Abstract
BackgroundThe evaluation of automatic segmentation algorithms is commonly performed using geometric metrics. An analysis based on dosimetric parameters might be more relevant in clinical practice but is often lacking in the literature. The aim of this study was to investigate the impact of state-of-the-art 3D U-Net-generated organ delineations on dose optimization in radiation therapy (RT) for prostate cancer patients.MethodsA database of 69 computed tomography images with prostate, bladder, and rectum delineations was used for single-label 3D U-Net training with dice similarity coefficient (DSC)-based loss. Volumetric modulated arc therapy (VMAT) plans have been generated for both manual and automatic segmentations with the same optimization settings. These were chosen to give consistent plans when applying perturbations to the manual segmentations. Contours were evaluated in terms of DSC, average and 95% Hausdorff distance (HD). Dose distributions were evaluated with the manual segmentation as reference using dose volume histogram (DVH) parameters and a 3%/3 mm gamma-criterion with 10% dose cut-off. A Pearson correlation coefficient between DSC and dosimetric metrics, i.e. gamma index and DVH parameters, has been calculated.Results3D U-Net-based segmentation achieved a DSC of 0.87 (0.03) for prostate, 0.97 (0.01) for bladder and 0.89 (0.04) for rectum. The mean and 95% HD were below 1.6 (0.4) and below 5 (4) mm, respectively. The DVH parameters, V_{60/65/70,{mathrm{Gy}}} for the bladder and V_{50/65/70,{mathrm{Gy}}} for the rectum, showed agreement between dose distributions within pm , 5% and pm ,2%, respectively. The D_{98/2%} and V_{95%}, for prostate and its 3 mm expansion (surrogate clinical target volume) showed agreement with the reference dose distribution within 2% and 3 Gy with the exception of one case. The average gamma pass-rate was 85%. The comparison between geometric and dosimetric metrics showed no strong statistically significant correlation.ConclusionsThe 3D U-Net developed for this work achieved state-of-the-art geometrical performance. Analysis based on clinically relevant DVH parameters of VMAT plans demonstrated neither excessive dose increase to OARs nor substantial under/over-dosage of the target in all but one case. Yet the gamma analysis indicated several cases with low pass rates. The study highlighted the importance of adding dosimetric analysis to the standard geometric evaluation.
Highlights
The evaluation of automatic segmentation algorithms is commonly performed using geometric met‐ rics
Using plastimatch [21] images and segmentations were converted from the Digital imaging and communications in medicine (DICOM) radiation therapy (RT)-struct format, which is required by treatment planning systems and contouring software, into binary masks that are used during the neural network training
A 3D 3 dimensional U-Net architecture (U-Net) was successfully trained for organ segmentation on computed tomography (CT) images of the male pelvic region
Summary
The evaluation of automatic segmentation algorithms is commonly performed using geometric met‐ rics. Previous studies have shown that manual delineation is time-consuming (in the order of several minutes) and prone to inter- and intra-physician variability [6,7,8]. To address these problems, considerable scientific efforts have been made to develop efficient automatic segmentation tools. Milletari et al [13] proposed a 3D fully convolutional neural network architecture trained end-to-end on magnetic resonance (MR) prostate images, referred to as V-Net, and introduced a novel objective function based on the Dice similarity coefficient (DSC). Sultana et al [17] proposed a two-stage network combining U-Net and generative adversarial network (GAN) architectures [18] for structure localization followed by precise prediction of organ delineation
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