125I particle therapy for malignant tumors is a new technology developed in recent years. 125I particle therapy (also known as physical gamma knife) implanted between tumor tissues has been rapidly developed worldwide because of less damage to normal tissues and better curative effect. Radiation particle brachytherapy started in 1901 when PierreCurie invented radium tube, a radioactive particle that could be implanted in the body. However, it was not until the late 1980s, with the successful research of Treatment Planning System (TPS), that 125I particle therapy became a hot topic with increasingly widespread applications, and is now used in the treatment of prostate cancer, cervical cancer, osteosarcoma, head and neck tumors, pancreatic cancer, liver cancer and other tumors. Intratumoral implantation of radioactive particles is a newest method to treat tumors. It involves sealing radioactive 125I, which has the effect of killing tumor cells, in a tiny titanium shell called “sealed seed source” or “particle”. Then, according to the three-dimensional shape of the patient’s tumor, a set of special instruments is used to place the radioactive particles directly into the tumor like “mine-laying”. The gamma rays emitted can kill the tumor cells, thus achieving the same effect as scalpel removal, so it is also called “particle knife”. This is a minimally invasive technique supported by imaging technology, which can achieve high dose in the target area of tumor and no dose in the surrounding tissues; it has better efficacy for local progressive tumor that cannot be operated. 1.Principle of 125I particle therapy for tumor; Traditional radiation therapy (external radiation) technology is to irradiate cancerous tissues with high intensity radiation, and the surrounding normal tissues will inevitably be damaged to different degrees. At present, some new techniques such as conformal and emphasis radiotherapy techniques are carried out, which can reduce the amount of radiation to normal tissues to some extent. The implantation of radioactive particles is based on the tumor morphology, and the dose of radioactive particles and the distribution of particles are decided by the precise calculation of three-dimensional stereotactic planning system (TPS), and because the attenuation of radioactive particle intensity is inversely proportional to the square of the distance, the damage to the surrounding normal tissues is smaller and the normal tissues are better protected. 125I particles can continuously release γ-rays at low doses, by directly acting on the DNA of tumor cells, causing their double-strand breaks, in addition to directly ionizing water molecules in the body, generating free radicals, promoting apoptosis of tumor cells, sensitive tumor cells die rapidly, and insensitive quiescent cells once they enter the division phase, rapidly apoptosis under the continuous action of γ-rays, making tumor cells unable to reproduce. The physical properties of 125I particles; 125I particles are made of 4.5mm long, 0.8mm diameter titanium tube, which is equipped with a 3mm long silver rod with 125I adsorption, each particle contains a radiation dose of 0.4~1.0mci (average 0.6mci), with a half-life of 59.4 days, and it takes 20d to release 94% of the radiation dose. Specific table: 3. Indications for 125I particle therapy for tumors; (1) locally advanced tumors that cannot be surgically removed; (2) advanced age, impaired cardiopulmonary function that cannot tolerate surgery or unwilling to operate; (3) local external invasion, tumor residue expected during surgery; (4) cases with poor or failed external irradiation effect, etc.; (5) chemotherapy-resistant patients. 4: Advantages of 125I particle therapy (1) Complementary effect with surgery: the addition of radioactive particle implantation during surgery is the most common way, and the maximum effect is the overall tumor killing; (2) Improve the completeness of tumor cure: when the tumor infiltrates the adjacent important organs, even if the tumor can be removed, it is only a palliative surgery, and the prognosis is very poor. (3) Reduce surgical trauma, shorten operation time and reduce postoperative complications: intraoperative puncture technique of implanting radioactive particles causes minimal trauma to normal tissues, eliminates the need to fully reveal the surgical field and avoids blind extensive fat removal operation, and can reduce surgical trauma and the impact on normal tissues to a greater extent. The postoperative recovery of patients is smooth. In the palliative resection of tumor, the implantation of radioactive particles by puncture technique is simple and does not need to worry about the residual size of tumor, which greatly reduces the degree of accidental injury to normal tissues, reduces the incidence of postoperative complications, and reduces the difficulty and workload of postoperative care for patients. (5) the effect of preserving the function and morphology of the body; (6) the complementary effect with chemotherapy. 5: Why use radioactive 125I particles? Radioactive 125I particles have 4 characteristics: Firstly, it has low radiation energy and does not damage the surrounding healthy tissues. The second is that the irradiation distance is short, only 1.7 cm, which is not harmful to doctors and families. Thirdly, it can continue to kill tumor cells every second for 24 hours without stopping. The fourth is the long duration of particle action, which can be up to 180 days. 6: What are the advantages of “particle knife” compared with traditional external radiotherapy? (1) Although the external radiotherapy equipment is progressing rapidly, the treatment of intrathoracic malignant tumors that move up and down by breathing still has the defect of uneven radiation dose, and the intensity of its radiation source is too large, which causes more obvious complications to the patient’s body; while the particles of “particle knife” are directly planted in the tumor, which is not affected by the activity and rarely damages the normal tissues. The particles of “particle knife” are directly implanted in the tumor, which is not affected by the activity and rarely damages the normal tissue. (2) External radiotherapy cannot avoid the shortcomings of “fractional short-time”. External radiotherapy can only treat cells in one part of the tumor reproduction cycle. After the irradiation, the tumor cells in other phases can still recover their reproduction ability soon. The tumor cells can be stimulated by any stimulation to change from quiescent cells to active cells, and the cell multiplication time is significantly shortened, so they can still grow rapidly in the interval between irradiation sessions, which directly affects the therapeutic effect of external radiotherapy. In contrast, the particles of “Particle Knife” can effectively irradiate tumor cells in the tumor body for up to 180 days, which can kill all tumor cells. 7: How many times does particle implantation need to be done? In principle, only a single implantation is needed for a tumor, not another or repeated implantation, so it is also called permanent radioactive implantation. In clinical practice, 10% of tumor patients are not satisfied with the implantation because the tumor site is obscured by the rib. After 2-6 months of review, a small amount of replanting particles can be considered necessary. 8: How long do the implanted particles work? After the radioactive particles are implanted into the tumor, they start to release gamma rays uninterruptedly to kill the tumor cells continuously, and the effective killing time is 180 days. 9: When is the most obvious effect? The most obvious effect is in the first and second month after radioactive particle implantation. 70% or more of the tumors can shrink by more than half, and about 30% disappear completely. Six months after the operation is the best time to evaluate the particle implantation. 10: What happens when the implanted particles fail? The shell of radioactive 125I particles is made of titanium alloy, which is highly compatible with the human body and can exist permanently in the body without causing rejection reactions.