Dmax As A Predictor Of Normal Tissue Toxicity: A Cautionary Note In Extrapolating Clinical Experiences Across Radiosurgury Platforms

Keywords: Dose, gamma knife, dose planning, radiosurgery, radiation injury

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     With the increased reach of the Perfexion, Gamma Knife treatment centers are likely to extrapolate extracranial normal tissue dose constraints from clinical experiences with other radiosurgery platforms.  
     In this study we seek to explore the variation of normal tissue maximum dose calculations (Dmax) both within GammaPlan and between other treatment planning systems.
     Within GammaPlan the dose calculation matrix grid size for seven consecutive pituitary adenoma patients was modified from the clinical plan. The variation in Dmax is reported for each case. Separately, a sample case enrolled in a clinical trial of spinal radiosurgery was replanned utilizing three linac radiosurgery platforms (Cyberknife, Tomotherapy HiArt2, Novalis). The treatment planning goals were to deliver 24 Gy to the GTV while limiting the spinal cord maximum dose to less than 14 Gy and no more than 0.5 cc of the spinal cord receiving more than 10 Gy. For all systems, the dose calculation grid spacing in the longitudinal direction was the CT slice spacing, 3 mm. The dose distributions were exported in DICOM-RT format. Dose volume histograms were obtained using several methods: (1) directly from the respective planning systems, (2) Eclipse via import of the DICOM-RT dose distributions, (3) CERR, and (4) by interpolating the dose distributions onto a 0.24 mm x 0.24 mm x 0.25 mm grid in custom MatLab software.
     Within GammaPlan changing the dose calculation matrix size resulted in 0-0.1 Gy difference to optic apparatus Dmax in five of the seven cases.  In two cases differences in 0.3 Gy and 0.4 Gy (5%) were observed.  For the linac spinal plans, the largest difference between the calculation methods was 11% for Dmax, and 8% for D0.35cc.
     This is a retrospective study.
     Differences in DVH calculation methods utilized for different radiosurgery platforms result in clinically significant differences in the calculation of Dmax.  These differences may be lower in magnitude within GammaPlan. 
     This observation has significant implications for extrapolations of normal tissue toxicity between radiosurgery platforms and for studies comparing dose distributions between radiosurgery platforms.


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