Treatment plan adaptation for MRI-guided radiotherapy using solely MRI data: a CT-based simulation study.

An integrated MRI-accelerator system provides MRI images before and during irradiation. Our purpose is to investigate the feasibility of treatment plan adaptation using solely MRI data, which lack density information. In this study we used CT data to quantify the tissue density effect. Treatment planning was performed for five prostate cancer patients. We simulated correction of a 3, 5, 7 and 10 mm prostate shift relative to the body contour in the anterior, posterior, superior and inferior directions. We applied the original treatment plan to each corrected prostate shift and recalculated the dose distribution using the same monitor units (MU). We calculated the dose differences with and without density information. The latter mimics geometrically correct MRI data. Physical path lengths, available in MRI data, are used to perform MU rescaling per beam and are shown to be of more importance than tissue densities for treatment plan adaptation in prostate cancer. As the change in the physical path length of the central beam axis is representative of the entire beam, MU rescaling based on central beam axis information works fine. In conclusion, MRI data could be used for treatment plan adaptation in prostate cancer provided that the images are geometrically correct.