model used was developed by Thames and Nilsson for continuous radiation (Fig. 1): BED = biologically effective dose D = total dose g = continuous repair factor / = the ratio of the tissue-specific linear and quadratic survival parameters t = exposure duration = recovery constant T = repair half-time We assumed a value of 1.5 for / . Response was defined as excellent, good, or fair pain relief. Logistic regression was used to model the logit of response as a function of t. The resulting coefficient was converted to an estimated probability of response at the range of typical treatment times, 25 60 min. The estimated probabilities were compared to yield the estimated difference of BED from 25 60 min. This difference was used to back-calculate a clinical value for T . Results: The estimated difference of BED from 25-60 minutes was 11%. The calculated value T = 1.28 hrs. Conclusions: With GKRS, the trigeminal nerve might respond more like an acute-reacting tissue and an / of 10 should be used, which changes the absolute values for each BED, but does not change their ratio. The value of T =1.28 hrs should be interpreted as a clinical value in the context of GKRS. The combined targeting error based on diameter of the nerve and mean deviation between imaging and mechanical measurement on a stereotactic MRI scan is ~3.6 mm. Pain recurrence may thus be related to a partial miss of the target (i.e. sublethal irradiation).
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