Methods of radiation therapy

Main types of radiation therapy

Globally, radiation therapy can be divided into two broad categories depending on the location of the radiation source relative to the patient.

1. External beam radiotherapy (External beam radiotherapy). The radiation source is located outside the patient's body. This type includes:
2. Conventional external beam therapy.
3. Three-dimensional conformal radiation therapy (3D-CRT).
4. Intensity-modulated radiation therapy (IMRT).
5. Particle therapy (utilizing protons, electrons, neutrons, carbon ions).

1. Brachytherapy (Brachytherapy). This is a contact method in which a radioactive isotope is placed directly inside the affected organ or on its surface. The main principle is strict dose localization around the source with a sharp drop-off at a distance, which maximally protects surrounding healthy tissue. Insertion can be automated (Afterloader) or manual (e.g., $^{125}$I seeds in prostate cancer).

Planning and imaging

A key step before starting irradiation is the precise determination of the target volume. To ensure the radiation hits the exact target, tumor margins must be contoured based on clinical, histological, and imaging data.

• Gross tumor volume (GTV) — the visible boundaries of the pathology. A safety margin is always added to it to cover potentially hidden malignant cells.
• Image fusion. For supreme precision, deformable image registration (CT, MRI, PET) is used, making it possible to account for both anatomical and physiological features (e.g., diffusion-weighted MRI to assess cellular density).
• Radiomics and radiogenomics. Images are analyzed by algorithms as "big data" to extract quantitative tumor characteristics and correlate them with the genetic profile, helping to personalize treatment.

Modern high-precision methods

Intensity-modulated radiation therapy (IMRT) makes it not only possible to shape the radiation beam as needed, but also to vary the radiation intensity within the beam itself. This allows the beam to "wrap around" critical organs (such as the spinal cord) and deliver a maximum dose to irregularly shaped tumors. The method requires constant visual monitoring, especially for areas subject to cardiorespiratory motion (lungs).

Stereotactic body radiation therapy (SBRT) utilizes multiple beams intersecting in three dimensions, often employing robotic systems (e.g., CyberKnife). The equipment automatically compensates for patient movement. Due to submillimeter precision, the dose is delivered over a very short course — ultrahypofractionation.

Intraarterial treatment methods (radioembolization)

For the treatment of certain tumors (e.g., hepatocellular carcinoma), radioembolization or selective internal radiation therapy (SIRT) is used.

This is a hybrid of vascular embolization and brachytherapy: glass or polymer microspheres containing the radioisotope Yttrium-90 ($^{90}Y$) are injected into the arteries supplying the liver tumor. Yttrium-90 emits beta particles, delivering a colossal dose of radiation locally to the tumor while sparing the liver parenchyma.

Class and exam questions

What else to study on this topic

• Stages of radiation therapy: from simulator to treatment verification
• IMRT delivery technologies: tomotherapy and volumetric modulated arc therapy
• Features of biologically optimized radiation therapy using prostate cancer as an example
• Algorithm for HCC treatment selection according to the Barcelona Clinic Liver Cancer staging system
• Technique and complications of transarterial chemoembolization (TACE)

Other topics in the "Diagnostic Radiology in Oncology" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.