ESTRO 36 Abstract Book

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S71 ESTRO 36 2017
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is recommended 48- 60 Gy in three fractions for lesions
with a diameter ≤3 cm, while for lesions with a diameter
>3 cm a higher prescription dose, such as 60- 75 Gy is
necessary to obtain similar local control [5].
3. Spine The goal of spinal SBRT is local control and pain
control. Several authors have reported that the 1-year
local control rate ranges 80- 98% and provides pain relief.
Therefore, several dose/fractionation schedules, such as
24 Gy in 1 fraction or 27 or 30 Gy in 3 fractions have been
used and the optimal dose/fraction schedule is still
unclear.
2) Comparison between surgery and SBRT for extracranial
oligometastases
According to several guidelines, surgery for extracranial
oligometastases is still standard practice because of lack
of evidence that SBRT has clinical advantages.
A retrospective analysis comparing surgery with SBRT for
110 patients with pulmonary oligometastases
demonstrated that 3-years overall survival rates were 62%
for surgery and 60% for SBRT (p = 0.43) [6]. The authors
concluded survival after surgery was not better than after
SBRT although SBRT should be the second choice after
surgery. However, no randomized trials have been
conducted, and prospective randomized studies are
required to define the effectiveness of each modality.
3) Cost-effectiveness
Extracranial oligometastases have been usually managed
with systemic therapy with or without surgery. However,
systemic therapy, including molecular targeted drugs, is
expensive. A cost-effectiveness analysis using a Markov
modelling approach demonstrated that video-assisted
thoracic surgery wedge resection or SBRT could be cost-
effective in selected patients with pulmonary
oligometastases [7]. Increases in medical expenses are a
social problem worldwide, but it can be said that SBRT is
a promising modality in this aspect.
(References) [
1] Lewis SL, Porceddu S, Nakamura N, et al. Am J Clin
Oncol 2015.
[2] Shultz DB, Filippi AR, Thariat J, et al. J Thorac Oncol
2014; 9: 1426-1433.
[3] Ashworth A, Rodrigues G, Boldt G, et al. Lung Cancer
2013; 82: 197-203.
[4] Binkley MS, Trakul N, Jacobs LS, et al. IJROBP 2015;
92:1044-1052.
[5] Scorsetti M, Clerici E and Comito T. J Gastrointestes
Oncol 2014; 5: 190-197.
[6] Widder J, Klinkenberg TJ, Ubbels JF, et al. Radiother
Oncol 2013; 107: 409-413.
[7] Lester-Coll NH, Rutter CE, Bledsoe TJ, et al. IJROBP
2016; 95: 663- 672.
Proffered Papers: Best of particles
OC-0149 Lateral response heterogeneity of Bragg peak
ion chambers for narrow-beam photon &proton
dosimetry
P. Kuess1 , T. Böhlen2, W. Lechner1, A. Elia2, D. Georg 1, H.
Palmans2
1 Medizinische Universität Wien Medical University of
Vienna, Department of Radiation Oncology and Christian
Doppler Laboratory for Medical Radiation Research for
Radiation Oncology, Vienna, Austria
2 EBG MedAustron GmbH, Medical Physics, Wiener
Neustadt, Austria
Purpose or Objective
A large area ionization chamber (LAIC) can be used to
measure output factors of narrow beams. In principle,
dose area product measurements are an alternative to
central-axis point dose measurements. Using an LAIC
requires detailed information on the uniformity of the
signal response across its sensitive area.
Material and Methods
8 LAICs (sensitive area with nominal diameter of 81.6mm)
were investigated in this study, 4 of type PTW-34070
(LAICThick ) and 4 of type PTW-34080 (LAICThin) with water-
equivalent entrance window thicknesses of 4mm and
0.7mm, respectively. Measurements were performed in an
X-ray unit (YXLON) using peak voltages of 100-200kVp and
a collimated beam of 3.1mm FWHM. The LAICs were
mounted on the moving mechanism of an MP3-P (PTW) and
moved with a step size of 5mm to measure the chamber’s
response at lateral positions. To account for beam
positions where only a fraction of the beam overlapped
with the sensitive area of the LAIC, a corrected response
was calculated as the basis for determining relative
response as a function of radial distance from the
centre. The impact of a heterogeneous LAIC response,
based on the obtained response maps was henceforth
investigated for small field photon beams (as small as
6x6mm²) and proton pencil beams (FWHM=8mm).
Results
A pronounced heterogeneity of the spatial responses was
observed in both the thick and thin window LAICs. These
heterogeneities could be calculated as a function of the
radial coordinate as there was no pronounced angular
dependency. All 4 LAIC Thick followed a monotonously
increasing response towards the chamber centre, while
the absolute response values varied up to 1.5%, excluding
the 2mm borders of the LAICs. In contrast the LAICThin
trends were not uniform and responses varied by up to 10%
(Fig 1). Investigating absolute dosimetry for a proton
pencil beam the signal varies with a systematic offset
between 2.4% and 4.1% for LAIC Thick and between -9.5% and
9.4% for LAICThin . For relative dosimetry (e.g. depth-dose
profiles) the increase of beam size with increasing depth
was investigated as the influencing factor. Systematic
response variation by 0.4% and 1% at the most were found
for the investigated LAICs. The systematic offset for
absolute dose measurements for decreasing photon field
size showed that for 6x6mm² field sizes the response was
systematically 2.5-4.5% higher for LAIC Thick. For LAICThin the
response varies even over a range of 20%. The entrance
window thickness was evaluated to be constant within
measurement uncertainty by performing measurement at
multiple peak voltages.