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Radiat Oncol. 2019 Apr 25;14(1):70. doi: 10.1186/s13014-019-1274-4.

Assessment of portal image resolution improvement using an external aluminum target and polystyrene electron filter.

Author information

1
Department of Radiation Oncology, Dongguk University Gyeongju Hospital, Gyeongju, South Korea.
2
Department of Radiation Oncology, Daegu Fatima Hospital, Daegu, South Korea.
3
Department of Radiation Oncology, Semyung Christianity Hospital, Pohang, South Korea.
4
Department of Radiation Oncology, Keimyung University College of Medicine, Daegu, South Korea. ykoh@dsmc.or.kr.
5
Department of Radiation Oncology, Dongguk University School of Medicine, Gyeongju, South Korea. opencagejhs@gmail.com.
6
Department of Medical Sciences, Radiation Oncology, Graduate School of Ajou University, Suwon, South Korea. opencagejhs@gmail.com.

Abstract

BACKGROUND:

In this study, an external 8 mm thick aluminum target was installed on the upper accessory tray mount of a medical linear accelerator head. The purpose of this study was to determine the effects of the external aluminum target beam (Al-target beam) on the portal image quality by analyzing the spatial and contrast resolutions. In addition, the image resolutions with the Al-target beams were compared with those of conventional 6 megavoltage (MV) images.

METHODS:

The optimized Al-target beam was calculated using Monte Carlo simulations. To validate the simulations, the percentage depth dose and lateral profiles were measured and compared with the modeled dose distributions. A PTW resolution phantom was used for imaging to assess the image resolution. The spatial resolution was quantified by determining the modulation transfer function. The contrast resolution was determined by a fine contrast difference between the 27 measurement areas. The spatial and contrast resolutions were compared with the those of conventional portal images.

RESULTS:

The measured and calculated percentage depth dose of the Al-target beam were consistent within 1.6%. The correspondence of measured and modelled profiles was evaluated by gamma analysis (3%, 3 mm) and all gamma values inside the field were less than one. The critical spatial frequencies (f50) of the images obtained with the Al-target beam and conventional imaging beam were 0.745 lp/mm and 0.451 lp/mm, respectively. The limiting spatial frequencies (f10) for the Al-target beam image and the conventional portal image were 2.39 lp/mm and 1.82 lp/mm, respectively. The Al-target beam resolved the smaller and lower contrast objects better than that of the MV photon beam.

CONCLUSION:

The Al-target beams generated by the simple target installation method provided better spatial and contrast resolutions than those of the conventional 6 MV imaging beam.

KEYWORDS:

Contrast and details; Contrast resolution; Low-Z target; Portal imaging; Spatial resolution

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