Treatment Parameters Optimization to Compensate for Interfractional Anatomy Variability and Intrafractional Tumor Motion.

Romain Brevet,Marco Durante,Christoph Bert,Christian Graeff,Daniel Richter

doi:10.3389/fonc.2015.00291

Abstract

Scanned ion beam therapy of lung tumors is severely limited in its clinical applicability by intrafractional organ motion, interference effects between beam and tumor motion (interplay), as well as interfractional anatomic changes. To compensate for dose deterioration caused by intrafractional motion, motion mitigation techniques, such as gating, have been developed. However, optimization of the treatment parameters is needed to further improve target dose coverage and normal tissue sparing. The aim of this study was to determine treatment-planning parameters that permit to recover good target coverage for each fraction of lung tumor treatments. For 9 lung tumor patients from MD Anderson Cancer Center (Houston, Texas), a total of 70 weekly time-resolved computed tomography (4DCT) datasets, which depict the evolution of the patient anatomy over the several fractions of the treatment, were available. Using the GSI in-house treatment planning system TRiP4D, 4D simulations were performed on each weekly 4DCT for each patient using gating and optimization of a single treatment plan based on a planning CT acquired prior to treatment. The impact on target dose coverage (V 95%,CTV) of variations in focus size and length of the gating window, as well as different additional margins and the number of fields was analyzed. It appeared that interfractional variability could potentially have a larger impact on V 95%,CTV than intrafractional motion. However, among the investigated parameters, the use of a large beam spot size, a short gating window, additional margins, and multiple fields permitted to obtain an average V 95%,CTV of 96.5%. In the presented study, it was shown that optimized treatment parameters have an important impact on target dose coverage in the treatment of moving tumors. Indeed, intrafractional motion occurring during the treatment of lung tumors and interfractional variability were best mitigated using a large focus, a short gating window, additional margins, and three fields.

Highlights

Treating moving targets, such as non-small cell lung cancer (NSCLC) tumors, using photon radiation therapy has been investigated [1] and is being clinically used nowadays combined to real-time tracking [2, 3]
Target definition including tumor motion, size, and position [22], as well as range-adapted margins, were discussed [15, 23,24,25] and implemented [26]. Those studies concentrated on intrafractional motion compensation, meaning that the possible anatomic variability between the time of the treatment planning CT and treatment or between fractions was not taken into account
The 4D0 simulations show the effect of intrafractional tumor motion on target dose coverage: V95 decreases with a large variability for the smaller focus sizes

Summary

Introduction

Treating moving targets, such as non-small cell lung cancer (NSCLC) tumors, using photon radiation therapy has been investigated [1] and is being clinically used nowadays combined to real-time tracking [2, 3]. Using heavy-ion scanned beam therapy has shown many advantages compared to conventional radiotherapy [4, 5] by reducing the number of fields, which have to be used as well as the dose delivered to the organs at risk (OARs) in the vicinity of the tumor It demands high precision and accuracy when applied to moving tumors because of the possible dose delivery errors induced by range shifts themselves due to intrafractional motion, interfractional anatomic changes, and patient misalignments [6, 7]. Target definition including tumor motion, size, and position [22], as well as range-adapted margins, were discussed [15, 23,24,25] and implemented [26] Those studies concentrated on intrafractional motion compensation, meaning that the possible anatomic variability between the time of the treatment planning CT and treatment or between fractions was not taken into account.

Objectives

Methods

Results

Conclusion