Evaluation of a multi-atlas CT synthesis approach for MRI-only radiotherapy treatment planning

F Guerreiro,N Burgos,A Dunlop,K Wong,I Petkar,C Nutting,K Harrington,S Bhide,K Newbold,D Dearnaley,N.M Desouza,V.A Morgan,J Mcclelland,S Nill,M.J Cardoso,S Ourselin,U Oelfke,A.C Knopf

doi:10.1016/j.ejmp.2017.02.017

Abstract

Background and purposeComputed tomography (CT) imaging is the current gold standard for radiotherapy treatment planning (RTP). The establishment of a magnetic resonance imaging (MRI) only RTP workflow requires the generation of a synthetic CT (sCT) for dose calculation. This study evaluates the feasibility of using a multi-atlas sCT synthesis approach (sCTa) for head and neck and prostate patients. Material and methodsThe multi-atlas method was based on pairs of non-rigidly aligned MR and CT images. The sCTa was obtained by registering the MRI atlases to the patient’s MRI and by fusing the mapped atlases according to morphological similarity to the patient. For comparison, a bulk density assignment approach (sCTbda) was also evaluated. The sCTbda was obtained by assigning density values to MRI tissue classes (air, bone and soft-tissue). After evaluating the synthesis accuracy of the sCTs (mean absolute error), sCT-based delineations were geometrically compared to the CT-based delineations. Clinical plans were re-calculated on both sCTs and a dose-volume histogram and a gamma analysis was performed using the CT dose as ground truth. ResultsResults showed that both sCTs were suitable to perform clinical dose calculations with mean dose differences less than 1% for both the planning target volume and the organs at risk. However, only the sCTa provided an accurate and automatic delineation of bone. ConclusionsCombining MR delineations with our multi-atlas CT synthesis method could enable MRI-only treatment planning and thus improve the dosimetric and geometric accuracy of the treatment, and reduce the number of imaging procedures.

Highlights

Cancer treatment with radiotherapy requires information regarding the patient’s anatomy, such as the organs and tumour’s location and the tissue attenuation properties necessary for dose calculations
X-ray computed tomography (CT) is the current gold standard for radiotherapy treatment planning (RTP) mainly because CT intensity values expressed in Hounsfield units (HU) can be correlated with tissue electron densities
The mean absolute error (MAE) obtained within the external contour for the prostate patients was on average 49.8 ± 4.6 HU, which is lower than the error obtained by Kim et al [20] (74.3 ± 10.9 HU) and is of the same order as the MAE obtained by Siversson et al [25], Dowling et al [19] and Andreasen et al [24] (36.5 ± 4.1 HU, 40.5 ± 8.2 HU and 54 ± 8 HU, respectively), when taking into account the fact that the images used in the present study had a lower resolution

Summary

Introduction

Cancer treatment with radiotherapy requires information regarding the patient’s anatomy, such as the organs and tumour’s location and the tissue attenuation properties necessary for dose calculations. Because of its limited soft-tissue contrast, CT imaging can prevent precise and reliable tumour location, in regions such as the brain, head and neck (H&N) or prostate. To overcome this limitation, magnetic resonance imaging (MRI) is being integrated into the radiotherapy workflow. Magnetic resonance imaging (MRI) is being integrated into the radiotherapy workflow By virtue of their excellent soft-tissue contrast, MR images improve the target volume definition [1,2]. This study evaluates the feasibility of using a multi-atlas sCT synthesis approach (sCTa) for head and neck and prostate patients. Conclusions: Combining MR delineations with our multi-atlas CT synthesis method could enable MRIonly treatment planning and improve the dosimetric and geometric accuracy of the treatment, and reduce the number of imaging procedures

Methods

Results

Discussion

Conclusion