Radiation oncology is essential in the treatment of cancer, with approximately fifty percent of oncology patients undergoing radiotherapy (RT) at some stage of their disease management. Significant progress is being made in the diagnosis, surgical, medical, and radiotherapeutic treatment of cancer. Consequently, an increasing number of cancer patients will live long after undergoing RT. Radiotherapy invariably involves exposure of healthy tissue to ionizing radiation, leading to potential acute and chronic adverse effects and, in some instances, the induction of secondary malignancies. The overall aim of this thesis was to investigate the feasibility of using alternative RT techniques to reduce the calculated risks of radiation-induced toxicity and secondary cancer in the RT of intrathoracic tumors compared to standard RT. Additionally, the thesis aimed to assess the feasibility of cardiac magnetic resonance imaging (CMR) for detecting early radiation-induced cardiac changes in comparison to myocardial perfusion scintigraphy (MPS). Study I: In this comparative dosimetric analysis, we evaluated the feasibility of proton beam therapy (PBT) against intensity-modulated radiation therapy (IMRT) and three-dimensional conformal radiotherapy (3D-CRT) in reducing radiation doses to relevant organs at risk (OARs) in the treatment of thymic tumors. Twelve patients who had previously received 3D-CRT were selected. Retrospective IMRT plans and PBT plans were generated for each patient. The dosimetric comparison demonstrated that PBT significantly reduced the mean doses to numerous OARs, such as the heart, lungs, breasts and esophagus. Utilizing the Lyman-Kutcher-Burman (LKB) model for the calculation of normal tissue complication probability (NTCP), we found that PBT plans could reduce the risks of pneumonitis, esophagitis, and myelopathy when compared to 3DCRT and IMRT. Study II: The goal of the second study was to determine whether myocardial perfusion scintigraphy (MPS) changes in lung cancer patients treated with RT were ...