6. Treatment Planning

Treatment planning is a process to design radiation beams that yield the optimum balance between high dose conformation to the target tumor and sparing of normal tissue, and to evaluate the resultant RBE-weighted doses to the patient. Generally, treatment planning for ion-beam therapy is conceptually the same as that for proton-beam therapy with the major difference arising from the variability of the relative biological effectiveness (RBE). In protonbeam therapy, the RBE is recommended to be a fixed constant. While this is an approximation, which may be clinically useful for proton-beam therapy, it is not considered a practical approach today by most ionbeam therapy centers. The RBE in a field of ions varies as a function of energy, penetration depth, absorbed dose per fraction, tissue type, clinical endpoint and other quantities. In principle, all these dependencies should be considered. Unlike for intensity modulated radiation therapy (IMRT) with photons, a greater dependency of the delivered dose on the delivery parameters of dedicated proton and ion beam machines exists. Consequently, it is a standard in proton and carbon ion-beam therapy, that a direct optimization of the delivery parameters is performed with respect to RBE-weighted dose. In IMRT, the absorbed dose distribution is optimized first and in a second step, the delivery parameters are tailored to this absorbed dose. As a result, a list of all available beam delivery parameters is used by the treatment planning system when optimizing the absorbed dose distribution. The ion-beam-delivery systems to be discussed here are the 3D beam scanning system (active system) and the conventional technique using beam shaping elements like range modulators, range shifters, compensators, scattering systems (or wobblers), and collimators (passive system). A derivative beam-delivery method that dynamically uses a multi-leaf collimator in combination with a range shifter (layer-stacking method) will be briefly discussed. The ion beam delivery parameters to be optimized by the treatment planning system (TPS) for a scanning-beam delivery are, typically, the beam energy for each scan, the scan spot positions, and the spot size. The selected beam intensity level may then be adjusted in an intermediate step by the treatment control system, according to the capabilities of the machine and monitoring system.