A comprehensive dosimetric study on switching from a Type-B to a Type-C dose algorithm for modern lung SBRT

BACKGROUND: Type-C dose algorithms provide more accurate dosimetry for lung SBRT treatment planning. However, because current dosimetric protocols were developed based on conventional algorithms, its applicability for the new generation algorithms needs to be determined. Previous studies on this issue used small sample sizes and reached discordant conclusions. Our study assessed dose calculation of a Type-C algorithm with current dosimetric protocols in a large patient cohort, in order to demonstrate the dosimetric impacts and necessary treatment planning steps of switching from a Type-B to a Type-C dose algorithm for lung SBRT planning. METHODS: Fifty-two lung SBRT patients were included, each planned using coplanar VMAT arcs, normalized to D(95%) = prescription dose using a Type-B algorithm. These were compared against three Type-C plans: re-calculated plans (identical plan parameters), re-normalized plans (D(95%) = prescription dose), and re-optimized plans. Dosimetric endpoints were extracted and compared among the four plans, including RTOG dosimetric criteria: (R(100%), R(50%), D(2cm), V(105%), and lung V(20)), PTV D(min), D(max,) D(mean,) V(%) and D(90%), PTV coverage (V(100%)), homogeneity index (HI), and Paddick conformity index (PCI). RESULTS: Re-calculated Type-C plans resulted in decreased PTV D(min) with a mean difference of 5.2% and increased D(max) with a mean difference of 3.1%, similar or improved RTOG dose compliance, but compromised PTV coverage (mean D(95%) and V(100%) reduction of 2.5 and 8.1%, respectively). Seven plans had >5% D(95%) reduction (maximum reduction = 16.7%), and 18 plans had >5% V(100%) reduction (maximum reduction = 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 11 plans, in R(50%), D(2cm), and R(100%). 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%). CONCLUSIONS: 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.