1, the basic principle of gamma knife: Gamma knife treatment relies on gamma rays, gamma rays belong to the penetrating ability of high-energy rays, thick concrete walls or metal plates to block gamma rays. Large doses of gamma rays whole body irradiation, will quickly lead to death. Gamma rays from the gamma knife come from the radioactive element cobalt 60 (Cu60). For safety, cobalt 60 is enclosed in a heavy metal container to form a radioactive source, and then the gamma rays are directed through a small circular hole to form a narrow beam of gamma rays. Many beams of gamma rays are distributed around a sphere and converge on the center of the sphere to produce a focal point, which is the center of the radiation source of the gamma knife. Then the lesion to be treated is placed in the focus of the radiation source center for irradiation, which is the basic principle of the gamma knife.
2, the positioning principle of the gamma knife: how to position the gamma knife, how to fix the lesion in the focal point? Gamma knife positioning is achieved through two sets of positioning coordinate system, one is the coordinate system of the radiation source, this coordinate system is fixed on the radiation source, the center is the focus of the radiation source; second is the cranial positioning frame coordinate system, this coordinate system is fixed on the head, through the tomographic scan (CT/MRI) can determine the coordinate value of the lesion. By converting the coordinate values of the two coordinate systems, it is possible to fix the lesion in the central focus of the radiation source very accurately and perform gamma irradiation.
3, gamma knife treatment plan principle: gamma knife gamma rays focused on the central point, each irradiation shot will form a focal spot similar to a sphere. The size of the focal spot varies, with a diameter of 4mm, 8mm, 14mm, 18mm, 22mm, etc.. However, the shape of the lesion is different. Only by using multiple shots of irradiation, the focal spots with different positions and sizes fuse with each other, so that the scope of radiation irradiation is consistent with the size and shape of the lesion, which is called three-dimensional conformal. In fact, Gamma Knife is a highly three-dimensional conformal treatment, and its degree of conformality is much higher than the three-dimensional conformality in radiotherapy.
To develop a Gamma Knife treatment plan is to transmit the images of the lesion to a computer planning system, simulate the treatment in the computer, and determine the treatment parameters: including the number of shots to be irradiated, the coordinate value of each shot, the focal spot size of each irradiation (collimator size), and the duration of each irradiation (irradiation dose). These treatment parameters are then transmitted to the Gamma Knife control system to perform the treatment. The real treatment process is a process of changing the coordinates, changing the collimator size, and conducting different times of irradiation.
4, the biological principle of Gamma Knife treatment: Gamma Knife can control the lesion without injuring the surrounding normal brain tissue, because of its principle of focused irradiation, in the surrounding brain tissue to withstand a small dose of the situation, large dose irradiation lesion. Gamma knife treatment of different diseases, including various brain tumors (benign and malignant), vascular malformations, trigeminal neuralgia, epilepsy, etc., its biological principles are different.
(1) malignant tumors: malignant tumor cells are very sensitive to radiation, and the cells are quickly necrotic and absorbed after gamma knife irradiation, and the tumor shrinks and disappears in a short time. Such as metastases, lymphomas, germ cell tumors.
(2) benign tumors: benign tumor cells, not very sensitive to radiation irradiation, gamma knife appropriate dose of irradiation, not to kill tumor cells, but to inhibit tumor cells, so that they lose the ability to proliferate. Tumor cells no longer reproduce and the tumor stops increasing in size. As time lengthens, the existing tumor cells will apoptosis, disappear and absorb themselves, and the tumor volume will gradually shrink, but the latent period is long, and the specific length of time is related to the growth rate of the tumor before treatment. For fast growing tumors, the latency period of shrinkage is short; for slow growing tumors, the latency period of shrinkage is long. Most benign tumors, such as meningioma, auditory neuroma, pituitary adenoma, etc., shrinkage occurs after 1-3 years. If the tumor shrinks significantly in 1-2 months after treatment, it needs to be paid high attention and may not be benign tumor. Rather, it is a malignant tumor.
(3) Cerebrovascular malformation: Cerebrovascular malformation is the congenital formation of malformed blood vessels in the brain. Rupture and bleeding of malformed blood vessels is the greatest threat of cerebrovascular malformation, and this threat increases significantly from the age of 40-50, so it should be treated as soon as possible after detection. The main cerebrovascular malformations that can be treated by Gamma Knife are arteriovenous malformations and cavernous hemangiomas. After Gamma Knife irradiation, the cells lining the malformed vessels will proliferate, leading to a reduction in the inner diameter of the vessel and eventual occlusion, thus eliminating the potential threat of rupture and bleeding. Vascular endothelial hyperplasia occurs slowly, mainly appearing six months-2 years after treatment, but the overall occlusion rate can reach 80-95%.
(4) Trigeminal neuralgia: The pathogenesis of trigeminal neuralgia is not well understood. There are secondary trigeminal neuralgia caused by brain tumors, etc., but most of them are primary trigeminal neuralgia for which no cause can be found. The efficiency of gamma knife treatment for trigeminal neuralgia is over 90%, of which 80% can be completely cured. The principle of cure is to cause gradual atrophy of the trigeminal nerve roots, and the pain disappears without affecting the normal function of the trigeminal nerve. The process of nerve atrophy is slow and therefore takes effect slowly, mostly after 1-2 months, but the efficacy is long-lasting and almost lifelong.
(5) epilepsy: epilepsy can also choose head gamma knife treatment, especially secondary epilepsy with obvious foci, such as epilepsy caused by softening foci, hemangiomas, etc. Gamma knife treatment is certainly effective. Gamma knife small doses of local irradiation can change the firing characteristics of nerve cells around the lesion, thus controlling epilepsy.