Why is treatment plan adaptation needed?
During a long course of radiation therapy, the patient's anatomy does not remain static. The dimensions of the tumor itself change, as well as the topography of adjacent structures. This is especially pronounced in pelvic organs.
With the standard approach, physicians have to apply large margins around the clinical target volume (Clinical Target Volume, CTV), forming a wide planning target volume (Planning Target Volume, PTV). Margins can reach up to 20 mm. This ensures that the tumor receives the required dose even with displacement, but inevitably leads to the irradiation of a large volume of healthy tissue.
A striking example is organs with high variability, such as the urinary bladder and the cervix. The volume of the bladder can change day by day despite strict preparation protocols and hydration regimens. Furthermore, during cervical cancer treatment, the degree of filling of the bladder and rectum can displace the uterus by 3–4 cm, drawing loops of the small intestine into the irradiation zone. Adaptive therapy solves this problem through daily image guidance (image guidance).
Main strategies of adaptive therapy
To tailor radiation to the current anatomy, cone-beam CT (Cone Beam CT, CBCT) is used. It allows soft tissues to be visualized immediately before the fraction. There are two main operational algorithms:
- "Plan of the day". During the markup stage, physicians create a whole library of plans for different organ states. For example, in cervical cancer, three scans are performed: with an empty, half-full, and full bladder, prescribing individual volumes for each case. Immediately before the session, a scan is taken, and the system selects the plan from the library that best matches the current picture.
- Composite volume. This strategy involves collecting daily cone-beam CT data during the first week of treatment. Based on these, the average value of the clinical target volume is calculated. A small margin (usually up to 10 mm) is added to this averaged value. The resulting volume is used for all remaining sessions, which significantly reduces the radiation load on organs at risk.
Target tracking and respiratory gating
Thoracic and abdominal organs are constantly in motion due to respiration. To accurately deliver a dose to peripheral lung tumors and other mobile targets, specialized monitoring methods are applied. These include breath-hold techniques, mechanical diaphragmatic shinting, and respiratory gating (respiratory gating). With gating, the device automatically turns off the beam if the displacement of the chest or tumor exceeds the allowable threshold.
For complex targets in stereotactic body radiation therapy (SBRT), real-time tracking systems are used. Robotic units, such as CyberKnife, use dual control:
- X-ray: built-in X-ray tubes periodically image the position of fiducial markers (fiducial markers) implanted directly into the tumor.
- Optical: special cameras continuously track the movement of the patient's skin surface during breathing.
The software combines this data, calculating a mapping function (mapping function). Thanks to this, the robotic arm dynamically adjusts the beam direction, literally chasing the tumor.
Integration of MRI and biological information
The newest milestone in the development of guidance is MRI-IGRT technology. It combines a linear accelerator (linear accelerator) and a magnetic resonance tomographer in a single unit. The main advantage is the supreme soft-tissue visualization quality of the pelvis, which is unattainable with conventional CT.
Integration comes with challenges:
- Engineering aspect: the magnetic field interferes with the accelerator's electronics, so it is placed outside the Faraday cage.
- Physical aspect: the magnetic field does not affect photons, but deflects secondary electrons. Due to the Lorentz force at tissue-air interfaces, the dose can increase by 30%. Intensity-modulated techniques (IMRT) are used to compensate for this effect.
In addition to anatomy, modern systems allow biological conformity to be accounted for. It is known that tumor microenvironment areas with oxygen deficiency (hypoxia) exhibit marked radioresistance. By integrating functional imaging data (e.g., PET with 18F-misonidazole or BOLD-MRI), physicians can selectively increase the dose in resistant areas and decrease it in healthy tissues.
Briefly: answers to topical questions
What is the difference between the clinical target volume (CTV) and the planning target volume (PTV)?
The difference between the clinical target volume (Clinical Target Volume, CTV) and the planning target volume (Planning Target Volume, PTV) lies in the factors considered for expanding the radiation boundaries.
| Target volume | Definition and considered factors |
|---|---|
| CTV | Formed by expanding the macroscopic tumor volume to account for potential microscopic spread of tumor cells into surrounding tissues. |
| PTV | Created by further expanding the CTV with the addition of a margin to account for patient setup errors and tumor motion during treatment. |
Which radiopharmaceuticals are used for PET diagnosis of tumor hypoxia?
For PET diagnosis of tumor hypoxia, 18F-misonidazole (18F-misonidazole) is used, which allows visualization of hypoxic areas of the tumor.
Which pelvic structures act as organs at risk in radiation therapy for cervical cancer?
During radiation therapy for cervical cancer, the following adjacent pelvic structures act as organs at risk:
- Bladder (organ at risk) — its filling degree affects uterine displacement.
- Rectum (organ at risk) — its volume also alters the position of the uterus during treatment.
In addition, target displacement heavily impacts the volume of the small intestine that may enter the irradiation field.
Class and exam questions
Why is a single treatment plan insufficient when irradiating the bladder?
The volume of the bladder changes constantly, even if the patient strictly follows the hydration regimen. Because of this, surrounding structures are displaced, and a static plan will lead either to under-irradiation of the tumor or over-irradiation of healthy tissues.
How is the issue of the MRI magnetic field's effect on the linear accelerator resolved?
The accelerator is placed outside the Faraday cage to isolate complex electronics. Meanwhile, the non-uniform dose distribution caused by secondary electron deflection is compensated for by intensity-modulated radiation therapy (IMRT) algorithms.
Why are fiducial markers implanted into the tumor?
Fiducial markers serve as contrast internal landmarks. X-ray systems read their position during the session, allowing the robotic unit to track actual target displacements and direct the beam precisely to the target.
What else to study on this topic
- Mathematical modeling of biological conformity
- Physical principles of the electron return effect (Lorentz force) during MR guidance
- Software algorithms for calculating composite volume from cone-beam CT scans
- Specific features of using BOLD-MRI to detect tumor hypoxia
- Operation technique of optical body surface tracking systems in SBRT
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