MR-only Radiotherapy of prostate cancer

Abstract

Radiotherapy is a local approach that involves the use of ionising radiation by exploiting its cell-killing effect to cure cancer. This effect, however, is not specific to damage only cancerous cells and spare healthy cells. Therefore, developments in radiotherapy aimed at reducing treatment uncertainties such that therapeutic radiation dose may be delivered to a malignant tumour while decreasing the dose received by healthy tissues. The recent advances in imaging techniques impacted and radiotherapy strategies enormously by facilitating the personalisation of the treatments and the dose to healthy tissues. To date, computed tomography (CT) is considered the primary modality in radiotherapy, providing a personalised patient model to forecast the radiation attenuation in human beings. Also, magnetic resonance imaging (MRI) is increasingly used in radiotherapy planning, especially for tumour delineation, owing to its superior soft-tissue contrast compared with CT. In Western countries, prostate cancer is one of the most commonly diagnosed cancer in men and radiotherapy is one of the curative treatments for prostate cancer patients. Since the prostate is a gland constituted of soft tissue, the advantage of using MRI for radiotherapy is recognised: it facilitates precision of target delineation. The current pathway for prostate cancer radiotherapy consists of patient simulation both on CT and MRI. Unfortunately, image registration is then required potentially introducing systematic spatial uncertainties and errors during treatment planning given the fact that CT and MR are acquired at a different moment and potentially affected by set-up differences. To remove potential inter-scan differences “MR-only radiotherapy” has been proposed. Also, MR-only radiotherapy offers practical and logistical advantages as simplifying the pathway by reducing the overall treatment cost and workload and lowering patient exposure to ionising radiation. In MR-only radiotherapy, sole MR images are used for delineation, dose planning and as reference images for patient positioning during irradiation. However, MRI does not provide information to forecast the attenuation of radiation in the body, and it may suffer geometrical distortion impacting the accuracy of patient positioning. To overcome such challenges, this thesis investigated strategies to perform MRI-based prostate radiotherapy facilitating future clinical implementation. First, the thesis focused on the evaluation of the dosimetric accuracy obtainable during MR-based planning studies. Embracing the clinical perspective, the accuracy of MR-based dose calculation has been investigated using a commercially available solution for sCT generation called MRCAT (Philips Healthcare). MR-based dose calculation for photon and proton radiotherapy was studied demonstrating its accuracy. Second, it was assessed whether image-guided radiotherapy (IGRT) is feasible when patient setup correction relies solely on MRI as a reference imaging modality. Considering that IGRT for prostate patients is based on gold fiducial marker implanted in the prostate. It has been evaluated the inter-observer localisation of the marker. Also, an automatic method has been proposed leading to a detection rate comparable to a manual observer. Ultimately, the work presented in this thesis contributed to shaping the clinical implementation of the MR-only pathway in Utrecht aiming at ensuring a safe introduction of MR-only radiotherapy for prostate cancer patients.