Radiation Physics forms the foundation of many modern medical applications in the diagnosis and treatment of cancer. Radiotherapy relies on the use of ionizing radiation to deliver an effective therapeutic dose to the tumor while minimizing radiation exposure to the surrounding healthy tissues. Advances in this field have significantly improved treatment precision, clinical outcomes, and the reduction of treatment-related side effects. Physical and Biological Mechanisms of Radiation Interaction : Radiation beams, including photons and charged particles, interact with biological tissues through mechanisms that cause damage to the DNA of cancer cells. This occurs either through the direct effect, in which high-energy particles or photons interact directly with DNA and cause chemical bond damage, or through the indirect effect, in which radiation interacts with water molecules within the cell through a process known as radiolysis of water. This process generates free radicals that attack and damage the genetic material. The absorbed radiation dose is measured in Gray (Gy), where 1 Gy represents the absorption of 1 joule of energy per kilogram of matter (1 Gy = 1 J/kg). Radiation Beam Guidance and Modulation Techniques : Radiotherapy has undergone significant advances in beam delivery and modulation techniques to better conform the radiation dose to the three-dimensional shape of the tumor. Intensity-Modulated Radiation Therapy (IMRT) is one of the most important techniques, using a Multileaf Collimator (MLC) to control beam intensity and distribution. This allows the radiation dose to be concentrated on the tumor while reducing exposure to nearby sensitive organs. Volumetric Modulated Arc Therapy (VMAT) uses continuous rotational movement of the linear accelerator around the patient while simultaneously varying the rotation speed, dose rate, and multileaf collimator aperture. This enables a more precise and conformal dose distribution. Image-Guided Radiation Therapy (IGRT), meanwhile, uses imaging techniques such as Cone-Beam Computed Tomography (CBCT) to verify the position of the tumor and patient before treatment and correct any positional changes caused by movement or respiration. Particle Therapy and the Bragg Peak : Charged-particle therapy, including proton and carbon-ion therapy, represents an important development in radiotherapy. These particles have physical properties that allow radiation doses to be delivered at specific depths within the body. This characteristic is known as the Bragg Peak, where most of the particle energy is deposited at a defined depth that can be adjusted to match the location of the tumor, followed by a sharp reduction in dose. This helps minimize the exit dose and protect healthy tissues located beyond the tumor. Computational Modeling and Adaptive Radiotherapy : Computational modeling has become an important component of radiotherapy treatment planning. Monte Carlo Simulation is considered one of the most accurate methods for calculating dose distribution, as it simulates the trajectories of large numbers of particles as they travel through different tissues. This provides high accuracy, particularly in heterogeneous regions such as the interfaces between bone and lung tissue. Adaptive Radiotherapy is another modern approach that aims to modify the treatment plan according to changes occurring in the patient during the course of treatment, such as tumor shrinkage, changes in body weight, or changes in organ position. Artificial intelligence and deep-learning algorithms are contributing to the development of this technique by accelerating tumor and organ delineation, dose recalculation, and treatment-plan optimization. Conclusion : Radiation Physics provides the scientific foundation for the development of radiotherapy. Modern techniques have improved the precision of radiation delivery to tumors while reducing exposure to healthy tissues.With continued advances in particle therapy, computational modeling, and artificial intelligence, radiotherapy is moving toward greater precision and personalization, enhancing treatment effectiveness while reducing treatment-related complications. Zahraa Jawad. Al Mustaqbal University The First University In Iraq