Room: AAPM ePoster Library
Purpose: To develop a volume-independent conformity metric called the Gaussian Weighted Conformity Index (GWCI) for evaluating the quality of stereotactic radiosurgery/radiotherapy (SRS/SRT) plans for small brain tumors.
Methods: The GWCI is calculated by the following steps: 1) calculating signed bi-directional distance between the prescription isodose line and the target contour (positive distance represents under-dose), 2) assigning different weight to Gaussian functions which have heights one and center zero, depending on the under- and over-dose (Gaussian weighting), 3) multiplying the distance and the Gaussian weighting on each point, and 4) averaging the results of step 3. The GWCI can penalize tumor under-/over-dosing by changing the Gaussian weightings. Tumor under-dosing was penalized one to five times more heavily than the prescription isodose falling outside the tumor (over-dose). Five weighting combinations were tested for finding the optimal weighting values. A total of 40 targets from 18 patients, who have received Gamma-knife SRS/SRT, were chronically selected to test this proof-of-concept. The DICOM RT dose and structure files were exported directly from the planning system to inhouse Matlab software for analysis. RTOG conformity indices (CI) were also calculated to evaluate the proposed GWCIs.
Results: Among five test weighting combinations, penalizing the under-dose twice than over-dose showed the optimal range of GWCIs (0.991±0.003). A GWCI of 0.985 was used as a volume-independent cutoff for plan conformity for under-dose penalizing twice than over-dose.
Conclusion: Limitations of current conformity indices become apparent when applied to SRS/SRT treatment plans. A GWCI metric was successfully developed which can be used to accurately score the quality of an individual treatment plan while eliminating small volume effects.