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Optimization of IMRT QA with EBT Gafchromic films
F. Schneider 1), M. Polednik 1), D. Wolff 1), V.
Steil 1), A. Delana2), F. Lohr 1), F. Wenz 1), L.
Menegotti 2) 1) Department of Radiation Oncology,
Mannheim Medical Center, University of
Heidelberg 2) Ospedale S. Chiara, Trento, Fisica
Sanitaria
Introduction I. IMRT QA Due to the increasing
quantity and complexity of IMRT plans, the
requirements for the individual patient plan
Quality assurance (QA) for IMRT rise as well.
Unfortunately, at the same time, conventional
radiographic film loses its importance in a
clinical setting due to widespread implementation
of filmless departments. The possibility to use
radiographic film such as EDR2 (Kodak) will
therefore be reduced in the future. Since most
digital radiography systems are not suitable for
QA due to their low resolution, other solutions
have to be found. The self developing
radiochromic dosimetry film Gafchromic EBT (ISP)
may be a good alternative for IMRT QA. II.
Requirements for a dosimetry film A dosimetry
film, which is used for IMRT QA, has to produce
constant results within one batch and the
correlation of optical density (OD) and dose has
to be unequivocal. It should also be dose rate
independent and the absorption characteristics
should be similar to water. III. Goal of this
verification The Gafchromic EBT films were
evaluated to check if they would satisfy the
requirements to qualify them for dosimetry.

• Material and methods
• I. Implementation
• To check the constancy within one batch, an IMRT
  field, which consists of six rectangular
  segments, was created with a planning system.
  This field covers a dose spread between 1.4Gy and
  8.4Gy. With this calibration field two films per
  batch were irradiated and the resulting
  calibration curves were compared with each other.
• To check the absorption characteristics, one film
  was positioned, with its top edge flush to the
  water surface, in a water phantom. This film was
  irradiated parallel to the radiation direction
  with a 5cm x 5cm field. The consequential Depth
  Dose Curves (DDC) have been compared with linear
  accelerators basis data.
• To decide the dose rate independence two series
  of measurements were done. Each of them contained
  four films which have been irradiated with
  different dose rates (55, 108, 217, 441 MU/min).
  All films were irradiated with a 10cm x 10cm
  field and 300MU in the depth of 4cm (SSD 96) of a
  slab phantom.
• To assess the OD change depending on storage
  conditions and time two films were irradiated
  with a 10cm x 10cm field and 100MU. After the
  exposure one film was stored in an opaque cover
  and one on the table. They were scanned several
  times within one month to compare the OD.
• II. Material
• Gafchromic EBT films (ISP, Wayne, USA)
• Linear accelerator with 6MV photons Elekta
  Synergy
• Planning system PrecisePLAN (Elekta, Crawley,
  GB)
• Water phantom 4322 (PTW, Freiburg, Germany)
• Slab phantom RW3 (PTW, Freiburg, Germany)
• III. Method of the film development and analysis
• The Gafchromic films consists of a
  microcrystalline, radiation sensitive monomer
  that is dispersed in a gelatin matrix and coated
  onto polyester film base. When the active
  monomeric component is exposed to ionizing
  radiation, a polymerization reaction is
  initiated, resulting in the production of a dye
  polymer.
• All films were scanned after 16 hours with a
  Epson Expression 1680 Pro flat bed scanner and
  were analyzed with the evaluation software
  VeriSoft (PTW).

Results I. Constancy The constancy (the
relative deviation between the calibration
curves) within one batch is 0.17 0.13,
between two batches 1.22 0.33. II. Absorption
characteristics The comparison of the film DDC
with the basis data shows a relative
deviation of 1,15 0,76. III. Dose rate
independence Both curves show a maximum
relative dependence (dose rate to OD) of 0,46
0,28. IV. Changing of OD after
irradiation The film changes its OD due to the
developing time and its storage conditions up to
10.
Discussion I. Gafchromic EBT films Based on the
fact that results with Gafchromic EBT films are
constant within one batch and the correlation of
optical density and dose is unequivocal (see
Results, diagram I.), the requirements for a
reliable film dosimetry are fulfilled. The films
show a low dose rate dependence and their
absorption characteristics are similar to water
which is also important for IMRT QA. But exposed
films should be stored in an opaque cover and
should be scanned after the same developing time
(1h), since the OD of a film changes due to the
storage conditions and its developing time. II.
Scanner Choice of scanner and scanning procedure
are, however, essential. When the correct
protocol was not followed closely, the deviation
could reach 30. Among the critical issues are
the time between two scans (should be gt two
minutes), scanning the area of interest in the
middle of the scanning field (10cm x 20cm),
making sure the scanning section is as small as
possible, and the deactivation of the automatic
color correction. Under these circumstances the
maximum deviation could be reduced to 3.
e-mail frank.schneider_at_radonk.ma.uni-heidelberg.d
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