A study of the interplay effect for VMAT SBRT using a four-axes motion phantom

Abstract

Purpose: To assess the accuracy of volumetric modulated arc therapy (VMAT) stereotactic body radiation therapy (SBRT) when treating moving targets (such as lung or liver lesions), focusing on the impact of the interplay effect in the event of complex breathing motion and when a gating window is used.

Methods: A dedicated programmable motion platform was implemented. This platform can carry large quality assurance (QA) phantoms and achieve complex three-dimensional (3D) motion. Volumetric modulated arc therapy SBRT plans were delivered with TrueBeam linac to this moving setup and the measured dose was compared to the computed one. Several parameters were assessed such as breathing period, dose rate, dose prescription, shape of the breathing pattern, the use of a planning target volume (PTV) margin, and the use of a gating window.

Results: Loss of dose coverage (D95%) was acceptable in most situations. The doses received by 95% of the CTV, D95% ( $CT{V}_m$ ) ranged from 94 to 101% (mean 98%) and the doses received by 2% of the CTV D2% ( $CT{V}_m$ ) ranged from 94% to 110% of the prescribed dose. A visible interplay effect was observed when no margin was used or when the number of breathing cycles during the treatment delivery was lower than 20.

Conclusions: In our clinical context, treating lung and liver lesions using VMAT SBRT is reasonable. The interplay effect was moderated and acceptable in all simulated situations.

Keywords: interplay; motion; radiotherapy; respiratory; stereotactic body radiation therapy (SBRT); volumetric modulated arc therapy (VMAT).

Fig. 1

The programmable motion platform 4X has four independent and programmable motion axes. One axis (a) is dedicated to achieving a vertical motion for the secondary tray (for RGSC external marker). The three others are to perform x, y, and z motions for a main carbon tray (b). For example it can carry the PTW Octavius four‐dimensional phantom (c).

Fig. 2

Breathing pattern according to the CURIE model: the part between vertical lines of this curve [see Eq. (2)] is repeated as a breathing pattern.

Fig. 3

Superior–Inferior dose profiles passing through the planning target volume center for $B_{5,n}$ (up‐left), $B_{5,y}$ (up‐right) with 6FFF fields and $B_{5,n}$ with 6FF field (bottom‐left).

Fig. 4

Superior–Inferior dose profiles passing through the planning target volume center for different plans with a prescription of 4 Gy with (left) and without (right) gating.

Fig. 5

Relative difference (in %) within the clinical target volumes (CTV) region between computed and measured SI profiles. All curves are for motion period of 5 s and without gating. The first seven curves (a–g) are for a prescription of 11 Gy with a CTV‐PTV margin of 5 mm and the last one (C‐4 Gy) is for only 4 Gy with no margin, see Fig. 3).