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Effectiveness of Computer Controlled Wedge Filter in Reduction of Penumbra Width in Siemens ONCOR Linear Accelerator

Aim: This work discusses the variation in penumbra in presence of computer controlled wedge field as compare to open field during radiation therapy. Treatment of the target region with a remedial dose is serious for treating several cancer types; to that end, wedge filters are generally used to improve dose uniformity to the target volume. Earlier, wedges considered for this purpose were metallic/physical and were made of high-density materials such as steel or lead. Afterward, nonphysical/computer controlled wedges were introduced; these improved the dose regularity using computer systems instead of physical materials. As wedge systems develop, however, they each still have their advantages and downsides. While using metallic wedges, it is difficult to normalize the generation of secondary radiation resulting from the collision of the radiation beam with the wedge body; conversely, virtual/nonphysical wedges do not create any secondary radiation because there is no physical intervention with the radiations. On the opposite hand, virtual wedges are less suitable for treating moving tumors, like those within the lung, and physical wedges have better dose coverage to the target volume than virtual wedges.

The use of VW (Virtual wedge) is an important segment of radiotherapy the use of wedge increases the uniformity of dose in the target volume. The understanding of penumbra is essential before Treatment Planning System (TPS). Penumbra is the scattering of photon beams at the edges of collimator jaws. The aim of our study is to measure the width of penumbra width in open field (in plane direction) and in virtual wedged-field. The variation in penumbra width by introducing the PW as a function of field size, wedge angle, depth and beam energy was observed and analyzed statistically.

Materials and methods: This experiment was carried out on Siemens ONCOR impression linear accelerator (Linac). The widths of penumbra were measured by using LDA 99 detector. During our work the source to surface distance was kept 100 cm. The square field sizes on which we worked were 10 × 10 cm2, 15 × 15 cm2 and 20 × 20 cm2. Three different depth inside the water phantom Dmax (depth at which the maximum dose is obtained), 10 cm and 20 cm for the virtual wedge angles 15° and 60°, 45°, 30°. The width of penumbra are taken for both photon energies 15 MV and 6 MV, tissue equivalent water phantom IBA blue water phantom inside which all the observations were taken. The width of penumbra in virtual wedged field is subtracted from the penumbra width in open field in in-plane direction. The variation in penumbra width as a function of beam energy, field size, depth and wedge angle were analyzed statistically by using statistical package for social sciences (SPSS).

Results: Analysis of variance (ANOVA) shows that the mean variation in the penumbral width is not significant statistically with the change in depth and beam energy, statistically significant with the change in field size and highly significant with the change in wedge angle.

Conclusion: Our study gives the statistical significance of wedge angle, field size, depth and beam energy on penumbra reduction in presence of virtual wedges. This study is useful in treatment planning in different virtual wedge angles, depth, field size and beam energy. This also describes why VW is better to use in reduction of penumbra width.


 Ilyas N, Farrukh S, Jaffry MA

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