Author: Derek Matthew Garcia, Mallory Glenn, Stephen F. Kry, Ruitao Lin, Dennis S. Mackin, Tucker J. Netherton 👨🔬
Affiliation: MD Anderson Cancer Center, The University of Texas MD Anderson Cancer Center, Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, University of Texas’ MD Anderson Cancer Center, The University of Texas MD Anderson Cancer Center, Houston 🌍
Purpose: This study aims to investigate the feasibility of implementing clinically relevant quality metrics for evaluating IROC SRS phantom plans, including conformity index (CI) and homogeneity index (HI).
Methods: Eighteen IROC SRS phantom irradiations from 2024 and various institutions were retrospectively analyzed. All irradiations were planned to deliver 25 Gy using 6 MV photon beams. Film-based dose distributions were used to calculate CI and HI in coronal and sagittal planes, employing a 1.9 cm diameter target volume centered in the high-dose region. These metrics were compared with their 3D counterparts derived from the corresponding treatment plans. Additionally, gamma index percentage pixels passing were compared with CI and HI for the film-based metrics. Pearson correlation analysis was performed to assess the agreement between measured and planned quality index values.
Results: Film-based CI demonstrated moderate agreement with planned CI values. The correlation was stronger for the coronal plane (r = 0.511, p = 0.030) than for the sagittal plane (r = 0.443, p = 0.066), reflecting variability in film measurements between planes. Agreement between film-based and planned HI was weaker overall, with r = 0.243 (p = 0.331) for the coronal plane and r = 0.390 (p = 0.109) for the sagittal plane, indicating limited sensitivity of film-based HI to capture 3D dose variations. Comparisons of gamma index values with film-based CI and HI yielded negligible correlations (all r < 0.27, p > 0.28), highlighting limitations of the gamma index for evaluating plan quality.
Conclusion: This study demonstrates the potential of integrating CI and HI into IROC Houston’s SRS phantom evaluations as complementary tools to the gamma index. By focusing on metrics that directly assess clinically relevant dose distributions, these tools could enhance the accuracy of plan evaluations. Future research should focus on 3D analyses to further enhance clinical relevance and sensitivity.