SU‐E‐T‐892: Calculation Uncertainty in CyberKnife Dosimetric Parameters of Brainstem

Purpose: It is important to understand the effect of dose calculation on the dosimetric parameters of a critical structure, which are often used to evaluate the risk of toxicity. We present the variation of some dosimetric parameters for brainstem as computed by two different dose algorithms. Methods: Both Ray Tracing and Monte Carlodose algorithms are commissioned for the CyberKnife planning system at Cooper. Study was performed for twenty seven intracranial patients treated with CyberKnife. Dose calculation was carried out in Multiplan workstation by Monte Carlo algorithm with heterogeneity corrections as well as Ray Tracing algorithm without heterogeneity corrections. Brainstem was delineated by the attending radiation oncologist during planning process. The same prescription and optimization as the actual treatment were employed in dose calculation for each patient. Results: Principle dosimetric parameters (D50, D10, D1cc, D0.1cc and maximum dose) of brainstem are derived for each patient. The difference in percentage between Ray Tracing and Monte Carlo calculation is plotted. Significant variation has been observed for each dosimetric parameter between two different dose algorithms. Also plotted is the variation of those dosimetric parameters against the volume of brainstem. Conclusions: The value of dosimetric parameters can be considerately different depending on dose algorithm used in calculation. The evaluation of dose tolerance for a critical structure using those dosimetric parameters needs to be cautious about dose computing methods.