Abstract
Purpose:
To determine whether positional errors on the order of 0.5–1 mm have any effect on critical structure dose or target coverage for pituitary adenoma patients treated with the Gamma Knife Perfexion.
Methods:
Treatment plans for six pituitary adenoma patients were modified to include positional shifts of 0.5 mm and 1 mm for each shot and in each cardinal direction. Twelve new plans were created per patient, and changes in target coverage and maximum dose to surrounding critical structures were recorded. Of particular interest were cases in which dose to the optic apparatus exceeded a tolerance of 8 Gy or dose to the brainstem exceeded a tolerance of 10–12 Gy. Target volumes ranged from 0.8–4.7 cc, and prescription doses ranged from 12–20 Gy.
Results:
Target coverage was significantly affected for only the smallest lesion. In this case, a shift of 1 mm in the superior or inferior direction resulted in a loss of coverage equal to 10%. A shift of 0.5 mm in the superior direction resulted in a loss of coverage equal to 4%. Critical structure tolerances were exceeded in 6/36 altered plans applying a 1 mm shift, and 3/36 altered plans applying a 0.5 mm shift. The increase in critical structure dose was between 2.0–2.7 Gy for these cases.
Conclusion:
In the world of radiosurgery accuracy is often quoted as sub-millimeter. It is important to recognize, however, that even at this scale small errors can lead to situations where OAR tolerances are exceeded or target coverage is lost. The risk of such errors is dependent on the uncertainties involved with each step of the planning/treatment process. Future work will be devoted to better quantifying these uncertainties and defining the risk/benefit ratio for specific treatment approaches, i.e. margin vs no margin, optimization strategy, choice of prescription dose, etc.
