Role of imaging and treatment plan preparation
Planning begins with delineation of the tumor boundaries and critical structures. Since pathological borders on scans do not always coincide with true cellular edges, a safety margin (safety margin) is always added to the macroscopic Gross Tumour Volume, GTV. This forms the final target volume.
For maximum accuracy, deformable fusion of different modalities is applied:
- CT (CT): Serves as the baseline. Special software uses the Hounsfield units of each voxel in the CT scan to predict how tissues will attenuate the therapeutic beam.
- MRI (MRI): Ideal for soft tissue visualization (e.g., in prostate cancer).
- PET (PET) and dynamic CT: Integrate biological information into the plan—oxygenation levels, blood flow, and cell proliferation.
The Radiotherapy Pathway (The Radiotherapy Pathway) is strictly regulated: diagnosis $\rightarrow$ decision and consent $\rightarrow$ imaging (CT simulation) $\rightarrow$ contouring the target volume $\rightarrow$ dosimetric calculation $\rightarrow$ plan verification $\rightarrow$ treatment delivery with continuous monitoring.
External Beam Radiotherapy
This is the foundation of treatment (about 85% of procedures). Radiation is generated at a distance from the patient's body, most often in a linear accelerator.
- Three-Dimensional Conformal Radiation Therapy (3D-CRT): Forms beams based on 3D anatomical reconstruction. A significant drawback is that with a complex (e.g., horseshoe-shaped) target, adjacent organs at risk may still receive a high dose.
- Intensity-Modulated Radiation Therapy (IMRT): Solves the 3D-CRT problem. Beams not only take the desired shape but are also intensity-modulated directly during delivery. This makes it possible to literally "curve around" the spinal cord or brainstem. IMRT is implemented via static fields, Arc therapies, or tomotherapy.
A key tool in both cases is the multi-leaf collimator (Multi-leaf collimator, MLC). It consists of multiple tungsten leaves built into the accelerator head. The leaves move independently, blocking or transmitting radiation to precisely match the target contours.
Stereotactic Radiosurgery and Brachytherapy
When extraordinary precision is required, localized methods are used:
- Stereotactic Body Radiotherapy (SBRT): Uses modified accelerators (e.g., CyberKnife), directing multiple narrow beams from different points in space. The device automatically compensates for the patient's breathing. Ideal for intracranial tumors and small metastases.
- Brachytherapy (Brachytherapy): The source is placed directly into the tumor. The dose drops rapidly with distance from the isotope (inverse square law). Today, an automated afterloading system (Afterloader) is used: empty applicators are first placed under ultrasound or CT guidance, and then the radioactive source (most commonly iridium-192) is remotely driven through the catheters with precise time delays.
Particle Therapy
The use of heavy particles or electrons instead of classical photons provides unique physical advantages:
- Electrons: Have a finite, strictly defined range (about 1/3 of the beam energy). Used for skin tumors, as the dose beyond the target drops to zero.
- Protons: Exhibit the Bragg peak (Bragg peak) phenomenon. The particle passes through tissues with minimal energy deposition and releases all its potential in the last millimeters of its range. Tissues behind the tumor are not irradiated. Indispensable in pediatrics.
- Fast neutrons and carbon ions: Produce extremely dense ionization along their track. This type of therapy is capable of destroying even hypoxic (radiation-resistant) tumor cells.
Briefly: answers to topical questions
What standardized clinical volumes (GTV, CTV, PTV) are identified when contouring the target for radiation therapy?
Three main standardized volumes are distinguished when contouring the target for radiation therapy.
- Gross Tumour Volume (Gross Tumour Volume, GTV) — the visible tumor contour or volume.
- Clinical Target Volume (Clinical Target Volume, CTV) — includes the GTV and an additional margin to account for potential microscopic spread of tumor cells into surrounding tissues.
- Planning Target Volume (Planning Target Volume, PTV) — formed by expanding the CTV to account for patient setup errors, equipment geometric variability, and tumor motion during treatment.
The treatment plan is designed to cover the PTV with the required isodose.
Which radioactive isotopes are used as radiation sources in modern brachytherapy?
In modern brachytherapy, radioactive isotopes of iodine, palladium, and iridium are used as radiation sources.
- Iodine-125 (I-125) — used as implantable seeds for low-dose-rate brachytherapy, with a half-life of 59–60 days.
- Palladium-103 (Pd-103) — used for interstitial radiation therapy, with a half-life of 17 days.
- Iridium-192 (Iridium-192, Ir-192) — the standard isotope for automated afterloading and high-dose-rate temporary implant brachytherapy, with a half-life of about 72 days.
What methods and technologies are used to compensate for patient respiratory motion during stereotactic body radiation therapy (SBRT)?
To compensate for respiratory motion during stereotactic body radiation therapy (Stereotactic Body Radiotherapy, SBRT), motion minimization techniques and real-time tracking systems are used.
- Breath-holding techniques (breath-holding techniques) and mechanical immobilization (diaphragm gating) methods.
- Respiratory gating (Respiratory gating) — automatic shutoff of the therapeutic beam when the displacement threshold is exceeded.
- X-ray control — tracking the position of fiducial markers implanted in the tumor (fiducial markers).
- Optical control — continuous monitoring of the skin surface to detect respiratory motion.
Software combines X-ray and optical control data, calculating a mapping function for beam correction.
PET — Processes (oxygenation), CT — Contours and attenuation, MRI — Soft tissues. Remember this role distribution in deformable image fusion.
Class and exam questions
Why are margins added to the gross tumor volume (GTV)?
There is a natural contouring error, as well as a lack of absolute correlation between CT/MRI boundaries and the true pathomorphological edges of the tumor.
How does a multileaf collimator (MLC) work?
The tungsten leaves built into the linear accelerator move independently of each other, changing the beam shape and modulating its intensity directly during dose delivery.
What is the essence of the Bragg peak phenomenon in proton therapy?
Protons deposit the bulk of their energy at the very end of their physical range. This allows delivering a curative dose deep inside the body while leaving tissues located beyond the tumor completely unaffected.
How is personnel protected during modern brachytherapy?
Manual insertion of radioactive elements is no longer used (except for low-dose iodine-125). Afterloader systems are used: the patient is alone in the bunker, and the radioactive source is fed remotely into pre-placed applicators.
What else to study on this topic
- Arc Therapy and Tomotherapy Technology
- Features of low-dose-rate brachytherapy for prostate cancer using iodine-125 seeds
- Megavoltage imaging and cone-beam CT during treatment
- Impact of tumor hypoxia on radiotherapy efficacy
- Principles of 4D planning to account for cardiorespiratory motion
Master the entire topic in the "Diagnostic Radiology" course
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Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.