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Clinical Utility of Conformity Indices for Brain and Lung SBRT Plans

G Warrell

G Warrell1*, Z Xu2 , S Lee2 , T Biswas1,2 , M Machtay1,2 , T Podder1,2 , (1) Case Western Reserve University, Cleveland, OH, (2) University Hospitals Cleveland Medical Center, Cleveland, OH


SU-I-GPD-T-572 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: A variety of planning indices have been proposed to assess the conformity of the prescribed isodose line to the target volume in SRS and SBRT plans. In this study, we have evaluated several indices for better understanding of their relative merits when applied to clinical SBRT arc/VMAT plans for both brain and lung cases.

Methods: Several common conformity indices (CIs) were investigated with 17 brain and 20 lung SBRT plans used for treatment. Brain PTVs had an average volume of 19.7cc (range 0.7-54.4cc), while lung PTVs had an average volume of 42.2cc (range 8.8-98.0cc); prescribed dose was an average of 27.1Gy (3-5 fractions) for brain and uniformly 50Gy (4 or 5 fractions) for lung cases. Dosimetric plans were calculated by the dose-convolution algorithm in Philips Pinnacle (ver9.10), with a dose grid resolution of 2mm.

Results: The CI used by the RTOG (ratio of prescription isodose volume to PTV volume) was smaller (3.8% lower for brain, 3.4% lower for lung plans, p<0.01) and had a greater coefficient of variation (8.9% higher for brain, 28.6% higher for lung) than that defined by the ratio of prescription isodose volume to PTV volume receiving the prescription dose. The CI of Paddick et al. demonstrated a lesser coefficient of variation of absolute deviation from unity than the other CIs. All CIs investigated trended towards unity as the PTV volume increased. The dose gradient index decreased with increased PTV volume for brain plans and well-described (R²=0.89) by a power-law function, whereas a weak correlation (R²=0.33) was found for lung plans.

Conclusion: Use of the same indices may have unanticipated implications in treating different anatomical sites. The behavior of different definition of CIs needs to be well understood to avoid unintentional biases in the assessment of the quality of a clinical SBRT plan.

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