Program Information
Volume-Independent Conformity Index for Stereotactic Brain Tumors
S Kim1*, H Kim2 , S Lee3 , M Awan4 , D Rangaraj5 , Y Zheng6 , J Monroe7 , R Partel8 , S Lo9 , M Machtay10 , A Sloan11 , J Sohn12 , (1) Baylor Scott & White Health, Temple, TX, (2) National Cancer Center, Goyang-si, Gyeonggi-do, ,(3) University Hospitals Case Medical Center, Cleveland Heights, OH, (4) Case Western Reserve University, Cleveland, OH, (5) Baylor Scott & White Health, Temple, TX, (6) University Hospitals Case Medical Center, Cleveland, OH, (7) St. Anthony's Cancer Center, St. Louis, MO, (8) Case Western Reserve University Hospitals of Cleveland, OH, Cleveland, OH, (9) Case Western Reserve University, Cleveland, OH, (10) University Hospitals Case Medical Center, Cleveland, Ohio, (11) Case Western Reserve University Hospitals of Cleveland, Cleveland, OH, (12) Case Western University, Cleveland, OH
Presentations
SU-F-T-599 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose:To develop a volume-independent metric called the Gaussian Weighted Conformity Index (GWCI) for assessing conformality of stereotactic radiosurgery plans for small brain tumors.
Methods:The GWCI calculates bi-directional distance by searching for corresponding points between the prescription isodose line and tumor contour, assigning different scoring weights to tumor coverage with a score of 1.0 being ideal assuming an idealized Gaussian distribution of dose around the tumor. (Figure 1, left) The GWCI penalizes tumor under-dosing three times more heavily than the prescription isodose falling outside the tumor. (Figure 1, middle) A user interface was created to calculate GWCI from images and RT structures (Figure 1, right). Patients receiving radiosurgery were randomly selected and images and RT structures were exported to MiM (MiMVista, Cleveland, OH) to calculate traditional conformality indices (CI). CIs were calculated for 39 tumors from patients receiving Gamma Knife radiosurgery (GKSRS) and from 10 tumors from patients receiving linac-based stereotactic radiosurgery (L-SRS). GWCIs were calculated for 14 tumors from patients receiving GKSRS and for 10 tumors from patients receiving L-SRS.
Results:Conformality indices calculated from 39 GKSRS plans and 10 L-SRS plans are plotted in Figure 2 demonstrating that as tumour volume gets smaller, conformality index increases. GWCIs for 14 tumors were plotted against CIs and linear regression was performed (Figure 3) yielding GWCI = -.077*CI + 1.044 (R² = .52). Utilizing this regression, the corresponding GWCI to a traditionally-acceptable CI of 1.5 was calculated as 0.927.
Conclusion:Limitations of current conformity metrics become apparent when applied to radiosurgery treatment plans. A GWCI tool was successfully developed which can be used to accurately score the quality of an individual treatment plan while eliminating small volume effects. A GWCI of 0.93 may be used as a volume-independent cutoff for plan conformality.
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