ABPS Radiation Oncology key terms

The average energy a charged particle deposits per unit distance traveled through tissue. High-LET radiation such as alpha particles or carbon ions produces dense ionization tracks, causing more clustered DNA damage that is harder to repair.

The ratio of a reference photon dose to the test-radiation dose producing the same biological effect. It quantifies how much more damaging a given radiation type is compared with standard megavoltage x-rays.

A parameter from the linear-quadratic model describing curvature of the cell-survival curve, numerically equal to the dose at which linear and quadratic cell-killing contributions are equal. Low values indicate strong sensitivity to fraction size, as in prostate cancer or late-responding tissues.

A calculated value that converts a fractionated schedule into an equivalent dose for a chosen alpha/beta ratio, allowing comparison of regimens with different dose per fraction. It is used to compare and design altered fractionation schemes.

A normalized dose expressing any fractionation schedule as the biologically equivalent dose delivered in 2-Gy fractions for a specified alpha/beta. It allows summation of sequential treatment phases and comparison across protocols.

The increased proliferation of surviving tumor clonogens that occurs after radiation begins, shortening the effective doubling time. It is the rationale for avoiding treatment gaps and for accelerated fractionation in rapidly growing tumors such as head and neck squamous cell carcinoma.

The ratio of dose needed to achieve a given effect under hypoxic versus well-oxygenated conditions, typically about 2.5 to 3 for photons. It reflects that oxygen fixation of DNA radicals increases radiation damage.

Regions of a tumor with low oxygen tension caused by poor or disordered vasculature. Hypoxic cells are relatively radioresistant, and reoxygenation during a fractionated course restores sensitivity.

A classic principle stating that cells are most radiosensitive when they are rapidly dividing, undifferentiated, and have a long future proliferative career. It explains why stem-cell compartments and rapidly renewing tissues show early radiation injury.

A syndrome following large whole-body or near-whole-body exposures, with prodromal symptoms then hematopoietic, gastrointestinal, and possibly neurovascular subsyndromes depending on dose. It is a medical emergency requiring early triage and supportive care.

Reduction in blood counts from marrow irradiation, most often affecting neutrophils and platelets first because of their short lifespan. Management includes monitoring counts, growth factor support, and transfusion when clinically indicated.

A phenomenon in which cells not directly traversed by radiation show damage or altered behavior because of signals from irradiated neighbors. It is studied as a contributor to nontargeted effects and second malignancy risk.

The excess risk of a new cancer arising years after radiotherapy, related to dose, volume, age at exposure, and tissue sensitivity. It informs counseling, surveillance, and use of modern conformal techniques in long-lived patients.

The absorbed dose at a given depth expressed as a percentage of the maximum dose on the central axis, measured for a fixed source-to-surface distance. It falls off with depth through attenuation and inverse-square effects and is used in photon beam characterization.

The ratio of dose at a given depth to dose at the depth of maximum dose, both measured at the same source-to-axis distance. It is independent of source-to-surface distance and is used in isocentric dose calculations.

The distance from the radiation source to the machine isocenter, typically 100 cm for modern linear accelerators. Isocentric treatment delivery keeps this distance constant while the gantry rotates.

The superficial zone where megavoltage photon dose rises from the skin surface to maximum because of electron equilibrium and secondary electrons. It spares skin but complicates treatment of superficial targets.

A tissue-equivalent material placed on the skin to build up dose in the superficial layers or to compensate for irregular surface contours. It is used to bring the prescription depth to the surface for skin or scar irradiation.

Treatment using energetic electrons that deposit dose in a limited depth range and spare deep tissues. It is used for superficial lesions such as skin cancers and chest wall irradiation.

A stereotactic radiosurgery device using multiple cobalt-60 sources converging on an intracranial target. It delivers ablative single-fraction or hypofractionated treatment for small brain lesions without an incision.

Delivery of a very high dose in one or a few fractions to a small, precisely localized intracranial or spinal target using rigid immobilization and image guidance. It relies on steep dose gradients to spare adjacent critical structures.

Ablative, highly conformal irradiation of an extracranial target in a small number of fractions, requiring precise immobilization, image guidance, and motion management. It is used for small tumors of lung, liver, prostate, spine, and other sites.

The use of imaging before or during each treatment fraction to verify and correct patient and target position. It enables smaller margins and supports high-dose or hypofractionated delivery near critical structures.

A technique using inverse planning and dynamic or static multileaf collimator fields to create nonuniform beam fluence, producing highly conformal dose distributions. It improves target coverage while sparing nearby normal structures.

An IMRT variant in which the gantry rotates around the patient while the dose rate, multileaf collimator positions, and gantry speed vary continuously. It produces conformal plans with shorter delivery times than fixed-field IMRT.

A treatment-planning approach in which desired dose objectives and constraints are specified and an algorithm optimizes beam fluence to meet them. It contrasts with forward planning based on manually adjusted fields.

