At present there are around 30 ion beam therapy facilities operating all over the world. With several more being built and becoming operational during the next years, it is essential to focus research on the field of ion beam therapy. There are many challenges on the way to utilize ion beam therapy in its whole potential. One of the challenging areas is organ motion management, therefore advances in IGRT and ART are of particular interest. During fractionated radiotherapy, which usually takes several weeks, a patient is treated according to a treatment plan generated at the beginning of the treatment. However, the actual delivered dose to the patient can be different to the one originally planned, because of many factors (anatomical changes, positioning uncertainties, etc.), which may influence the delivered dose distribution. Adaptation of the treatment during the course of radiotherapy or choosing an appropriate planning approach is essential in order to successfully treat patients with ion beam therapy. However, the implementation of such methods needs further research and continuous development. This thesis is dedicated to adaptive radiotherapy strategies for ion beam therapy, which were assessed with treatment planning studies. It consists of 3 independent studies, each dealing with different areas of particle treatment, i.e. PTV margin definition, combined modality treatments and influence of the anatomical variations on dose distribution. Anatomical variations in the pelvic area can affect the delivered dose distribution in scanned proton beam therapy. The purpose of the first study was to investigate the most robust margin strategy to account for inter-fractional motion in low risk prostate cancer. With the use of one planning CT and 7 weekly repeated CBCTs, the following PTV approaches were investigated: conventional margin approach (PTV10mm), reduced margin recipe (PTVRed) and a patient specific margin (PTVHull). The dosimetric impact of organ motion on targets and OARs for all three approaches was evaluated. Results of the study showed that the patient specific margin is the most robust approach among all three compared, but also proved that repeated imaging is an useful tool to estimate inter-fractional motion and its impact on the dose distribution. The second study was conducted to compare particle treatment modalities (IMPT and IMIT) as boost options with advanced photon beam therapy (VMAT). This study was performed for two indications: locally advanced head-and-neck cancer (H&N) and patients diagnosed with high-risk prostate cancer (PC). For both, using VMAT, fast and effi cient treatments can be delivered. However, combining particles and photons spare OARs more beneficially compared to sole photon treatments. In this study, for H&N cases, not only main OARs (i.e. brainstem, spinal cord, parotid glands) were assessed, but we also evaluated various OAR (e.g. cochlea, middle ear, masticator space) that are currently discussed with respect to side effects after treatment, but usually not taken into consideration while optimizing the plan. Results showed that with advances in dose delivery techniques (especially a combination of VMAT+IMIT), nowadays, we are able to spare many more organs than it was possible with the conventional techniques. Consequently, modern radiotherapy is able to improve cancer treatment and can potentially increase the quality of life after radiation. For high precision dose deliveries such as scanned beam ion therapy, any inhomogeneities along the particles' path may change the outcome of the treatment. In respect to that conventional photon treatment is less sensitive and is expected to have more robust dose delivery. In the last study, a special focus was given to the impact of organ motion on the delivered dose and differences between VMAT and IMPT were studied. Moreover, the deformable registration of repeated CTs and dose accumulation was performed for H&N patients to assess the total dose delivered to the patient in two different scenarios: (1) when new plans were created and the dose was adapted to the current anatomy and (2) when no adaptation was performed. The assessment of the dose changes influenced by anatomical variations showed benefits of adaptive strategies for both modalities. However, those benefits were more apparent for individual patients, whereas for the averaged representation some of the information might have been smeared out. Moreover, the same anatomical variations within a patient resulted in very different dose distributions between IMPT and VMAT. Therefore, for successful implementation of ART into the clinic, among others, the adaptation strategy should be adjusted to needs of the chosen modality. The presented PhD thesis underlined different issues concerning ion beam therapy. Starting from the most robust margin strategy for low risk prostate cancer, advantages of the combined modality treatments for both prostate and head and neck cancer and finally consideration of adaptive RT strategies for particle treatments. Each of the studies highlighted the advantages of ion beam therapy as well as its limitations, but above all they proved that the experience taken from conventional photon therapy cannot be directly translated into the particle field. Regardless of the tumor itself, which remains the same, for ion beam therapy much more factors must be taken into the account, in comparison to photon beam therapy, as they are less significant and could be neglected. This thesis answers only few questions and much more research in this field needs to be conducted.