Radiotherapy is one of the principal applications of medical physics in cancer treatment. It uses ionizing radiation to damage the DNA of cancer cells and limit their ability to divide and proliferate. The effectiveness and safety of treatment depend on understanding the physical interactions of radiation with matter, accurately calculating absorbed dose and its distribution within tissues, and selecting an appropriate treatment technique. This article reviews the physical principles of radiotherapy, its major clinical applications, and the role of the medical physicist in treatment planning, dosimetry, equipment calibration, quality assurance, and radiation protection. It also discusses challenges associated with organ motion, anatomical changes, and uncertainty in dose calculation, while highlighting modern techniques designed to improve dose-delivery accuracy and reduce exposure to healthy tissues. The article emphasizes that integrating physical expertise with clinical decision-making and quality assurance programs is essential for safe and effective radiotherapy. Radiotherapy is an important modality in the management of many cancers. It may be used alone or in combination with surgery, chemotherapy, or other systemic treatments, depending on tumor type and stage and the patient's clinical condition. The treatment involves delivering a calculated dose of ionizing radiation to a defined target volume to damage cancer cells or limit their ability to proliferate, while minimizing the dose delivered to surrounding normal tissues . Medical physics plays a central role in translating the physical principles of radiation into measurable and verifiable clinical procedures. Treatment success depends not only on the total prescribed dose but also on its distribution within the body, beam-delivery accuracy, equipment performance, and anatomical changes and tissue properties. Modern radiotherapy therefore relies on collaboration among radiation oncologists, medical physicists, and radiation therapy technologists. Radiotherapy uses ionizing radiation, which has sufficient energy to ionize atoms and molecules. Radiation can cause direct damage to DNA or indirect damage through free radicals generated when radiation interacts with intracellular water molecules. Accumulated damage may inhibit cancer-cell division or cause cell death. However, normal cells may also be affected; consequently, tumor control must be balanced against the protection of healthy tissues . Absorbed dose is a fundamental quantity in radiotherapy. It is defined as the mean energy imparted by radiation per unit mass and is expressed as: Absorbed Dose D = dε / dm where D is the absorbed dose, dε is the mean energy imparted to the material, and dm is the mass of the material. The SI unit of absorbed dose is the gray (Gy), where 1 Gy equals 1 joule per kilogram (1 Gy = 1 J/kg). Medical physicists use dose calculations and dose-distribution analysis to verify target coverage and compliance with dose constraints for organs at risk. The interactions of photons, electrons, and protons with tissues depend on radiation type, energy, and tissue composition. These properties influence beam selection, penetration depth, and dose distribution. Several radiotherapy techniques are available, and the appropriate approach is selected according to tumor location, size, extent, treatment intent, and the patient's clinical condition. First, external beam radiotherapy (EBRT) delivers radiation beams generated outside the patient's body, commonly using a medical linear accelerator (LINAC). Techniques include three-dimensional conformal radiotherapy (3D-CRT), intensity-modulated radiation therapy (IMRT), image-guided radiation therapy (IGRT), and stereotactic body radiation therapy (SBRT). These approaches enable beam shaping, intensity modulation, and improved delivery accuracy . Second, brachytherapy involves placing a sealed radioactive source inside or near the tumor. This can deliver a high dose to the target while reducing the dose to tissues farther from the source. Accurate treatment depends on determining source position, activity, dwell time, and dose distribution . Third, proton therapy takes advantage of the concentration of energy deposition near the end of the proton range, known as the Bragg peak. This characteristic may reduce the dose beyond the target in selected cases. However, the clinical benefit depends on range accuracy, anatomical changes, and the specific treatment indication. Medical physicists contribute throughout the radiotherapy process to ensure accuracy and safety. During treatment planning, computed tomography (CT) data are used, and magnetic resonance imaging (MRI) or positron emission tomography (PET) may be incorporated when appropriate to define the target volume, identify organs at risk, and calculate dose distribution. In radiation dosimetry, ionization chambers and other suitable instruments are used to verify dose, calibrate equipment, and assess calculation systems. Medical physicists also contribute to pretreatment verification to confirm that the planned dose can be delivered accurately. Quality assurance (QA) includes periodic tests of machine-output constancy, geometric accuracy, and the performance of beam-shaping, imaging, and guidance systems. These procedures aim to identify technical deviations before they affect patient treatment. Medical physicists also support radiation protection, shielding assessment, occupational exposure monitoring, and compliance with established safety procedures . Dose distribution within the tumor and surrounding tissues is a critical determinant of treatment quality. An inadequate dose to part of the target volume may reduce tumor control, whereas exceeding dose constraints for healthy organs may increase the risk of complications. Dose–volume histograms (DVHs) describe the relationship between dose and the volume of tissue receiving it. They support assessment of target coverage, comparison of treatment plans, and evaluation of dose constraints for organs at risk. Image-guidance techniques and patient-position verification can reduce errors associated with positioning differences, organ motion, or changes in tumor size during treatment. Treatment-plan review and verification of patient data and machine parameters remain essential before and during treatment. Quality assurance is a systematic program of testing, measurement, and documentation designed to ensure that radiotherapy equipment and planning software operate within acceptable performance limits. It includes acceptance and commissioning tests, calibration, periodic checks, treatment-plan verification, review of measurement results, and documentation and correction of deviations. Radiation protection aims to prevent unnecessary exposure of patients, workers, and the public while maintaining the dose required to achieve the therapeutic objective. It includes appropriate shielding design, verification of radiation-source safety, occupational monitoring, and ongoing safety training. These activities should follow established professional standards and protocols and be implemented collaboratively by the clinical and physics teams . Clinical practice faces several challenges, including internal organ motion, anatomical changes during treatment, uncertainty in dose calculation, and the need to verify agreement between the calculated and delivered dose. These factors are particularly important in highly conformal techniques, where small deviations may affect target coverage or the dose received by healthy tissues. Current developments include adaptive radiotherapy, advanced image guidance, artificial intelligence-assisted organ delineation and plan evaluation, and improved dose-measurement and verification methods. These approaches may enhance planning efficiency and accuracy, but they require scientific and clinical validation and ongoing quality assurance before routine implementation. Radiotherapy is an advanced application of medical physics that integrates radiation physics, dosimetry, medical imaging, and computational methods to deliver an accurate therapeutic dose to a tumor. Medical physicists play a fundamental role in treatment planning, equipment calibration, dose verification, quality assurance, and radiation protection. Improving treatment outcomes depends on integrating technological advances with physical expertise and continuous clinical evaluation. Medical physics is therefore essential to balancing treatment effectiveness, patient safety, and high-quality healthcare.