Radiotherapy is the therapeutic use of ionising radiation to damage cancer cells and prevent them from continuing to proliferate. It is primarily a local or regional treatment because radiation is directed towards a defined anatomical target, although its biological effects extend through the irradiated volume. The central mechanism involves damage to DNA, either directly through ionisation or indirectly through reactive chemical species generated by radiation interactions with cellular molecules.
Radiation-induced DNA damage can include single-strand and double-strand breaks, with unrepaired or incorrectly repaired double-strand damage being particularly important for cell death. Cancer cells may be more vulnerable because of impaired DNA-repair capacity, rapid proliferation or other biological characteristics, although normal tissues within the treatment field can also be injured.
Radiotherapy can be used with curative intent, as an adjuvant after surgery, as neoadjuvant treatment in selected cancers, concurrently with systemic treatment, or palliatively to reduce symptoms from painful or threatening lesions. Modern radiotherapy aims to maximise dose to the tumour while limiting exposure to surrounding healthy structures.
External-beam Radiotherapy delivers radiation from a machine outside the body and is the most familiar form of therapeutic radiation. Treatment planning uses imaging to define the tumour and relevant organs at risk, after which a computer-based plan determines beam arrangements, energies and dose distributions. Techniques such as three-dimensional conformal radiotherapy, intensity-modulated radiotherapy and image-guided radiotherapy allow increasing precision in shaping the delivered dose.
Stereotactic techniques deliver highly focused radiation in a small number of sessions for selected tumours or metastases, whereas conventional fractionated treatment divides the total dose into multiple smaller fractions over several sessions. Fractionation exploits differences in the ability of normal and malignant tissues to repair sublethal damage between treatments and allows normal tissues time to recover. The appropriate dose and fractionation depend on tumour type, location, size, proximity to sensitive organs, treatment intent and previous radiation exposure. The goal is not simply to deliver the largest possible dose but to achieve an appropriate therapeutic ratio between tumour control and normal-tissue toxicity.
Brachytherapy is a form of Radiotherapy in which radioactive sources are placed within or close to the tumour. Because the radiation source is physically near the target, high doses can sometimes be delivered to the intended region while limiting exposure to more distant tissues. Brachytherapy has important applications in selected cancers, including some cervical, prostate, breast and skin malignancies, although the exact techniques vary.
Sources can be temporarily inserted and removed or, in certain circumstances, left in place according to the treatment design. The distinction between brachytherapy and external-beam treatment is therefore primarily the location of the radiation source. Systemic radiopharmaceutical therapy represents another approach in which a radioactive substance is administered so that it preferentially reaches cells expressing a particular target or accumulates in a particular tissue. This can provide radiation to disease that is distributed throughout the body. Examples of targeted radionuclide approaches exist for selected endocrine and neuroendocrine cancers and other molecularly defined settings. These therapies demonstrate that radiotherapy can be local, intracavitary, interstitial or systemically targeted rather than confined to an external machine.
Radiation effects depend on total dose, dose per fraction, treatment volume, radiation quality, tissue characteristics and timing. Normal tissues have different tolerances, and rapidly dividing tissues can show acute effects during or shortly after treatment. Depending on the treated region, acute effects may include skin reactions, mucosal inflammation, fatigue, gastrointestinal symptoms or temporary changes in blood counts. Late effects can develop months or years later and may involve fibrosis, vascular injury, endocrine dysfunction, organ-specific damage or other changes.
The probability and severity of toxicity depend strongly on the anatomical structures receiving significant dose. Modern planning therefore identifies organs at risk and applies dose constraints where appropriate. Some normal tissues have a limited capacity for recovery, making previous radiotherapy an important consideration if a region requires retreatment. Radiation can also affect fertility when reproductive organs or relevant hormonal structures receive significant doses. The technical challenge is consequently not merely targeting cancer but designing a dose distribution that reflects the biological sensitivity and functional importance of every tissue within and around the treatment field.
