The effect of user-defined variables on dosimetry consistency in Gamma Knife planning

We report a dosimetric variation caused by a user-defined variable for the Leksell Gamma Knife planning system. Treatment plans of 31 randomly selecte d patients were studied retrospectively to determine the dosimetric effects in the dose prescription and computation as a result of dose matrix positi oning in the Leksell Gamma Plan (LGP, Version 4.12). Phantom studies with i on chamber measurements were carried out to validate the accuracy of the co mputation results. An average overdose of 2% was found due to the variation s in the user-defined dose matrix position for the studied cases. In the ex treme, the overdose value was as high as 5% with an over-treatment time exc eeding 2 min. The phantom measurements were found to agree with the LGP cal culation within 0.5%. An adaptive method was developed and demonstrated in this study to eliminate such dosimetry variations.