MRI-based treatment plan simulation and adaptation for ion radiotherapy using a classification-based approach

摘要

Accurate treatment planning and simulation are required to benefit from the highly conformal irradiation of tumors in ion radiotherapy. The purpose of this study was to investigate the potential of MRI for treatment plan simulation and adaptation in ion radiotherapy using a classification-based approach.\udAt first, a tissue classification tool was developed and applied for deriving so-called pseudo CT numbers from MR images. Suitable MR sequences and parameters were evaluated in cross-validation studies of tissue samples and of patients with tumors in the head region. Then, ion radiotherapy treatment plans were optimized by using both MRI-based pseudo CT and reference CT and dose distributions were calculated on the reference CT. Finally, a target volume shift was simulated and employing a hybrid approach, a treatment plan adapted to the shift was optimized on the basis of a so-called corrected CT. The latter consisted of a reference CT, in which a volume around the target was replaced by pseudo CT values.\udThe derivation of pseudo CT values led to mean absolute errors in the range of 21 - 95 HU for tissue samples and 141 - 165 HU for patients. Most significant deviations appeared at transitions between different tissue types and at air cavities as well as compact bones. The former originated from partial volume effects and registration errors whereas the latter resulted from the low signal to noise ratio of bones in MR imaging. Simulations of ion radiotherapy treatment plans using pseudo CT revealed only small underdosages of a target volume compared to reference CT simulations. Deviations of the mean dose of target ranged from 0.8 - 3.1 % for tissue samples and 0.4 - 2.0 % for patients. Plans adapted to the target volume shift and optimized on the basis of a corrected CT exhibited an improved target dose coverage compared to non-adapted plans optimized on the basis of a reference CT. The obtained results for tissue samples and the head region of patients provided first evidence that MRI-based derivation of pseudo CT and treatment plan simulation for ion radiotherapy are feasible.