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Tommy Knöös

Sjukhusfysiker

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Photon beam quality variations of a flattening filter free linear accelerator

Författare

  • Dietmar Georg
  • Gabriele Kragl
  • Sacha Af Wetterstedt
  • Patrick McCavana
  • Brendan McClean
  • Tommy Knöös

Summary, in English

Purpose: Recently, there has been an increasing interest in operating conventional linear accelerators without a flattening filter. The aim of this study was to determine beam quality variations as a function of off-axis ray angle for unflattened beams. In addition, a comparison was made with the off-axis energy variation in flattened beams. Methods: Two Elekta Precise linear accelerators were modified in order to enable radiation delivery with and without the flattening filter in the beam line. At the Medical University Vienna (Vienna, Austria), half value layer (HVL) measurements were performed for 6 and 10 MV with an in-house developed device that can be easily mounted on the gantry. At St. Luke's Hospital (Dublin, Ireland), measurements were performed at 6 MV in narrow beam geometry with the gantry tilted around 270 degrees with pinhole collimators, an attenuator, and the chamber positioned on the table. All attenuation measurements were performed with ionization chambers and a buildup cap (2 mm brass) or a PMMA mini phantom (diameter 3 cm, measurement depth 2.5 cm). Results: For flattened 6 and 10 MV photon beams from the Elekta linac the relative HVL(theta) varies by about 11% for an off-axis ray angle theta=10 degrees. These results agree within +/-2% with a previously proposed generic off-axis energy correction. For unflattened beams, the variation was less than 5% in the whole range of off-axis ray angles up to 10 degrees. The difference in relative HVL data was less than 1% for unflattened beams at 6 and 10 MV. Conclusions: Off-axis energy variation is rather small in unflattened beams and less than half the one for flattened beams. Thus, ignoring the effect of off-axis energy variation for dose calculations in unflattened beams can be clinically justified. (C) 2010 American Association of Physicists in Medicine. [DOI: 10.1118/1.3264617]

Avdelning/ar

  • Medicinsk strålningsfysik, Lund

Publiceringsår

2010

Språk

Engelska

Sidor

49-53

Publikation/Tidskrift/Serie

Medical Physics

Volym

37

Issue

1

Dokumenttyp

Artikel i tidskrift

Förlag

American Association of Physicists in Medicine

Ämne

  • Biophysics

Nyckelord

  • half value layer
  • unflattened beams
  • off-axis energy variation

Status

Published

ISBN/ISSN/Övrigt

  • ISSN: 0094-2405