Medical Radiology - Lazar A.P. 2008

Radiation therapy
Radiation therapy planning

For each neoplasm, the radiation therapist determines the required radiation dose. To visualize the absorbed dose distribution, isodose curves are plotted on topographic diagrams of the irradiated body region. A line connecting points that receive the same amount of radiation energy is called an isodose. A curve connecting points with identical dose values is referred to as an isoline. Several such isolines located at varying distances from the radiation source constitute an isodose chart.

Isodoses from a point radiation source in a medium of uniform density are distributed concentrically at equal distances from the source. In human body Tissues of varying density—especially when using wedge filters and blocks—isodose curves take on different configurations. An isodose chart makes it possible to track the distribution of ionizing radiation within the patient's tissues during Radiation therapy. This is of paramount importance, as it requires knowing not only the absorbed dose in the target lesion, but also the degree of radiation delivered to surrounding healthy Organs and tissues.

Isodose charts indicate relative values as a percentage of the maximum absorbed dose (taken as 100%) rather than absolute absorbed doses ingrays. In clinical practice, dose distribution is planned so that the entire tumor is encompassed within the 100–80% isodose zone, the zone of subclinical tumor spread and regional metastasis falls within the 70–60% isodose, and healthy tissues receive no more than 50–30% of the dose.

In radiation therapy, a distinction is made between single and total tumor doses. The single focal dose (SFD) of ionizing radiation is the dose delivered to the patient's pathological lesion during a single Treatment session. The total focal dose (TFD) is the cumulative dose delivered to the pathological lesion over an entire course of treatment consisting of multiple sessions. The total focal dose for a course of radiation therapy is chosen based on clinical and radiobiological factors. The magnitude of the total focal dose is the sum of the individual absorbed doses and determines the total duration of irradiation in days—the course of radiation therapy.

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Fig. 328. Topographic-dosimetric chart with a coordinate grid for planning external beam gamma therapy for Lung Cancer. I–III — irradiation fields; A — single-field irradiation; B — two-field irradiation; C — three-field irradiation.

For each neoplasm, the radiation therapist prescribes the required total tumor dose, which depends on the histological Structure OF THE tumor. For example, malignant lymphoma requires 45–50 Gy, keratinizing Squamous Cell Carcinoma 60–80 Gy, adenocarcinoma 90–100 Gy, and osteogenic Sarcoma or melanoma 100–120 Gy.

Treatment planning requires a thorough understanding of the Anatomy of the area to be irradiated and the tissue structures within the radiation field. The exact Location OF THE tumor within the patient's body is precisely determined using radiography, Ultrasonography, computed tomography (CT), or Magnetic Resonance imaging (MRI). Clinical topography is then performed. Based on computed tomography scans, anteroposterior and lateral radiographs, as well as stereotactic devices, horizontal (transverse), sagittal, and frontal topographic diagrams of individual human body cross-sections can be constructed (Fig. 327). In most radiological departments, clinicians generally limit themselves to cross-sectional diagrams in the horizontal plane (Fig. 328).

Modern treatment planning systems (e.g., Teraplan) use specially developed software that allows for the automated, highly accurate calculation of the absorbed dose in the tumor for both external beam and intracavitary radiation therapy. Based on CT or MRI scans, the patient's outer contour, the target irradiation area, and the contours of critical organs are outlined, and irradiation fields are defined (Fig. 329). Data describing the treatment unit itself are entered into the system: apparatus specifications, source-to-Skin distance, dose rate, half-life for 60Co, depth of maximum dose, minimum and maximum radiation field size, geometric shape of wedge filters, and attenuation coefficients for wedge filters and blocks. Dosimetric beam characteristics measured in a Water or paraffin phantom are also entered: percentage depth doses for various square fields, scatter factors, beam profiles for multiple fields at various depths, for both static and rotational irradiation (Fig. 330). Based on these data, dose distribution and beam modeling are performed for the radiation therapy machine (gamma therapy unit or linear accelerator).

Fig. 329. Computed tomography scanner in the treatment room of a radiotherapy department featuring a linear accelerator.

For optimal spatial dose distribution, irradiation is performed using static or dynamic (moving) techniques. Static irradiation can be delivered through a single entrance field on the body surface (single-field irradiation) or through 2, 3, 4, or more fields (2-, 3-, 4-, or multi-field irradiation, respectively). The number and size of irradiation fields are selected individually, depending on the size, shape, and localization of the tumor. The choice of the number of fields also depends on the type and energy of radiation, the required single and total doses, and the size of the subclinical spread zone. In moving beam irradiation, the radiation source moves relative to the patient's body. These Methods include rotational, sector (or pendular), tangential, and convergent techniques. The advantage of all moving techniques is the concentration of the absorbed dose within the pathological lesion while reducing the absorbed dose in surrounding tissues and, particularly, the skin.

During radiosurgical treatment, precise targeted irradiation of small target tumors is performed using specialized stereotactic devices. One such device used for treating Brain neoplasms is the Gamma Knife. This unit utilizes precise guiding systems for three-dimensional (3D) radiotherapy using multiple gamma-ray sources. The Gamma Knife system is surpassed in certain respects by the CyberKnife, which generates radiation via a linear accelerator. The CyberKnife does not require source reloading and features an imaging system for treatment planning and delivery without The Need for a stereotactic frame. This unit is capable of delivering fractionated radiotherapy, precisely localizing dose delivery even during patient movement throughout treatment, and performing conformal therapy—tailoring the dose distribution to complex tumor geometries.

Fig. 330. Paraffin phantom for determining depth doses.

1 — radiation source;

2 — collimator;

3 — phantom;

4 — dosimeter.

The primary indications for radiation therapy are malignant diseases of various localizations: cancers of the skin, Lips, Tongue, breast, Larynx, Esophagus, rectum, Lungs, Urinary Bladder, Kidney, cervix and body of the Uterus, and brain, as well as lymphogranulomatosis and leukemia. Radiation therapy is also successfully used for certain non-malignant conditions, including inflammatory and neurological disorders, and degenerative-dystrophic lesions of The Musculoskeletal System.

Absolute and relative contraindications to radiation therapy can be distinguished. Absolute contraindications include: severe General condition of the patient, cachexia (emaciation), acute infectious and septic diseases, anemia, leukopenia, tumor breakdown, and widespread metastases. Relative contraindications include Heart, Liver, and kidney diseases, Diabetes Mellitus, and active Pulmonary Tuberculosis. Due to potential genetic consequences, radiation therapy should be prescribed with extreme caution to children and pregnant women.

In the radiation therapy plan, the radiation oncologist specifies:

1. Clinical Diagnosis, including the pathomorphological report.

2. Method of radiation therapy.

3. Total and single focal doses.

4. Number of fractions and duration of the radiation therapy course.

5. Number, localization, size of fields, and single dose per irradiation field.

6. Technical conditions of irradiation (source-to-skin distance, radiation source and its energy, exposure time, etc.).

7. Concomitant therapy that enhances the efficacy of radiation therapy and mitigates the patient's radiation reaction.