br Reduction of the patient

Reduction of the patient\'s radiation load The main reason why background radiation appears in the radiation room during proton therapy of the eye is that the majority of the protons are absorbed by the system generating the treatment beam, by the equipment and by the walls of the radiation room. As a rule, a proton beam with a diameter of 2–5mm (‘narrow’) is introduced into the radiation room and passes a number of devices that form the treatment beam [2,3]. The latter should correspond to the diameter of the tumor and can reach 4cm. During this process, over 90% of the protons are absorbed by the elements of the beam-generating system, the equipment and the walls of the room. Up to 10% of these protons cause nuclear reactions with atomic nuclei of the structural materials [4]; secondary particles and radionuclides are produced as a result of the reactions. It should be noted that neutrons pose the greatest risk of all secondary particles, since they have a high biological effectiveness and can cause additional activation of the materials in the radiation room. We have proposed and simulated a version of a ‘broad’ input beam whose diameter should correspond to the diameter of the irradiated region [5]. The computations were carried with the Geant 4.9.6 software package using the Monte Carlo method which allows to simulate how different types of radiation pass through different media. The program was tested for spatial distribution of the absorbed dose and for the formation of neutrons in tissue-equivalent materials. Simulations of proton beam transport were carried out for different diameters of the input proton beam with an Nanaomycin A cost of 60MeV:
In both cases, the same modified Bragg curve was created with the following parameters:
As a result of the simulations, it was established that the efficiency value was ε = 0.06 for the narrow beam and ε = 0.17 for the broad beam with identical characteristics of the absorbed dose in the irradiated region. The remaining part of the proton beam (1–ε) was absorbed by the materials of the radiation room, creating secondary particles and radionuclides as a result of nuclear reactions. The quantitative ratio of such protons η for cases of the narrow and the broad beam is determined by the following expression:
It follows from formula (1) that using a broad beam allows reducing the background radiation in the radiation room by approximately three times, in comparison with the traditional scheme involving a narrow beam. The second aspect allowing to reduce the radiation load on healthy organs under proton irradiation is related to the quality of the radiation therapy, in particular, with minimizing the dose for the organs adjacent to the tumor. Proton therapy in ophthalmic oncology typically involves radiation doses up to 60–70Gy, administered in 5 sessions [6]. In this case, the optic nerve, located directly behind the eye, is a critical structure. According to the Quantitative Analyses of Normal Tissue Effects in the Clinic (QUANTEC), the estimate of the risk of optic neuropathy developing in the optic nerve varies greatly with varying dosage. For example, the risk of complications is 3–7% for a maximum dose of 55–60Gy, reaching up to 20% for a dose above 60Gy [7]. Thus, in order to achieve a clinical effect with a minimal risk of complications, the absorbed dose must be determined as accurately as possible both in the region with a very high dose gradient and at the interface between two anatomical structures. The biological effect of tissue irradiation is known to depend not only on the amount of the absorbed dose, but also on the nature of the irradiation, namely, on the number of ionizations under the action of a particle in the critical volume of the cell or, correspondingly, on the magnitude of the linear energy transfer (LET) of the particle. In view of this, the concept of relative biological effectiveness (RBE) was introduced: