What Is Brachytherapy?

Brachytherapy is a form of radiation therapy in which a radioactive source is placed directly within or immediately adjacent to a tumor, delivering a concentrated dose of ionizing radiation at close range. The term derives from the Greek "brachy," meaning short distance, reflecting the defining characteristic of the technique. Unlike external beam radiation therapy, which directs beams from outside the body, brachytherapy confines the radiation field to the immediate vicinity of the source, exploiting the rapid dose falloff with distance to spare surrounding healthy tissue. The approach draws on nuclear physics, medical dosimetry, and radiation oncology, and is governed by dosimetry protocols published by the American Association of Physicists in Medicine (AAPM).

Radioactive isotopes used in brachytherapy include iridium-192, iodine-125, palladium-103, and cesium-131, each selected for its photon energy, half-life, and dose rate characteristics to match the clinical requirements of the target site and treatment intent. Treatment planning relies on three-dimensional imaging, most commonly CT or MRI, to delineate the tumor volume and calculate dose distributions before source placement.

Dose Delivery Modes

Brachytherapy is classified by dose rate into low dose rate (LDR) and high dose rate (HDR) modalities. In LDR brachytherapy, small radioactive seeds or wires are implanted and left in place for hours to days, delivering radiation continuously at rates typically below 2 Gy per hour. Prostate cancer treatment with iodine-125 seeds is one of the most common LDR applications, with permanent implants that remain as the isotope decays to inactivity. HDR brachytherapy uses a single high-activity iridium-192 source housed in a remote afterloading machine; the source travels through catheters positioned at the treatment site and dwells at programmed positions for precise intervals, completing treatment in minutes per fraction. The IAEA's brachytherapy program provides guidance on both modalities and supports their adoption in low- and middle-income countries as cost-effective cancer treatment infrastructure.

Radiation Effects on Tissue

The biological effect of brachytherapy depends on the absorbed dose, dose rate, fractionation schedule, and the radiosensitivity of the target tissue. At the cellular level, ionizing radiation damages DNA through direct strand breaks and through the production of reactive oxygen species in surrounding water. Rapidly dividing tumor cells are generally more susceptible to radiation-induced mitotic arrest and apoptosis than quiescent normal tissue, a selectivity that the dose distribution of brachytherapy reinforces by concentrating dose at the tumor. The linear-quadratic model, which parameterizes cellular response through the alpha/beta ratio of each tissue type, guides selection of dose per fraction to maximize tumor cell kill while protecting organs at risk. Radiation effects on surrounding structures, including fibrosis and vascular changes, are tracked as late toxicities in long-term outcomes data reported in journals such as Brachytherapy, the official journal of the American Brachytherapy Society.

Treatment Planning and Dosimetry

Accurate dosimetry is central to brachytherapy safety and efficacy. The AAPM Task Group 43 (TG-43) formalism provides the standard framework for calculating dose around individual seeds and sources, using measured dose-rate constants, radial dose functions, and anisotropy factors for each approved source model. Image-guided adaptive brachytherapy (IGABT), recommended by the Groupe Européen de Curiethérapie (GEC-ESTRO), incorporates repeat imaging during a course of treatment to account for tumor volume changes and to optimize source positions for each fraction. NCBI Bookshelf's StatPearls entry on brachytherapy summarizes the clinical workflow from patient selection through implant and dosimetric verification.

Applications

Brachytherapy has applications across multiple oncology specialties, including:

• Prostate cancer treatment with permanent seed implants or HDR fractions
• Cervical and uterine cancer treatment using intracavitary applicators
• Breast cancer partial irradiation as an accelerated post-surgical option
• Skin cancer and lip cancer surface applicator treatment
• Esophageal and bile duct cancer palliation using intraluminal catheters
• Ocular melanoma treatment with episcleral plaque applicators