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KMID : 1164220060180010021
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Journal of Korean Society for Radiotherapeutic Technology
2006 Volume.18 No. 1 p.21 ~ p.28
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The Variation of Surface Dose by Beam Spoiler in 10 MV Photon Beam from Linear Accelerator
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Bae Seong-Cheol
Lee Chul-Soo
Kim Jun-Ho
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Abstract
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Purpose: The purpose of this study is to find a optimal beam spoiler condition on the dose distribution near the surface, when treating a squamous cell carcinoma of the head and neck and a lymphatic region with 10 MV photon beam. The use of a optimal spoiler allows elivering high dose to a superficial tumor volume, while maintaining the skin-sparing effect in the area between the surface to the depth of 0.4 cm.
Materials and Methods: The lucite beam spoiler, which were a tissue equivalent, were made and placed between the surface and the photon collimators of linear accelerator. The surface-dose, the dose at the depth of 0.4 cm, and the maximum dose at the dmax were measured with a parallel-plate ionization chamber for field sizes using lucite spoilers with different thicknesses at varying skin-to-spoiler separation (SSS). In the same condition, the dose was measured with bolus and compared with beam spoiler.
Results: The spoiler increased the surface and build-up dose and shifted the depth of maximum dose toward the surface. With a 10 MV x-ray beam and a optimal beam spoiler when treating a patient, a similer build-up dose with a 6 MV photon beam could be achieved, while maintaining a certain amount of skin spring. But it was provided higher surface dose under SSS of less than 5 cm, the spoiler thickness of more than 1.8 cm or more, and larger field size than provided higher surface dose like bolus and obliterated the spin-sparing effect. the effects of the beam spoiler on beam profile was reduced with increasing depths.
Conclusion: The lucite spoiler allowed using of a 10 MV photon beam for the radiation treatment of head and neck caner by yielding secondary scattered electron on the surface. The dose at superficial depth was increased and the depth of maximum dose was moved to near the skin surface. Spoiling the 10 MV x-ray beam resulted in treatment plans that maintained dose homogeneity without the consequence of increased skin reaction or treat volume underdose for regions near the skin surface. In this, the optimal spoiler thickeness of 1.2 cm and 1.8 cm were found at SSS of 7 cm for field. The surface doses were measured 60% and 64% respectively. In addition, It showed so optimal that 94% and 94% at the depth of 0.4 cm and dmax respectively.
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KEYWORD
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lucite spoiler, spoiler-to-skin separation, head and neck RT, 10 MV X-ray
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