Prostate cancer is the most commonly diagnosed male cancer in developed countries.
After non-melanoma skin cancer, prostate cancer is the most commonly diagnosed male cancer in developed countries . Radiation therapy is a front-line treatment for over 50% of people diagnosed with cancer. It involves high-energy x-ray beams being sent from multiple directions to deliver radiation (dose) to a tumour to destroy cancer cells. A radiation treatment plan consists of mapping patient’s tumour (cure-related) while limiting the amount of radiation given to nearby healthy tissues (toxicity-related). This treatment plan is delivered in daily ‘fractions’, typically over a period of six weeks. During prostate cancer radiotherapy treatment there is a need to minimize the dosage applied to the bladder and rectum (to reduce post-treatment complications).
Fig. 1: Original axial CT image of the pelvis (left) and original axial MR image (right) from the same patient.
MRI-alone external beam radiation therapy for prostate cancer
Prostate radiation therapy dose planning is currently performed using computed tomography (CT) scan images which contain electron density information needed for patient dose calculations. MRI scans have vastly superior soft-tissue contrast than CT scans and offer the potential to better visualise and more accurately delineate the prostate border (Fig. 1). Prostate borders delineated on MRI scans have been shown to be more consistent between radiation oncologists,  and are smaller than on CT. The use of MRI in treatment planning should result in a reduction in margins added to account for delineation uncertainties and less normal tissues irradiated, reducing treatment toxicity.
Fig. 2: Atlas of the pelvis (prostate, bladder, rectum and bone) built from MRI scans from 39 patients.
In the proposed workflow the prostate and organs are automatically defined on the high contrast MRI scan using computer segmentation (organ delineation) algorithms. These algorithms use an “average” MRI scan that has been created from a set of patient MRI scans and is commonly known as an atlas (Fig. 2). This atlas is deformably “warped” (registered) to the patient’s MRI scan until all the tissues line up with the patient’s MRI scan tissues. As the organ boundaries are known in the MRI atlas, these boundaries are also warped onto the patient’s MRI scan and hence the patient’s organ boundaries are determined. The automatically defined prostate would then be inspected and adjusted if necessary by the radiation oncologist (an example automatic outline of the prostate is shown in Fig. 5).
ig. 3: MRI pelvic atlas with surface models of the main pelvic organs (bones, bladder, rectum and prostate).
Fig. 4: Pseudo-CT pelvic atlas with surface models of the main pelvic organs (bones, bladder, rectum and prostate). This pseudo-CT atlas corresponds the MRI atlas shown in Fig. 2 (allowing the mapping of organs and pseudo-CT Hounsfield Units to new MRI scans).
A benefit of the automatic organ delineation is to minimise inter-observer variation and uncertainty. In order to calculate dose, a pseudo-CT scan would be automatically created from the MRI scan with electron densities mapped to the tissues. This is performed by having a CT electron density atlas that corresponds exactly to the MRI atlas (Fig. 3 and 4). As the deformation for the MRI atlas to the patient’s MRI scan is known, the same deformation will work for the CT density atlas. The result is a pseudo-CT scan with electron densities mapped to the patient’s MRI scan. Using this pseudo-CT atlas, it is also possible to generate digitally reconstructed radiographs from MRI scans . Work to validate these pseudo-CTs for treatment planning is ongoing [6-9].
Fig. 5: Axial, Sagittal and Coronal views of a small field of view patient MRI where the prostate has been automatically defined from an atlas .
This project is a collaboration between the Australian e-Health Research Centre, the Department of Radiation Oncology, Calvary Mater Newcastle Hospital, and the University of Newcastle, and is partially funded by the Cancer Council NSW through Project Grants RG 07-06 and RG 11-05.
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