SU‐E‐T‐244: Should Treatment Time Be Included in Assessing the Quality of a Gamma Plan?

Propose: To establish a method of estimating nominal treatment time for a given treatment volume in order to facilitate the assessment of Gamma Knife treatment plan quality. Methods and Materials: The clinical plans of 90 patients with brain metastases and/or surgical cavities treated on our Leksell Gamma Knife PERFEXION unit between Nov. 2010 and Jan. 2011 were evaluated. Patients with other diagnosis, e.g. acoustic neuroma or meningioma, were not included. Each plan was prescribed by one of the six radiation oncologists using GammaPlan 8.3.1. After excluding the lesions whose dose grid was overlapped with other lesions', we analyzed 198 tumors from these 90 plans. The planned treatment (TX) time were normalized to the time needed if prescription (Rx) dose and dose rate were 20 Gy and 2.5 Gy/min, respectively. The relationship between the normalized TX time and TX volume was analyzed, and a curve was established where about half of the lesions have lower normalized TX time than the nominal time predicted by the curve. Results: A formula, which is 4.5 times the cubic root of the TX volume, is proposed to estimate the nominal TX time, which, after scaling back by the Rx dose and treatment dose rate, can be used to compare with the planned TX time. After satisfying other quality metrics, a lower planned TX time indicates that the plan is likely to have reached an optimum, while a much higher planned TX time, as determined by the upper threshold, also established by this study, may signify the need for a better plan. Conclusions: A good plan has high Coverage, high conformity, low Gradient Index, and low treatment time. Knowing the bounds of these metrics helps one determine if an optimum has been reached.