The use of probability maps to deal with the uncertainties in prostate cancer delineation

The use of probability maps to deal with the uncertainties in prostate cancer delineation - The increased use of new imaging techniques in radiotherapy raises new questions and challenges about how to use the images for treatment planning. While we know that a patient has cancer, the important issue is the exact location of the tumor and its biological characteristics. Patients receiving radiotherapy will not be operated on. Therefore, the tumor location and characteristics cannot be verified in clinical practice. For that reason we need to develop methods that deal with the uncertainties in the images.

Dynamic contrast-enhanced CT (DCE-CT) is an imaging technique that reflects blood flow and blood vessel leakage. Analysis of DCE-CT images shows that tumor regions within the prostate are typically characterized by increased blood flow and leakage with respect to healthy prostate tissue (Figure 1). However, there is some overlap in blood flow and leakage values of healthy and tumorous prostate tissue. Therefore one can never be 100% sure whether a region in the prostate is tumor or not. We propose a method that converts this intrinsic uncertainty in the imaging data in a delineation of regions that have a high risk of containing tumor and regions with an intermediate or low risk.

In our patient group of 29 patients we found an overlap in blood flow and leakage values. Because of this overlap, a simple threshold approach will fail in discriminating between healthy prostate tissue and prostate cancer. Therefore we apply a method that takes this overlap into account and calculates the probability that a voxel (a volume element of the image) is malignant. We converted the DCE-CT images into probability maps, reflecting the probability that a voxel contains tumor. In this way we were able to identify high-, intermediate- and low-risk regions for prostate cancer in all patients.

About 9,000 patients are diagnosed with prostate cancer in the Netherlands each year and over 2,000 patients die from it. Although radiotherapy is a successful treatment for prostate cancer, a considerable number of patients develop a local recurrence in the 5-10 years after treatment. These recurrences appear to occur at the same location as the original tumor was located. This suggests that a more aggressive treatment is necessary on these locations. Imaging prostate tumors with DCE-CT makes it possible to give this radiation micro-boost to the tumor while still sparing the healthy tissue.

In order to define the target volume for this micro-boost, malignant areas in the prostate must be imaged and delineated. With the method we developed, we were able to translate the uncertainties in the imaging into uncertainties in the radiation treatment planning process. The high-risk area may be interpreted as a target volume for the micro-boost, whereas for low-risk regions the regular dose could be prescribed. The intermediate-risk region in between, could be treated as a sort of margin around the target. This opens the way to focal therapies in which the macroscopic tumor is treated with a higher dose than surrounding microscopic disease.

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