ESTRO 36 Abstract Book

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S84 ESTRO 36 2017
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OC-0166 Fast 3D CBCT imaging for Lung SBRT: Is image
quality preserved ?
B. De Rijcke1, R. Van Geeteruyen1, E. De Rijcke1, Y.
Lievens1, E. Bogaert1
1 Ghent University Hospital, Radiation Oncology, Gent,
Belgium
Purpose or Objective
Irradiation of Early Stage Non-Small Cell Lung Cancer (ES-
NSCLC), through Stereotactic Body Radiotherapy (SBRT)
requires image guidance. At our institute double pre-
treatment CBCT, with manual registration is performed at
every fraction. Speeding up CBCT gantry rotation and
implementation of automated registration allows for
faster decision taking. It also offers the possibility of
intrafraction CBCT, without severe prolongation of
treatment time. In a first step we investigated the image
quality and performance of a CBCT protocol with lower
dose and faster acquisition time.
Material and Methods
Standard (S) and Fast (F) scan protocols only differed in
gantry speed (180°/min (S) and 360°/min (F)) and were
performed on XVI Elekta ® CBCT. For six patients receiving
lung SBRT (60Gy in 3 or 4 fractions) for upper lobe ES-
NSCLC, dual pre-treatment imaging consisted of a S scan
followed by a F scan. This resulted in 17 useful S and F
image sets. Tumor movement amplitude stayed below 1cm
(1) , removing the necessity for 4D-CBCT. All CBCT images
were retrospectively exported to Raystation ® (RaySearch
Laboratories, Sweden) for easy and blended side-by-side
evaluation. The resolution was 1x1x1mm3 for all scans. All
CBCT images were matched to planning CT. WW/WL was
set fixed per patient. Zooming was allowed.
Visual Grading Analysis (VGA) comprised well defined
criteria over the three planes (T, C, S), categorized in
three Image Quality (IQ) Focus groups: bony anatomy
(N=11), tumor characteristics (N=3) and anatomical
landmarks (N=7). Examples are: visualization of corpus
vertebrae (C, S plane), tumor edge (3 planes); carina
bifurcation (C, T plane). Scoring was done independently
by 3 routined RTTs. Possible answers were: equal, better
or worse for ‘upper’ scan (randomly assigned to F or S).
Data were analyzed using SPSS software v24 (IBM Corp.,
New York, NY).
Results
In 73.7 % of all cases, visualization of anatomical
structures was appreciated equally on S and F scans. When
differences emerged, visualization on F scan was
appreciated more in 71.3 % of the cases (71.8 % for bony
anatomy, 75.0 % for tumor characteristics and 67.2 % for
anatomical landmarks). Binary Logistic Regression in these
cases did not reveal significant dependence on patient (for
which BMI or tumor location are most relevant; however
not evaluated separately) (p=0,638), not on IQ focus group
(p=0,540) and not on reader (p=0,883). Thus, in 92.4 % of
all cases, image quality was scored equal or better for fast
imaging protocol compared to the standard protocol
(Figure 1).
Conclusion
Fast CBCT imaging can be safely used for ES-NSCLC tumors
with tumor movement amplitude < 1cm. In 73.7 % of the
cases there is no image quality loss and even more, in 18.8
% of the cases IQ of the fast scan is preferred compared to
the standard scan.
(1) Rit, S., et al., Comparative study of respiratory motion
correction techniques in cone-beam computed
tomography. Radiotherapy and Oncology, 2011. 100(3): p.
356-359
Symposium: Novel approaches in particle biology
SP-0167 The ESTRO initiative on biological effects of
particle therapy
B.S. Sørensen1
1Aarhus University Hospital, Exp. Clin. Oncology, Aarhus
C, Denmark
Particle therapy as cancer treatment, with either protons
or heavier ions, provide a more favourable dose
distribution compared to x-rays. While the physical
characteristics of particle radiation have been the aim of
intense research, less focus has been on the actual
biological responses particle irradiation gives rise to. One
of the biggest challenges for the radiobiology is the RBE,
with an increasing concern that the clinical used RBE of
1.1 is an oversimplification, as RBE is a complex quantity,
depending on both biological and physical parameters, as
dose, LET, biological models and endpoints. Most of the
available RBE data is in vitro data, and there is very
limited in vivo data available, although this is a more
appropriate reflection of the complex biological response.
There is a need for a systematic, large-scale setup to
thoroughly establish the RBE in a number of different
models, in a clinical relevant fractionated scheme. The
aim of the ESTRO initiative is to form a network of the
research and therapy facilities. This would open for the
possibility of standardising radiobiological experiments,
and coordinating the research in order to deliver the
needed experimental data.
SP-0168 RBE of protons
B. Jones1
1Jones Bleddyn, CRUK-MRC Oxford Institute- Department
of Oncology, Oxford, United Kingdom
Introduction. Increasing clinical use of proton therapy
(PT) is not simply an extension of photon radiotherapy
(RT), but requires more detailed knowledge of clinical
physics and radiobiology in order to achieve optimal
outcomes. A critical difference is that megavoltage RT has
linear energy transfer (LET) of around 0.22 -1, but
LET further increases towards and within proton Bragg
peaks. ‘Spread-out’ Bragg peaks (SOBP), depending on
their volume, normally have LET of 1-2 -1, but