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

Abstract

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 V₂₀), 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.

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