Basic planning algorithm
The radiation therapy process is strictly regulated and consists of three inseparable stages:
- Target volume definition (Target volume definition).
- Dosimetry (Dosimetry) — calculation of complex dose distribution.
- Dose delivery and verification (Delivery and verification).
CT is the basis of the entire process. Tomography visualizes bone and soft-tissue structures, but most importantly, it provides tissue density data in the form of Hounsfield units. This is necessary to calculate X-ray absorption coefficients. Based on CT series, the system generates digitally reconstructed radiographs (DRR) that simulate the projection of the therapeutic beam onto the patient's anatomy.
Definition of Irradiation Volumes
Target contouring is based on the concept of expanding volumes (using lung cancer as an example):
- GTV (Gross Tumour Volume) — true tumor margins visible on scans.
- CTV (Clinical Target Volume) — includes GTV plus microscopic spread of cancer cells into adjacent tissues.
- PTV (Planning Target Volume) — the final contour. This is CTV plus a margin for setup errors and physiological organ motion during the session. It is precisely this volume that must be completely encompassed by the required isodose.
Localization methods
Depending on the treatment goals, two main approaches are used:
- Virtual simulation (Virtual simulation). Used for simple palliative irradiation with one or two opposed fields. The tumor is contoured in 3D, and fields are modeled right on the screen. Pros: speed, good visualization, ability to perform image fusion.
- CT conformal planning (CT conformal planning). Data is imported into the planning system, where algorithms calculate complex dose distributions from multiple beams. The program's goal is to strike a balance: deliver a homogeneous dose to the tumor and minimize radiation exposure to organs at risk (OAR). For this, tissue compensators are often used, and beam energy, size, and shape are varied.
Image Fusion
CT alone does not always perfectly differentiate tissues. To maximize the value of contouring, CT is fused with other modalities:
- PET/CT: In bronchial tumors, it helps distinguish active malignant tissue from a collapsed lung. In lymphomas, it determines the volume of active nodes.
- MRI: Provides unsurpassed soft-tissue detail. In prostate cancer, it finds dominant foci for dose escalation (boost), and in CNS tumors, it provides clear lesion boundaries.
- In head and neck tumors, PET searches for small lymph node metastases, and MRI saves the situation when dental implants cause severe artifacts on CT.
Main challenges of fusion: differences in resolution, choosing between rigid and deformable fusion, spatial distortions (typical for MRI), and anatomical changes between different scans.
Dose delivery and organ motion
Patient positioning on the linear accelerator is performed using reference skin tattoos or fixing plastic masks. Machine settings include beam energy, shape modification (via a multileaf collimator), dose rate, and gantry tilt angles.
The main challenge of delivery is patient and organ mobility during the 6–8 week course. Couch mechanical errors cause an error of a couple of millimeters, whereas internal organs can shift radically.
Two types of motion are distinguished:
- Intrafraction (during a single session). Can be predictable (respiratory lung movements) and random (peristalsis and gas movement in the intestine).
- Interfraction (difference between daily sessions).
Since a CT plan is only a static "snapshot" of anatomy, accounting for motion is critically important. To achieve this, Image-Guided Radiation Therapy — IGRT (Image-Guided Radiation Therapy) — is used.
Briefly: answers to topical questions
What methods of respiratory motion compensation exist during lung cancer irradiation?
To compensate for respiratory motion during lung tumor irradiation, motion minimization, synchronization, and tracking methods are used.
- Breath-holding techniques (breath-holding techniques) — used to reduce target displacement during treatment.
- Mechanical immobilization methods — include mechanical diaphragmatic shinting.
- Respiratory gating (Respiratory gating) — automatic shut-off of the therapeutic beam when the displacement of the chest wall or the tumor itself exceeds a predetermined allowable threshold.
- Optical control — continuous tracking of the patient's skin surface to detect respiratory movements.
The matryoshka rule for targets: GTV (what we actually see) is nested inside CTV (plus invisible microscopy), and CTV is enclosed in PTV (the largest volume with a margin for technical shifts and respiration).
Class and exam questions
How do metal implants affect the radiation plan?
They block the passage of radiation. For example, in bladder cancer, the presence of bilateral hip prostheses precludes the use of lateral fields. Irradiation must be delivered solely from anterior and posterior directions, which creates a "pillar" of high dose and prevents the rectum from being spared.
What else to study on this topic
- Virtual simulation vs CT conformal planning
- Principles of Image-Guided Radiation Therapy (IGRT)
- Rigid and deformable image fusion
- Accounting for intrafraction and interfraction displacement
- Organs at risk (OAR) and their radiation tolerance
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