Alignment of multiradiation isocenters for megavoltage photon beam

Abstract

The accurate measurement of the linear accelerator (linac) radiation isocenter is critical, especially for stereotactic treatment. Traditional quality assurance (QA) procedure focuses on the measurement of single radiation isocenter, usually of 6 megavoltage (MV) photon beams. Single radiation isocenter is also commonly assumed in treatment planning systems (TPS). Due to different flattening filters and bending magnet and steering parameters, the radiation isocenter of one energy mode can deviate from another if no special effort was devoted. We present the first experience of the multiradiation isocenters alignment on an Elekta linac, as well as its corresponding QA procedure and clinical impact. An 8 mm ball‐bearing (BB) phantom was placed at the 6 MV radiation isocenter using an Elekta isocenter search algorithm, based on portal images. The 3D radiation isocenter shifts of other photon energy modes relative to the 6 MV were determined. Beam profile scanning for different field sizes was used as an independent method to determine the 2D multiradiation isocenters alignment. To quantify the impact of radiation isocenter offset on targeting accuracy, the 10 MV radiation isocenter was manually offset from that for 6 MV by adjusting the bending magnet current. Because our table isocenter was mechanically aligned to the 6 MV radiation isocenter, the deviation of the table isocentric rotation from the "shifted" 10 MV radiation isocenter after bending magnet adjustment was assessed. Winston‐Lutz test was also performed to confirm the overall radiation isocenter positioning accuracy for all photon energies. The portal image method showed the radiation isocenter of the 10 MV flattening filter‐free mode deviated from others before beam parameter adjustment. After the adjustment, the deviation was greatly improved from 0.96 to 0.35 mm relative to the 6 MV radiation isocenter. The same finding was confirmed by the profile‐scanning method. The maximum deviation of the table isocentric rotation from the 10 MV radiation isocenter was observed to linearly increase with the offset between 6 and 10 MV radiation isocenter; 1 mm radiation isocenter offset can translate to almost 2 mm maximum deviation of the table isocentric rotation from the 10 MV radiation isocenter. The alignment of the multiradiation isocenters is particularly important for high‐precision radiotherapy. Our study provides the medical physics community with a quantitative measure of the multiradiation isocenters alignment. A routine QA method should be considered, to examine the radiation isocenters alignment during the linac acceptance.PACS number: 87.55.Qr, 87.56.bd, 87.56.Fc

Development of a digital star‐shot analysis system for comparing radiation and imaging isocenters of proton treatment machine

“…To address this issue, many researchers have attempted to integrate multiple isocenter measurements into one system or procedure. [20][21][22][23] Note that Fuse et al developed a new device which can compute the offset value of the radiation isocenter from the imaging isocenter by using the radiosensitive film. 24 And modified Winston Lutz test could compare radiation isocenter to mechanical isocenter, 25 to imaging isocenter.…”

Technical note: Preprocessing of portal images to improve image quality of VMAT‐CT

“…In clinical quality assurance procedure, static open‐field EPID images of a ball‐bearing phantom at several gantry angles were evaluated to detect the isocenter shift 12 . However, the real VMAT delivery is more complicated because the gantry rotation speed changes dynamically 5,13,14 . We measured the open‐field EPID images in both clockwise and counterclockwise directions at several rotation speeds by changing the dose rates (100, 400, and 700 MU/min), and generated a set of flex maps.…”

“…12 However, the real VMAT delivery is more complicated because the gantry rotation speed changes dynamically. 5,13,14 We measured the open-field EPID images in both clockwise and counterclockwise directions at several rotation speeds by changing the dose rates (100, 400, and 700 MU/min), and generated a set of flex maps. Based on the rotation directions and speed, the flex map for a specific VMAT arc delivery could be interpolated from the set of flex maps we generated,and the acquired EPID image were corrected using the specific flex map.…”

Teste de Winston-Lutz: Uma análise quantitativa

“…Sendo: A medida da acurácia do isocentro no acelerador linear é crítica, e necessária para o controle de qualidade na radioterapia. No estudo 11 publicado em 2015 foi desenvolvido um teste com diferentes faixas de energia, observou um desvio máximo de 1.39 mm, para a faixa de energia de 10 MV.…”