SU‐E‐T‐535: Linac Based Multiple SRS Targets Plan Evaluation: Understanding Conformity Index Generated Form BrainLab IPlan RT Dose 4.1 SRS Planning System

Purpose:The study intent is to provide a practical method of extracting Conformity Index for multiple Stereotactic Radiosurgery (SRS) target plans generated from BrainLab iPlan RT Dose 4.1 planning system (CIiPlan).Methods:CIiPlan is the ratio between normal tissue volume (VN, IDL) and target volume (VPTV, IDL) encompassed by the prescription isodose line (IDL). BrainLab iPlan RT Dose 4.1 planning and physics documents lack details on how to retrieve and interpret CIiPlan parameters of multiple SRS targets. The CIiPlan values for multiple lesions are misleading and in most cases gave bigger values than what is reflected from the plan. To understand and demonstrate the underlying issues CIiPlan values of patient plans with multiple SRS targets were extracted using two different methods. Method 1: CIiPlan values were directly obtained with all target plans present (CITN,iPlan). Method 2: CIiPlan values were extracted by assigning 100% weighting for one of the targets and 0 weighting for the rest of the targets (CIIN,iPlan). This process was t repeated for each target. In both cases CIiPlan were read from DVH graphs at the prescription IDL.Results:CITN,iPlan values from Method 1 were larger than CIIN,iPlan values obtained using Method 2. The difference was pronounced for smaller lesions. Even though targets were far apart and were not covered with the same prescription IDL, CIiPlan values from Method 1 were based on normal tissue volume encompassed by 5 treatment IDL from all targets. Carefully analyzing CIiPlan values from the two methods we were able to provide appropriate CIiPlan formulas that gave good estimate of both methods values.Conclusion:CIiPlan for multiple SRS targets need be analyzed with great caution; especially if small lesions and big lesions are part of same plan. CIIN,iPlan values based on each target normal tissue volume encompassed by the prescription IDL should be used.