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γmean of 0.29±0.13 with 100% of tests within alert
criteria, and a mean γ% equal to 96.9±5.2% with 96.0% of
tests within alert criteria. In contrast to our past
experience of patients with head-neck and pelvic
treatments, where the systematic use of IVD revealed
some discrepancies due to major anatomical variations or
random anatomical changes in terms of filling of
rectum/bladder, no relevant discrepancies were detected
in SBRT patient. The results are supplied in quasi real-
time, with IVD tests performed and displayed after only 1
minute from the end of arc delivery. Figure 1 shows the
SOFTDISO user interface.
Conclusion
The present EPID-based IVD algorithm provided a fast and
accurate procedure for SBRT-VMAT delivery verification in
clinical routine, with results obtained 1 minute after each
arc delivery. This strategy allows physics and medical staff
to promptly act in case of major deviations of dose
delivery.
EP-1449 The effect of a build-up screen on superficial
dose in total body irradiation
L.S. Fog
1
1
Rigshospitalet, The Clinic of Oncology, Copenhagen,
Denmark
Purpose or Objective
In total body irradiation (TBI), a build-up screen is
typically positioned between the linac and the patient to
reduce the build-up effect in the patient skin. With the
implementation of step and shoot TBI (SS TBI), the dose
conformity is considerably improved compared with TBI
delivered with open fields. Thus, the delivery of an
accurate skin dose becomes pertinent. We measured and
calculated skin dose for a range of TBI conditions.
Material and Methods
The dose was measured in a 20 cm thick solid water
phantom using a NACP parallel plate chamber, and a PTW
Unidos electrometer. The energy response from the
chamber contributed no more than 5% to the
measurement uncertainty (Phys Med Biol. 2001
Aug;46(8):2107-17).
The dose was measured at a depth of 1 mm in the
phantom; for a range of SSDs (340-440 cm); for 6 and
18MV; and for open jaw fields (used in conventional TBI
treatments) and MLC defined fields (used in SS IMRT); and
with and without a lucite build-up screen of 16 mm
thickness, placed 20 cm from the phantom. The MLC fields
were created with a 3 cm distance from the phantom edge
to the field edge when projected to the isocentre. The
chamber was calibrated by measurements under standard
reference conditions.
The doses were calculated using Eclipse™ (Varian Medical
Systems, Palo Alto), AAA algorithm, v.13.6, with a 1 mm
calculation grid.
The difference between measured and calculated doses
Δ
meas, calc
, between jaw and MLC fields Δ
jaw, MLC
, and with
and without build-up screenΔ
b,no b
were determined.
Results
For jaw fields, Δ
meas, calc
is reduced from 0-22% to 0-9%
when using a build-up screen (table 1).
For MLC fields, Δ
meas, calc
increases from 10-31% to 4-48%
when using a build-up screen.
With the exception of the scenario with jaws and a build-
up screen, Δ
meas, calc
increased with SSD.
The considerable Δ
meas, calc
values could arise from
inaccurate beam modelling in the build-up region; with
the modelling less accurate for MLC fields. For MLC
fields,Δ
meas, calc
increased with increasing SSD, perhaps due
to an underestimation of the scatter contribution from
MLC fields at extended SSD.
The jaw fields gave rise to a greater dose than the MLC
fields. The calculation underestimated Δ
jaw, MLC
with a
build-up screen, while it overestimated it without a build-
up screen.
Doses were greater with than without a build-up screen in
all the scenarios investigated. Δ
b,no b
was not considerably
different between the calculation and the measurement.
Δ
b,no
b
was greater for 18X than for 6X .
Conclusion
The presence of a build-up screen increases superficial
phantom dose. However, differences of up to 48% exist
between calculated and measured doses at the phantom
surface. These differences generally increase with SSD and
depend on beam energy and field type (jaw vs MLC) in a
complex way. The modelling of scatter from MLC fields at
large SSDs appears to be a particular challenge.
EP-1450 Implementation of dosimetry equipment and
phantoms in clinical practice of light ion beam
therapy.
L. Grevillot
1
, J. Osorio
1
, V. Letellier
1
, R. Dreindl
1
, A.
Elia
2
, H. Fuchs
3
, A. Carlino
4
, S. Vatnitsky
1
, H. Palmans
5
,
M. Stock
1
1
EBG MedAustron GmbH, Medical Physics, Wiener
Neustadt, Austria
2
EBG MedAustron GmbH / University of Lyon France,
Medical Physics, Wiener Neustadt, Austria
3
EBG MedAustron GmbH / Medical University of Vienna,
Medical Physics, Wiener Neustadt, Austria
4
EBG MedAustron GmbH / University of Palermo Italy,
Medical Physics, Wiener Neustadt, Austria
5
EBG MedAustron GmbH / National Physical Laboratory
UK, Medical Physics, Wiener Neustadt, Austria
Purpose or Objective
QA equipment (water phantoms, films, ionization
chambers, anthropomorphic phantoms, etc.) is generally