Abstract
In photon radiotherapy, respiratory-induced target motion can be accounted for by internal target volumes (ITV) or mid-ventilation target volumes(midV) defined on the basis of four-dimensional computed tomography (4D-CT). Intrinsic limitations of these approaches can result in target volumes that are not representative for the gross tumor volume (GTV) motion over the course of treatment. To address these limitations, we propose a novel patient-specific ITV definition method based on real-time 4D magnetic resonance imaging (rt-4DMRI). Three lung cancer patients underwent weekly rt-4DMRI scans. A total of 24 datasets were included in this retrospective study. The GTV was contoured on breath-hold MR images and propagated to all rt-4DMRI images by deformable image registration. Different targets were created for the first (reference) imaging sessions: ITVs encompassing all GTV positions over the complete (ITV ) or partial acquisition time ( ), ITVs including only voxels with a GTV probability-of-presence (POP) of at least 5% ( ) or 10% ( ), and the mid-ventilation GTV position. Reference planning target volumes ( ) were created by adding margins around the ITVs and midV target volumes. The geometrical overlap of the with from the six to eight subsequent imaging sessions on days n was quantified in terms of the Dice similarity coefficient (DSC), sensitivity [SE: ( )/ ] and precision [PRE: ( )/ ] as surrogates for target coverage and normal tissue sparing. Patient-specific analysis yielded a high variance of the overlap values of , when different periods within the reference imaging session were sampled. The mid-ventilation-based PTVs were smaller than the ITV-based PTVs. While the SE was high for patients with small breathing pattern variations, changes of the median breathing amplitudes in different imaging sessions led to inferior SE values for the mid-ventilation PTV for one patient. In contrast, and showed higher SE values with a higher robustness against interfractional changes, at the cost of larger target volumes. The results indicate that rt-4DMRI could be valuable for the definition of target volumes based on the GTV POP to achieve a higher robustness against interfractional changes than feasible with today's 4D-CT-based target definition concepts.
Highlights
In high-precision radiotherapy (RT) of lung tumors, target motion due to respiration remains a predominant challenge.[1]
We describe how a 4DMRI dataset (4DMRI)-based internal target volumes (ITV) can be defined based on the probability-of-presence (POP) of the gross tumor volumes (GTVs) to reduce random uncertainties
The median number of breathing cycles recorded in the 4DMRI sessions was 15.5 over an acquisition time of 80 s
Summary
In high-precision radiotherapy (RT) of lung tumors, target motion due to respiration remains a predominant challenge.[1]. The clinical benefit of many of these methods remains to be proven.[3,4] For these reasons, passive motion-management (PMM) techniques are still primarily used clinically, in particular for conventionally fractionated RT.[1,10] The use of internal target volumes (ITV) as a motionencompassing method is described in Report 83 of the International Commission on Radiation Units and Measurements (ICRU).[11] Assessment of the range of motion by four-dimensional computed tomography (4D-CT) imaging has become the clinical standard-of-care.[4,12] The ITV is ideally obtained from the union of all gross tumor volumes (GTVs) delineated on the datasets at the different breathing phases.[7] It ideally includes all possible positions of the GTV throughout the course of treatment.[13] The ITV is expanded by margins to account for interfractional changes and patient setup uncertainties to create the planning target volume (PTV). The resulting PTVs are typically smaller than corresponding ITVs.[4,16] the midV concept has the potential to reduce the integral dose to the lungs, its clinical implementation is typically limited to academic RT centers.[17]
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