Dosimetric Evaluation of the Tissue Inhomogeneity effect in Gamma Knife Treatment Planning

Abstract

Purpose/Objective(s): Treatment planning using a single isocenter for multiple brain metastases with conformal dose distribution is challenging in terms of plan quality, planner variance, and planning efficiency. Automated planning techniques are currently available to provide efficient planning and consistent plan quality. This study evaluates two automated treatment planning techniques for multiple brain metastases using a single isocenter. Materials/Methods: Eighteen patients with a total of 96 lesions who underwent treatment for multiple brain metastases (3-10 lesions) were included in this retrospective study. The clinical plans were originally generated with VMAT technique in a treatment planning system. Using the same beam geometry, these patients were replanned with a knowledgebased planning (KBP) SRS model based on 100 prior monoisocentric SRS plans. The same cohort of patients were replanned with the Multiple Brain Mets (MBM) SRS module in a commercially available SRS planning software using noncoplanar dynamic conformal arcs with the same numbers of couch kicks. Both techniques were designed to automatically generate volumetrically optimized plans for all metastases using a single isocenter. All the plans were optimized and calculated only once without further manual tuning. The MBM plans were imported into the TPS to compare with the corresponding KBP plans on the same platform. Plan evaluation metrics including the PTV coverage, conformity index (CI), gradient index (GI), total monitor units (MUs), planning time, brain V 12Gy , V 8Gy , and V 5Gy were recorded. Planning times were recorded from initial PTV assignment for optimization to the end of dose calculation. Comparisons of the KBP and MBM plans were performed using the two-tailed paired student t-test. Results: For KBP vs. MBM plans, PTV coverage mean AE standard deviation values were (99.2% AE 1.2%) vs. (98.9% AE 1.6%); CI were 1.5 AE 0.4 vs. 1.5 AE 0.2; GI were 3.0 AE 1.6 vs. 3.1 AE 1.9. Planning time mean AE standard deviation values were 19.2 AE 3.8 vs. 6.3 AE 1.2 minutes (p < 0.05); MUs were 2366 AE 1496 vs. 2892 AE 1828 for KBP and MBM plans, respectively (p < 0.05). Brain V 12Gy Z (64.9 AE 34.2) cc vs. (66.1 AE 50.7) cc; V 8Gy Z (172.6 AE 97.1) cc vs. (146.5 AE 109.9) cc; V 5Gy Z (422.8 AE 216.9) cc vs. (324.8 AE 205.5 cc) (p < 0.05). Conclusion: Both KBP and MBM automated planning techniques produced clinically acceptable plans and yielded comparable PTV coverage, CI, and GI values. In cases with irregularly shaped lesions, KBP plans provided better PTV coverage vs. MBM plans. KBP plans took significantly longer to plan but shorter treatment times due to fewer MUs required vs. MBM plans. Brain V 12Gy were comparable, but MBM plans spared healthy brain better vs. KBP plans in terms of brain V 8Gy and V 5Gy .