Role of irradiation in the metastatic process
In modern oncology, patient management tactics depend on the stage of the disease. While surgery and radiation are radical for localized tumors, in metastatic disease (metastatic disease) the goals change.
The main task of radiation therapy at this stage is palliative treatment. It aims to reduce the severity of clinical symptoms (e.g., bone pain) and prevent severe complications. Unlike pharmacotherapy, which systemically reduces tumor burden, radiation works locally.
For palliative purposes, traditional external beam radiotherapy (conventional external beam radiotherapy) is frequently used. With this approach, the irradiation volume is defined by simple geometric parameters (forming a "box" volume). Precise tumor contouring is not performed here, as long-term toxicity to normal tissues takes a back seat in the palliative setting.
Types of external beam radiotherapy
External beam radiotherapy (external beam radiotherapy) is the basic method in which the radiation source is located at a distance from the patient's body. Depending on physical properties, electromagnetic (X-rays and gamma rays) and corpuscular (electrons, protons) radiation are distinguished.
The depth of beam penetration depends directly on its energy:
- Low-energy rays (60–300 keV) are used for superficial lesions, such as skin metastases.
- Megavoltage rays and electrons (6–18 MeV) are used in most procedures to target deeply seated lesions. They are generated using linear accelerators (linear accelerators).
A modern alternative to the traditional approach is 3D conformal radiotherapy (3D conformal radiotherapy). It uses computed tomography data to create a volumetric model of the target (target volume). Using multileaf collimators, the beam profile is adapted to the tumor shape, allowing the required dose to be safely delivered while protecting surrounding organs.
Imaging and treatment planning
The effectiveness of radiation therapy critically depends on imaging quality. The transition from 2D techniques to high-precision 3D and 4D technologies has been made possible by integrating diagnostic images into the planning process.
The basis of planning is CT simulation. Specialized software analyzes tissue X-ray density (in Hounsfield units) voxel by voxel. This allows precise calculation of how tissues will attenuate the therapeutic beam.
To improve accuracy, anatomical CT scans are combined with functional imaging data:
- FDG-PET shows the metabolic activity of lesions.
- Dynamic contrast-enhanced MRI serves as a marker of angiogenesis and vascular permeability.
- BOLD-MRI helps assess the volume of hypoxic blood in the tumor.
The final treatment plan looks like a color dose map overlaid on CT slices. The physician visually assesses whether the metastasis will receive a lethal radiation dose while sparing healthy structures.
Stereotactic radiotherapy and ablation
In the presence of a limited number of metastases (oligometastases), the role of focal methods such as stereotactic ablative radiotherapy (SABR) and percutaneous ablation (radiofrequency or cryoablation) increases.
For example, in lung metastases (often from colorectal cancer), ablative techniques are used in elderly patients with comorbidities. For successful treatment, lesions must meet strict criteria:
- Quantity: less than 3 per one half of the chest (hemithorax).
- Size: up to 35 mm in diameter.
- Localization: in well-aerated tissue, at least 2–3 cm away from the structures of the lung root.
The efficacy of such local control exceeds 90%, and the 5-year overall survival reaches 50%, which is comparable to the results of more traumatic surgical resections.
To memorize the selection criteria for pulmonary metastases for ablation, use the "rule of threes": no more than 3 lesions, size up to ~3 cm (35 mm), distance from the lung root of at least 3 cm.
Class and exam questions
What is the difference between radical and palliative radiation therapy in metastases?
Radical therapy aims at complete tumor eradication, whereas palliative therapy is used in widespread metastases to relieve symptoms (e.g., pain) and prevent severe complications.
Why is CT simulation necessary before starting treatment?
It allows for creating an accurate 3D model of the patient's anatomy, contouring the target, and calculating beam attenuation by tissues in order to deliver the maximum dose to the tumor while protecting healthy organs.
What types of radiation are used in external beam therapy?
Electromagnetic (X-rays and gamma rays) and corpuscular (electrons, protons) radiation are used. Their depth of tissue penetration depends on the beam energy.
What else to study on this topic
- Physical principles and application of PET-CT in oncology
- Percutaneous ablation methods: radiofrequency and cryoablation
- Functional MRI: diffusion-weighted imaging and BOLD contrast
- Palliative stenting in inoperable esophageal cancer
- Intensity-modulated radiation therapy (IMRT)
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