What is radiation therapy?
Radiotherapy is the most important treatment for tumors of the central nervous system after surgery. As an independent discipline, radiation therapy (including radio-oncology) is a relatively young subject. In former times radiation therapy or radio-oncology was led under the heading of “radiology“, which summarized the individual disciplines of diagnostic radiology, radiation therapy and nuclear medicine. Through intensive research by physicians, biologists and physicists, an independent discipline has developed over the past few years, which has achieved an optimized overall treatment for brain tumors in close cooperation with the other participating departments, especially neurosurgery and neurology.
The development of modern irradiation devices (linear accelerators) has created the prerequisite for irradiating tumors located in the depth of the body. As a result, neighboring organs and the skin surface are largely spared. An essential prerequisite for carrying out optimized radiation therapy is the introduction of computer-assisted irradiation planning systems, which achieve an individually directed radiation, with the aim of optimizing the healing rates and largely reducing any side effects. The patient is placed in a virtual three-dimensional coordinate system, the rays focus the tumor area from different spatial directions. For this purpose, however, it is important to identify the tumor exactly.
The modern imaging methods are capable of this: the tumor can be exactly defined by normal tissue, so that high-precision irradiation techniques have developed over recent years. In contrast to systemic drug therapy, radiation therapy is a purely local measure, ie it only acts in the area of the irradiation field. This applies both to the desired tumor-destroying effect and to the undesirable side effects.
Today, the medical radiation is generated by ultra-modern “linear accelerators“. This results in a “high-energy X-ray radiation “, which is capable of penetrating into larger body depths. Modern irradiation planning systems can focus this radiation in the desired target area with the aid of the modern imaging methods. In this case, different radiotherapeutic fields are used which are radiated from a wide variety of individually oriented directions.
Mode of action
Radiation is directed to achieve primarily the tumor tissue and to preserve normal, healthy tissue. In general, however, an effect on the whole living tissue is exerted. Every tissue, including tumor and normal healthy tissue, is composed of individual cells. Normal cells as well as tumor cells are subject to a certain cell division, which ultimately leads to tissue multiplication. In normal tissue, the supply by cell division and the death rate through cell aging are subject to a fluid process, which is in an equilibrium of cell formation and death. This balance is disturbed in the tumor tissue. Tumor cells can grow unhindered and ultimately form a tumor.
Radiation is able to hinder this cell division process. Tumor cells can no longer divide and perish. In contrast, normal cells can recover from radiation and are not killed. In the case of radiotherapy of tumors of the central nervous system, tumor tissue is destroyed and normal tissue is spared. This separation effect between tumor cell killing and cell recovery of healthy tissue is particularly well utilized when the irradiation is divided into several small individual doses. Therefore, a radiation treatment is very often distributed over several therapy wells, on each individual treatment day only small amounts of the irradiation are administered.
The radiation biologic effect of the radiation is technically exploited and optimized by:
- Application of modern irradiation techniques and thus preservation of normal tissue (as low dose load as possible)
- An optimal detection of the tumor by applying modern irradiation techniques (maximum dose).
- Irradiation planning and irradiation fields
The selection of therapies is based on the biological properties of the tumors. In essence, two properties are distinguished. Some tumors tend to grow locally locally infiltratively into the surrounding tissue and to re-grow in situ after local removal (local recurrence). Among these tumors are mainly the gliomas, which are distinguished into the low- and high-malignant forms, the ependymomas and the craniopharyngeal.
Other tumors are characterized by the fact that they are metastasized via the cerebral water passages. These tumor forms occur predominantly in childhood. These include primarily the medulloblastomas , the primitively neurectodermal tumors of the cerebral hemisphere (PNET) and the germ cell tumors, but rarely also the malignant variant of the ependymomas . Tumors, which have arisen outside the central nervous system, can produce deposits in the brain tissue ( brain metastases ). Leukemia may also manifest within the central nervous system. In this case, the leukemias show tumor distributions along the inner and outer cerebral water paths within the head.
Essentially, therefore, the target volumes or the resulting irradiation fields are distinguished in 3 concepts.
- Radiotherapy of the enlarged tumor region
- Radiotherapy of the whole head also including the meninges (meninges)
- Radiation treatment of the entire cerebrospinal fluid space (synonymously neuro axis / craniospinal axis).
After selection of therapies or necessary target volumes, the corresponding therapy technique is decided. Radiation treatments of the extended tumor region require computer-assisted radiation planning, which results in individual therapy plans, which in turn consist of different individually aligned and configured therapies. The radiation treatment of the whole brain takes place via two therapeutic fields, which are irradiated laterally and have their geometrical center in the center of the head (counter-fields with common axis of rotation) (isocenter).
In radiotherapy of the entire cerebrospinal fluid space, the entire head is irradiated according to the above-mentioned technique. The spinal cord can be treated with radiation via one or two standing panels, which are irradiated directly from the rear. The therapeutic field size is based on the anatomical conditions, ie here corresponding to the width of the spinal canal. Therapeutic therapies are linked with special techniques in such a way that a uniform dose distribution is achieved within the entire central nervous system. Today, increasingly modern computer-assisted methods are being used to allow accurate field positioning and dose calculation.
Modern irradiation planning systems allow today an optimal adaptation of the Therapiefelder to the target area and a blanking of normal tissue. Diagnostic imaging such as nuclear magnetic resonance tomography and computed tomography , in the future also positron emission tomography (PET), are integrated into the planning systems and permit a reliable identification of tumor and normal tissue. The modern technologies are subject to a constant development with the possibility to modify the intensity of the irradiation within the tumor. As a result, a further, more individual adaptation of the irradiation fields is achieved.