On the pitfalls of PTV in lung SBRT using type-B dose engine: an analysis of PTV and worst case scenario concepts for treatment plan optimization

Abstract

Background: PTV concept is presumed to introduce excessive and inconsistent GTV dose in lung stereotactic body radiotherapy (SBRT). That GTV median dose prescription (D 50) and robust optimization are viable PTV-free solution (ICRU 91 report) to harmonize the GTV dose was investigated by comparisons with PTV-based SBRT plans. Methods: Thirteen SBRT plans were optimized for 54 Gy/3 fractions and prescribed (i) to 95% of the PTV (D 95) expanded 5 mm from the ITV on the averaged intensity project (AIP) CT, i.e., PTVITV, (ii) to D 95 of PTV derived from the van Herk (VH)'s margin recipe on the mid-ventilation (MidV)-CT, i.e., PTVVH, (iii) to ITV D 98 by worst case scenario (WCS) optimization on AIP,i.e., WCSITV and (iv) to GTV D 98 by WCS using all 4DCT images, i.e., WCSGTV. These plans were subsequently recalculated on all 4DCT images and deformably summed on the MidV-CT. The dose differences between these plans were compared for the GTV and selected normal organs by the Friedman tests while the variability was compared by the Levene's tests. The phase-to-phase changes of GTV dose through the respiration were assessed as an indirect measure of the possible increase of photon fluence owing to the type-B dose engine. Finally, all plans were renormalized to GTV D 50 and all the dosimetric analyses were repeated to assess the relative influences of the SBRT planning concept and prescription method on the variability of target dose. Results: By coverage prescriptions (i) to (iv), significantly smaller chest wall volume receiving ≥30 Gy (CWV30) and normal lung ≥20 Gy (NLV20Gy) were achieved by WCSITV and WCSGTV compared to PTVITV and PTVVH (p > 0.05). These plans differed significantly in the recalculated and summed GTV D 2, D 50 and D 98 (p < 0.05). The inter-patient variability of all GTV dose parameters is however equal between these plans (Levene's tests; p > 0.05). Renormalizing these plans to GTV D 50 reduces their differences in GTV D 2, and D 98 to insignificant level (p > 0.05) and their inter-patient variability of all GTV dose parameters. None of these plans showed significant differences in GTV D 2, D 50 and D 98 between respiratory phases, nor their inter-phase variability is significant. Conclusion: Inconsistent GTV dose is not unique to PTV concept but occurs to other PTV-free concept in lung SBRT. GTV D 50 renormalization effectively harmonizes the target dose among patients and SBRT concepts of geometric uncertainty management. © 2020 The Author(s).