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A Comprehensive Clinical Study On Switching From Type-B to Type-C Dose Algorithms for Modern Lung SBRT
C Zhou1*, N Bennion2 , R Ma2 , X Liang3 , S Wang2 , K Zvolanek2 , M Hyun2 , S Zhou2 , W Zhen2 , C Lin2 , A Wahl2 , D Zheng2 , (1) University of Chicago, Chicago, IL, (2) University of Nebraska Medical Center, Omaha, NE, (3) UFPTI, Jacksonville, FL
Presentations
PO-BPC-Exhibit Hall-19 (Saturday, March 18, 2017) Room: Exhibit Hall
Purpose: To investigate the impact of switching from Type-B to Type-C dose algorithms for lung SBRT planning on a large patient cohort.
Methods: Fifty-two lung SBRT patients were included. The original Type-B plans used coplanar VMAT arcs normalized to D₉₅=Prescription Dose. These were compared against three Type-C plans: re-calculated plans (identical plan parameters), re-normalized plans (D₉₅=PrescriptionDose), and re-optimized plans. Dosimetric endpoints were extracted and compared among the four plans, including RTOG dose criteria(R100%, R50%, D2cm, V105% and lung V20), PTV Dmin, Dmean, Dmax, D₉₅ and D₉₀, PTV coverage(V100%), homogeneity index(HI), and Paddick conformity index(PCI). Patient follow-up data was also examined to assess clinical significance and correlation between local failure and target coverage reduction.
Results: Re-calculated Type-C plans resulted in decreased PTV Dmin(mean difference=5.2%) and increased Dmax(mean difference=3.1%), with similar or improved RTOG dose compliance, but compromised PTV coverage (mean D₉₅ and V₁₀₀ reduction of 2.5% and 8.1%, respectively). Seven plans had >5% D₉₅ reduction (max=16.7%), and eighteen had >5% V₁₀₀ reduction (max=60.0%). Re-normalized Type-C plans restored target coverage, but yielded degraded plan conformity (average PCI reduction 4.0%), and RTOG dosimetric criteria deviation worsened in eleven plans, mostly in R50% and D2cm. Except for one case, re-optimized Type-C plans restored RTOG compliance achieved by the original Type-B plans, resulting in similar dosimetric values but slightly higher target dose heterogeneity (mean HI increase=13.2%). Two cases of local failures were identified and no lung toxicity was observed with a median(range) follow-up of 9(3-23) months. Reductions in D₉₅ and V₁₀₀ for the PTV were < 3% for the two failure cases.
Conclusion: Type-B SBRT lung plans considerably overestimate target coverage for some patients, necessitating Type-C re-normalization or re-optimization. Current RTOG dosimetric criteria appear to remain appropriate. A larger cohort with longer follow-up is needed to illustrate the clinical significance of the dosimetric effects.
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