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

Background: PTV concept is presumed to introduce excessive and inconsistent GTV dose in lung stereotactic body radiotherapy (SBRT). That GTV median dose prescription (D50) 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 (D95) expanded 5 mm from the ITV on the averaged intensity project (AIP) CT, i.e., PTVITV, (ii) to D95 of PTV derived from the van Herk (VH)‘s margin recipe on the mid–ventilation (MidV)–CT, i.e., PTVVH, (iii) to ITV D98 by worst case scenario (WCS) optimization on AIP,i.e., WCSITV and (iv) to GTV D98 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 D50 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 D2, D50 and D98 (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 D50 reduces their differences in GTV D2, and D98 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 D2, D50 and D98 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 D50 renormalization effectively harmonizes the target dose among patients and SBRT concepts of geometric uncertainty management.


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