Speaker
Description
The study of dose leakage is a vital topic because the number of patients who have developed delayed radiation injuries is increasing every year [1].
The linear accelerator Varian Halcyon V1.0 (Varian Medical Systems, Palo Alto, California, USA) [2] without collimation shutters has recently been put into operation and is now gaining popularity in oncology clinics. The Halcyon MLC system features a unique stepped two-layer design consisting of a distal and proximal lobe layer. The primary and secondary collimators are fixed, there is no flattering filter [3-4].
Similar studies were carried out on the Varian TrueBeam STx accelerator, which combines the capabilities of radiation therapy and radiosurgery with millimeter precision. The TrueBeam STx accelerator is equipped with a high resolution multi-leaf collimator (HD120 MLC), which has two blocks of 60 wolfram leaves each [5].
The aim of this study is to experimentally measure dose leakage from the Varian Halcyon multileaf collimator and HD120 MLC. To achieve this goal, the following experiments were carried out on the Varian Halcyon and Varian TrueBeam STx linear accelerator with an energy of boundary photons of 6 MeV:
1. Study of the influence of the field size on dose leakage. IMRT plans were created to irradiate the phantom with fields of various sizes in the Eclipse planning system. The dose profile was measured with an IC Profiler SunNuclear array detector and an SNC125c ionization chamber. A Solid Water GAMMEX water equivalent phantom and a 3D Scanner SunNeclear water phantom were used.
2. Measurement of dose leakage at a small field size. In this experiment, an irradiation plan for the 3D Scanner SunNuclear water phantom was created. The multi-leaf collimator was positioned in such a way as to create a rectangular beam with a size of 1×2 〖cm〗^2. An ionization chamber of the SNC125c type was used.
The experiment showed that despite the fact that the same dose of 200 monitor units was applied in all experiments, the dose leakage in the collimator becomes larger with increasing field size. For the Varian Halcyon collimator at a distance of 16 cm from the central axis, doses of 0.003, 0.012, 0.029, 0.052, 0.092, 0.128 Gy are observed for fields with dimensions of 5x5, 10x10, 15x15, 20x20, 25x25, and 28x28 〖cm〗^2, respectively. For the HD120 MLC collimator at a distance of 16 cm from the central axis, doses of 0.014, 0.33, 0.62, 0.96% of the maximum dose are observed for fields with dimensions of 10x10, 16x16, 20x20 and 24x24 〖cm〗^2, respectively.
The results of the current study are important for understanding how field size affects dose leakage.
Literature
- Takam R, Bezak E, Marcu LG, Yeoh E. Out-of-field neutron and leakage photon exposures and the associated risk of second cancers in high-energy photon radiotherapy: current status. Radiat Res. 2011;176:508–520;
2Varian Medical Systems. Halcyon Brochure. 2017 URL: https://www.varian.com/sites/default/files/resource_attachments/Halcyon_brochure_RAD10443B_092417.pdf. (Accessed: 06/08/2020). - Lim TY, Dragojević I, Hoffman D, Flores-Martinez E, Kim GY. Characterization of the Halcyon™ multi-leaf collimator system. J Appl Clin Med Phys. 2019; 20:106–114;
- NCRP, NCRP Report 170. Second Primary Cancers and Cardiovascular Disease After Radiation Therapy. Bethesda, Md: National Council on Radiation Protection and Measurements; 2011;
5.Varian TrueBeam STx Specifications:
https://www.equiphos.com/wp-content/uploads/2015/06/TrueBeam_STx_Specifications_10093D_0112.pdf
| Section | 8. Nuclear technology and methods in medicine, radioecology |
|---|---|
| The speaker is a student or young scientist | Yes |
