Feasibility of a TPS-integrated method to incorporate tumor motion in the margin recipe

Abstract

Background and Purpose: There are several alternatives to the widespread ITV strategy in order to account for breathing-induced motion in PTV margins. The most sophisticated one includes the generation of a motion-compensated CT scan with the CTV placed in its average position -the midposition approach (MidP). In such configuration, PTV margins integrate breathing as another random error. Despite overall irradiated volume reduction, such approach is barely used in clinical practice because of its dependence to deformable registration and its unavailability in commercial treatment planning systems. As an alternative, the mid-ventilation approach (MidV) selects the phase in the 4D-CT scan that is the closest to the MidP, with a residual error accounted for in the PTV margin. We propose a treatment planning system-integrated strategy, aiming at better approximating the MidP approach without its drawbacks: Hybrid MidV-MidP approach, i.e. the delineation on the MidV-CT and translation at the mid-position coordinates using treatment planning system built-in capabilities.Material and Methods: Forty-five lung lesions treated with stereotactic radiotherapy were selected. PTV was defined using MidP, MidV, Hybrid MidV-MidP and ITV strategies. Margin definitions were adapted and resulting PTVs were compared.Results: Hybrid MidV-MidP showed similar target volume and location than the MidP and confirmed that margin-incorporated tumor motion strategies lead to significantly smaller PTVs than the ITV with mean reduction of 26±7%. Conclusion:We report on the successful implementation of a pseudo-MidP solution without its inherent drawbacks. It answers the need for TPS-embedded tumor motion range identification and related margin's component calculation.

Incorporation of tumor motion directionality in margin recipe: The directional MidP strategy

“…Tumor motion being the only error treated differently by the proposed method, this may be due to its larger weight with regard to the other geometrical uncertainties. It could thus be conjectured that the dosimetric advantage of directional MidP margins becomes also apparent for motions of lower amplitudes in case of larger coefficient β e.g., normofractionated treatment of lung tumors [9], smaller penumbra e.g., liver SBRT [39] or less significant set-up [40] and systematic uncertainties [41][42]. Nevertheless, tumors with similar amplitudes can benefit very differently from ).…”