Amplitude gating for a coached breathing approach in respiratory gated 10 MV flattening filter-free VMAT delivery.

Francis Viel,Cheryl Duzenli,Ermias Gete,Richard Lee

doi:10.1120/jacmp.v16i4.5350

Abstract

The purpose of this study was to investigate amplitude gating combined with a coached breathing strategy for 10 MV flattening filter‐free (FFF) volumetric‐modulated arc therapy (VMAT) on the Varian TrueBeam linac. Ten patient plans for VMAT SABR liver were created using the Eclipse treatment planning system (TPS). The verification plans were then transferred to a CT‐scanned Quasar phantom and delivered on a TrueBeam linac using a 10 MV FFF beam and Varian's real‐time position management (RPM) system for respiratory gating based on breathing amplitude. Breathing traces were acquired from ten patients using two kinds of breathing patterns: free breathing and an interrupted (~5 s pause) end of exhale coached breathing pattern. Ion chamber and Gafchromic film measurements were acquired for a gated delivery while the phantom moved under the described breathing patterns, as well as for a nongated stationary phantom delivery. The gate window was set to obtain a range of residual target motion from 2–5 mm. All gated deliveries on a moving phantom have been shown to be dosimetrically equivalent to the nongated deliveries on a static phantom, with differences in point dose measurements under 1% and average gamma 2%/2 mm agreement above 98.7%. Comparison with the treatment planning system also resulted in good agreement, with differences in point‐dose measurements under 2.5% and average gamma 3%/3 mm agreement of 97%. The use of a coached breathing pattern significantly increases the duty cycle, compared with free breathing, and allows for shorter treatment times. Patients' free‐breathing patterns contain considerable variability and, although dosimetric results for gated delivery may be acceptable, it is difficult to achieve efficient treatment delivery. A coached breathing pattern combined with a 5 mm amplitude gate, resulted in both high‐quality dose distributions and overall shortest gated beam delivery times.PACS number: 87.55.Qr

Highlights

79 Viel et al.: Amplitude gating for coached breathing gated flattening filter-free mode (FFF) volumetric-modulated arc therapy (VMAT) delivery to a specific portion of the respiratory cycle
To compensate for this limitation, the idea of coupling respiratory gating with a fast delivery technique, such as volumetric-modulated arc therapy (VMAT), has increasingly been adopted as the treatment of choice for moving tumors in the liver and the pancreas.[5,6,7,8] In addition, the new advancements in flattening filter-free mode (FFF) for modulated treatments allow for an increased dose rate, significantly reducing the treatment time for treatments delivering high doses per fraction, such as stereotactic ablative radiation therapy (SABR).(9) The combination of reduced margins and gated VMAT could permit treatment of a select group of hepatocellular carcinoma (HCC) patients who were previously ineligible for radiation therapy due to larger lesions.[10]
Continuing development of the quality assurance (QA) procedure for amplitude gated FFF VMAT is ongoing as we have identified some limitations of the technique during the course of this study

Summary

Introduction

79 Viel et al.: Amplitude gating for coached breathing gated FFF VMAT delivery to a specific portion of the respiratory cycle. Respiratory motion during intensity modulation, such as IMRT or VMAT, presents an additional challenge over 3D conformal therapy as dose gradients are no longer limited to the edges of the field and vary with time. In these cases, tumor and organs at risk are likely to move through high-dose gradients even within the gate. For this reason it is important to consider the residual motion when performing dosimetric measurements

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