Radiotherapy can be delivered with several different clinical intentions. Definitive Radiotherapy may be the principal curative treatment for cancers that can be controlled locally without surgery or when surgery would cause disproportionate functional damage. Adjuvant radiotherapy can eradicate microscopic residual disease after surgery and reduce local or regional recurrence risk in appropriately selected cancers. Neoadjuvant treatment can shrink or biologically modify a tumour before surgery.
Concurrent chemoradiotherapy uses systemic medicines alongside radiation when evidence indicates that combined treatment improves local control. Palliative radiotherapy can relieve symptoms caused by metastatic or locally advanced disease, including pain, bleeding, obstruction or compression of critical structures. Palliative schedules may use fewer treatment sessions than curative courses because the objective is rapid symptom relief with limited treatment burden. The appropriate approach depends on expected survival, symptom severity, anatomical risk, prior treatment and the person's goals. The same radiation technology can therefore serve very different clinical purposes depending on the disease context.
Radiotherapy requires careful planning and quality assurance because small geometric errors can affect the relationship between the prescribed dose and the tissues actually exposed. Patients may undergo simulation imaging in a reproducible treatment position, sometimes with immobilisation devices to minimise movement. Target volumes are defined according to the visible tumour, known or suspected microscopic extension and appropriate margins for uncertainties such as organ motion.
Image guidance can verify positioning before or during treatment and can sometimes adapt the plan to changes in anatomy. Respiratory motion is relevant for tumours in the chest and upper abdomen, while changes in bladder or bowel filling can affect pelvic anatomy. Adaptive radiotherapy attempts to modify treatment plans when anatomy changes sufficiently to affect the intended dose distribution. These technical developments reflect the principle that precision is multidimensional: accurate imaging, target definition, patient positioning, machine calibration, dose calculation and treatment delivery all contribute to safety. Radiation oncology therefore combines clinical medicine with physics, engineering, imaging and computer-based planning.
The effectiveness of Radiotherapy is influenced by tumour biology and the ability of cells to repair radiation damage. Oxygen can increase the effectiveness of certain types of radiation damage because oxygen stabilises some radiation-induced molecular lesions. Hypoxic tumour regions can therefore be relatively resistant to radiation, and strategies to overcome hypoxia have been investigated. Tumour cell-cycle distribution, DNA-repair pathways and the tumour microenvironment also affect radiosensitivity. Some molecular alterations increase sensitivity to DNA-damaging treatment, while others promote resistance.
Fractionation schedules are designed partly around these biological principles and the differing repair characteristics of normal and malignant tissues. Radiobiology provides mathematical and conceptual frameworks for understanding how dose and fractionation influence tumour control probability and normal-tissue complication probability. These principles explain why simply increasing the total dose is not necessarily the best strategy. Modern radiotherapy research seeks to combine improved physical precision with biological information, potentially allowing treatment intensity to be adapted to tumour characteristics rather than relying solely on anatomical boundaries.
The history of Radiotherapy began after the discovery of X-rays and radioactivity and developed alongside major advances in radiation physics, imaging and oncology. Early treatments were limited by imprecise dose measurement and substantial normal-tissue toxicity, but improved machines, dosimetry and planning progressively increased safety and effectiveness. Linear accelerators became important sources of high-energy external radiation, while computerised planning and advanced imaging enabled increasingly conformal treatment.
Contemporary techniques can sculpt dose around complex targets and spare nearby critical structures more effectively than earlier methods. Nevertheless, precision does not eliminate toxicity because microscopic disease can extend beyond visible tumour and unavoidable dose may reach normal tissues. Radiation can also produce secondary malignancies at low frequency many years after exposure, an important consideration particularly for younger patients with long expected survival. Modern practice therefore weighs immediate tumour-control benefits against both short- and long-term risks. Radiotherapy remains a major cancer treatment because ionising radiation can control many local and regional malignancies, but its success depends on careful biological, anatomical and technical optimisation rather than radiation dose alone.
