Study of Flattening Filter Effects on the Off-Axis Ratio and the Dose Delivered by 6 MV Photon Beam Produced by Varian Clinac 2100 in a Water Phantom



This work presents a study of the effects of a medical linear accelerator flattening filter on the dose delivered. The focus is on the off-axis ratios (OARs) of the photon beam at different depths along beam central axis in a water phantom and for different irradiation field sizes.

The dose profiles of a 6 MV photon beam in a water phantom were measured for a source-to-surface distance of 100 cm; irradiation field sizes of 3 × 3 cm², 6 × 6 cm², 10 × 10 cm², 15 × 15 cm², 20 × 20 cm², and 25 × 25 cm²; and depths along the central beam axis of 1.5 cm, 10 cm, 30 cm. All measurements were performed under the same technical and clinical and by using a Varian Clinac 2100 linear accelerator to produce the photon beam.

The purpose of this study was to investigate the effect of the flattening filter on the delivered dose, firstly as a function of the irradiation field size, and secondly as a function of the depth in the water phantom along the central beam axis.

The results of this study demonstrate that the filter thickness affects the beam energy spectrum and, therefore, the delivered dose. The peaks of the dose profiles reflect the effects of flattening filter on the beam dosimetry, and it was found that the peak amplitude increases as the filter zone thickness decreases at the beam edge; in these peaks, the dose becomes greater than 100% of the dose along the central beam axis.

These peaks were observed to gradually decrease with depth. This effect is due partly to an in-scatter/out-scatter imbalance at the edge of the beam and partly to the fact that the radiation quality becomes progressively softer (the attenuation coefficient increases) away from the central axis, because of the effects of the flattening filter shape.


Flattening filter, Dose profile, X-rays, Dosimetry, Linear accelerator

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ISSN: 2509-2065

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