Towards MRI-guided radiotherapy in early-stage breast cancer patients

At the Department of Radiation Oncology of the UMC Utrecht, an MRI-linear accelerator (MRI-linac) is being developed. This hybrid MRI radiotherapy system features an 8 MV linear accelerator rotating around a 1.5 T cylindrical MRI scanner. This combination enables direct visualization of the radiotherapy target directly during the radiation treatment. Accordingly, the future use of this machine would considerably increase treatment accuracy by decreasing uncertainties concerning target position and shape for various tumor sites. In this thesis, the potential for an MRI-guided radiation treatment in early-stage breast cancer patients was investigated, as breast cancer is the most common malignancy among women worldwide. Firstly, treatment volumes in breast-conserving therapy were evaluated. It was shown that neither surgical excision volumes nor irradiated tumor bed volumes did relate to the tumor diameter. Before further investigating the potential for MRI-guided breast radiotherapy, the effects of the presence of a magnetic field on the skin dose were determined in a treatment planning study. It was shown that for standard whole breast radiotherapy the skin dose would significantly increase in the presence of a magnetic field. This was not the case for partial breast irradiation, which was therefore further studied in the subsequent chapters. Consequently, it was investigated whether MRI-guided target definition for partial breast irradiation could be improved by the use of MRI. It was shown that the use of MRI in addition to CT did not improve the low consistency of the standard postoperative CT-guided tumor bed definition among observers. Moreover, it was shown that the use of postoperative MRI as a single imaging modality could significantly decrease consistency among observers when implemented in its current format, and accordingly, reduce treatment precision. Preoperative MRI-guided target definition showed to be highly consistent among observers and preoperative treatment volumes were shown to be small and stable. In a subsequent planning study, MRI-guided single fraction radiotherapy with an integrated ablative boost to the breast tumor appeared to be dosimetrically feasible using VMAT. The results of this thesis offer possible future avenues for studying MRI-guided breast radiotherapy in a future MRI-linac setting. This could be a first step towards a more accessible and patient-friendly treatment for selected early-stage breast cancer patients.