Both methods showed an instance where the ground truth volume grew, yet they reported tumor shrinkage. The kNN method failed to correctly predict tumor growth in Patient 1, from Repeat Scan 1 to 2. Since the kNN volumes are based on multiple trials, it is difficult to assign a specific cause. The KB method failed to predict tumor growth in Patient 2, from the Baseline scan to Repeat Scan 1. According to pathology reports, the Baseline scan contained a significant amount of fluid, possibly hemmorage, which artificially brightened regions surrounding the tumor in the PD scan and made the border between non-tumor and tumor pixels unusually diffuse. This distorted the histogram from which the initial tumor segmentation was based, resulting in significant overestimation of tumor volume. In Repeat Scan 1, however, not only had the fluid disappeared, but pathology reports noted a slight decrease in gadolinium enhancement. Thus, the initial overestimation followed by the decreased gadolinium enhancement caused the trend to appear to be tumor shrinkage instead of growth. Patient 2 had received significant treatment (surgery and radiation therapy) prior to scanning, making the tumor boundaries particularly difficult to detect. In fact, a review of the pathology reports showed that radiologist estimations of the tumor volume had to be revised.
Finally, Figure 13 shows examples of the KB system's correspondence to hand-labeled tumor in slices. Figures 13(a-c) show a worst case segmentation, while (d-f) and (g-i) show an average and best case segmentation respectively. All three examples are from the test set.
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