29 Pseudo-CT generation for MRI-only radiotherapy treatment planning: comparison between patch-based, atlas-based and bulk density methods

Abstract

Introduction Several methods have been developed to generate pseudo-CT (pCT) from MRI for dose calculation. The objective of this study was to compare an original non-local mean patch-basatlas-based method (ABM) and a bulk density method (BDM) (Fig.1). Methods Thirty-nine patients received VMAT for prostate cancer. T2-weighted MR images were acquired in addition to the planning CT images. pCT were generated from MRI by three methods: an original non-local mean PBM, ABM [1] (leave-one-out strategy) and BDM (water–air-bone density assignment). The PBM was carried out with feature extraction and approximate nearest neighbor search in a training cohort of 25 patients. The methods were compared in a validation cohort of 14 patients. Imaging endpoints were mean absolute error (MAE) and mean error (ME) of Hounsfield unit (HU) from voxel-wise comparisons between pCT and reference CT. Dosimetric endpoints consisted on the absolute mean DVH differences between the reference CT and the pCTs, for the organs at risk and PTV. 3D gamma index analyses (local, 1%/1 mm) were also performed. Results The PBM shows significantly less difference in MAE and ME compared to the ABM and BDM. The differences between pCT from PBM and CT were 0.7% for prostate PTV V 95 % , 0.5% for rectum V 70 Gy and 0.2% for bladder V 50 Gy . Compared to ABM, PBM showed significantly lower DVH difference: for the prostate PTV (from 70 to 78 Gy), the rectum (from 8.5 to 29 Gy, 40 to 48 Gy and 61 to 73 Gy) and the bladder (from 12 to 78 Gy). All the gamma passrate values were significantly higher with the PBM, than those of ABM and BDM. Conclusions In order to generate pCT from MRI for dose calculation, our non-local mean PBM provides lower imaging and dosimetric uncertainties than ABM and BDM. Download : Download high-res image (495KB) Download : Download full-size image