Techniques such as gating, breath-hold, abdominal compression, or real-time tracking used to account for target movement during imaging and treatment. It reduces the internal margin needed for tumors affected by breathing or peristalsis.

The visible or palpable extent of tumor, determined by imaging and clinical examination. It is the starting volume from which clinical and planning target volumes are derived.

The GTV plus surrounding tissue suspected to contain microscopic tumor spread. It represents the volume that must receive the prescribed dose to achieve tumor control.

The CTV expanded to account for setup error, organ motion, and other geometric uncertainties. The prescription dose is usually specified to the PTV to ensure the CTV receives adequate dose.

Normal tissues whose radiation sensitivity or functional importance may influence treatment planning or prescribed dose. Dose constraints for OARs are used to limit toxicity.

A graphical summary showing the fraction of a structure's volume receiving at least a given dose. It is used to evaluate target coverage and organ-at-risk constraints in a treatment plan.

A modeled estimate of the probability that a specified normal-tissue complication will occur for a given dose distribution. It is used in plan comparison and biologically based optimization.

Radiation therapy delivered by placing radioactive sources inside or adjacent to a tumor. Its steep dose falloff spares distant normal tissue and allows high local dose in a short overall treatment time.

Brachytherapy delivered with a high-activity source at dose rates above roughly 12 Gy per hour, usually by remote afterloading in fractions. It allows outpatient treatment and precise source positioning.

Brachytherapy using permanently or temporarily implanted low-activity sources delivering dose over hours to days at rates typically below 2 Gy per hour. Common examples include prostate seeds and gynecologic tandem-and-ovoid implants.

Verification of the activity or air-kerma strength of a sealed radioactive source against its stated value before clinical use. It is a required quality-assurance step because errors directly alter delivered dose.

Systemic administration of iodine-131, which concentrates in thyroid tissue, to ablate remnant thyroid after surgery or treat iodine-avid disease. It requires radiation-safety counseling and thyroid hormone withdrawal or stimulation.

Targeted delivery of a radionuclide bound to a carrier that accumulates in tumor or specific tissue. Examples include iodine-131 for thyroid disease and radium-223 for bone metastases.

A qualified individual responsible for implementing a facility's radiation protection program, including licensing, monitoring, and safe use of radioactive materials. The role is defined in regulations governing byproduct material.

The principle that radiation exposure to workers, patients, and the public should be kept as low as reasonably achievable, considering economic and social factors. It guides shielding, workflow, and procedural design.

The three basic methods of reducing radiation exposure: minimizing time near a source, maximizing distance, and interposing absorbing material. They apply to occupational protection and to handling of radioactive patients.

Delivery of chemotherapy during a course of radiotherapy so that the two modalities interact to enhance tumor kill. It is standard for several locally advanced cancers but increases acute toxicity.

An agent given with radiation to increase tumor cell killing or overcome radioresistance, such as chemotherapy drugs or oxygen-mimetic compounds. The goal is improved therapeutic ratio.

An inflammatory reaction occurring in a previously irradiated area after administration of a systemic agent such as certain chemotherapies or targeted drugs. It is typically managed by stopping the offending drug and supportive care.

Acute skin injury from radiotherapy ranging from erythema and dry desquamation to moist desquamation and ulceration. Management includes gentle skin care, topical agents, and avoidance of irritants.

Painful inflammation and ulceration of the oral mucosa caused by radiation to the head and neck or by chemotherapy. It impairs nutrition and increases infection risk, requiring oral care and analgesia.

Dry mouth from salivary gland irradiation, causing dental problems, dysphagia, and reduced quality of life. Sparing of parotid and submandibular glands with conformal techniques reduces its severity.

Extradural tumor compressing the spinal cord or cauda equina, causing pain, weakness, sensory loss, and bowel or bladder dysfunction. It is an oncologic emergency requiring urgent steroids and decompression or radiotherapy.

Obstruction of the superior vena cava by tumor or thrombus, producing facial swelling, dyspnea, and distended neck veins. Management includes supportive measures and urgent radiotherapy or stenting depending on histology and acuity.

Radiation given primarily to relieve symptoms such as pain, bleeding, or obstruction rather than to cure. Short courses are often appropriate, with fractionation chosen to balance benefit and treatment burden.

A fractionation schedule that shortens overall treatment time by delivering more than one fraction per day or fewer rest days. It counters tumor repopulation but increases acute normal-tissue effects.

Any departure from conventional 1.8 to 2 Gy once daily, five days per week, including hyperfractionation, accelerated schedules, and hypofractionation. The aim is to improve tumor control or reduce late toxicity.

The process of explaining a proposed radiation treatment, its expected benefits, alternatives, and material risks, and documenting the patient's agreement. It respects autonomy and supports shared decision-making.

A collaborative process in which clinicians and patients exchange information about options, benefits, and preferences to reach a treatment choice. It is central to patient-centered oncology care.

Discussion and documentation of a patient's goals and preferences for future care, including resuscitation status and surrogate decision-makers. It guides treatment decisions when the patient loses capacity.