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addition, we found that irradiation is inducing CSC marker
and CSC properties in a dose- and time-dependent
manner. This irradiation-induced CSC-plasticity was
attributed to the modulation of the histone methylation
code. Within the present study we analyzed a panel of
secreted cytokines and their corresponding cytokine
receptors in the radioresistant prostate cancer sublines, in
a s.c. xenotransplantation model, in ex vivo irradiated
primary prostate cancer biopsies and in blood samples of
prostate cancer patients during the course of radiotherapy
and found, for example, the CXCR4-CXCL12 signaling to be
involved in the CSC maintenance and the induction of
prostate cancer radioresistance.
Conclusion
Our studies suggest that the combination of irradiation
with cytokine signaling modulation, especially the CXCR4-
CXCL12 signaling, may increase the cytotoxic effects of
irradiation in prostate cancer cells. The expression
profiling of proteins involved in the cytokine signaling can
be used to predict clinical outcome of prostate cancer
patients after radiotherapy.
Proffered Papers: New technologies for imaging and
therapy
OC-0437 Scatter imaging: promising modality for image
guided ablation radiotherapy for lung cancer patients
J. Chu
1
, G. Redler
1
, G. Cifter
1
, K. Jones
1
, J. Turian
1
1
Rush University Medical Center, Department of Radiation
Oncology, Chicago IL, USA
Purpose or Objective
Early stage lung cancers can be effectively treated by
stereotactic ablation radiation therapy (SABR). Successful
treatment requires hypofractionation and large dose per
fraction (up to 20 Gy) while maintaining a high level of
accuracy (≤1.0mm). By imaging the photons that are
Compton-scattered out of the treatment beam, real-time,
non-invasive monitoring of the tumor location may be
possible. To assess the potential of this modality, we have
obtained scatter images of static and movable tumor
phantoms, and calculated images from CT-based Monte
Carlo simulations.
Material and Methods
Compton scatter is a natural by-product of external beam
radiation therapy. The scattered radiation contains
information about the patient anatomy and the transient
tumor location. An embedded tumor in a Quasar
respiratory motion phantom (Modus Medical Devices Inc.)
was programmed to move linearly over 2.5cm. While
irradiating the embedded tumor using a 6MV Varian
TrueBeam linear accelerator, experimental scatter images
were measured with a Varian PaxScan flat panel detector
and a pinhole collimator. Tumor centroid locations were
then measured from various scatter images and compared
with the expected values. Monte Carlo N-Particle (MCNP)
code was used to simulate scatter images from phantoms
and patient CT images using 10 - 1000MU, or 0.5 – 50
second time scales. The quality of the images was assessed
to determine their potential for tumor localization during
treatment.
Results
The measured tumor centroid locations agreed with the
expected values to within 1mm, which is adequately
accurate for clinical tumor tracking. Lung tumor phantom
images showed excellent signal and contrast. The
contrast-to-noise ratio ranged from 3.4 to 15.1 for scatter
images acquired with 0.5 to 50s. The attached figures
below show CT and simulated scatter images
(corresponding to the red shaded region in CT) for both
inhale and exhale breathing phases. The scatter images
clearly show variation of tumor and diaphragm locations
for two breathing phases. Other pertinent anatomical
structures, such as chest wall, heart, and lung are also
clearly visible.
Conclusion
This study has demonstrated the feasibility of using
scatter imaging to track lung tumor movement during
SABR treatments. The potential benefits may include real-
time, good quality image guidance without additional
radiation. We are optimistic that improvements in the
detector and collimation system, will lead to better image
quality, more accurate tumor localizations, and possible
3D reconstruction of patient’s anatomy within the
irradiated region.
OC-0438 The impact of a 1.5 T MR-Linac fringe field
on neighbouring linear accelerators.
T. Perik
1
, J. Kaas
1
, F. Wittkamper
1
1
The Netherlands Cancer Institute, Department of
Radiation Oncology, Amsterdam, The Netherlands
Purpose or Objective
In our institution a clinical prototype of the MR-
Linac(MRL)(Elekta AB, Stockholm, Sweden) was installed
in an existing treatment room. The MRL, which has a field
strength of 1.5 T, is neighboured by 3 clinical Elekta
accelerators at a distance (Isocenter MRL to gun linac) of
7.5 and 5.5 and 11 meters. The peripheral magnetic field
outside of the magnet core of the MRL, the fringe field,
may influence the beam steering of accelerators in
adjacent treatment rooms. This influence for a pre-
clinical prototype was described by Kok et al. in 2009. The
aim of this study is to investigate the influence of the
significantly reduced fringe field of the clinical prototype
on the beam steering of its neighbouring accelerators.
Material and Methods
A STARCHECK MAXI detector array (PTW Freiburg,
Germany) was mounted on all the neighbouring
accelerators with a frame that puts the array on isocentre
height. An inclinometer was attached to the gantry to
acquire a gantry rotation signal. For every available
energy, two 360 degree arcs (clockwise and counter
clockwise) were irradiated with a 40x40 field. A
measurement of beam profile was acquired in movie mode
at a frame rate 2.5 Hz. Beam symmetry (IEC) was
determined for every frame.
These measurements were done before and after ramping
up the MRL magnet, and a 3
rd
time after adjusting the
look-up tables (LUT) which correct the beam steering by
applying a gantry-angle dependent current to the steering
coils (2R and 2T). These LUTs were adjusted using the
accelerator internal monitor chamber.
Results
A change in beam symmetry as a function of gantry angle,
before and after ramping the magnet, of up to 4% (Linac
A) and 1% (Linac B) is observed, causing beam symmetry
on both linacs to be out of tolerance (IEC 102%). Linac C
did not show any significant change. Figure 2 shows the
LUT before ramping the magnet (pre) and after
adjustment (post) and the difference for Linac A for the
10 MV beam. After adjustment of the beam steering on
Linac A and B, the symmetry was within tolerance for all
gantry angles. Adjusting the LUTs took 1.5 hours per linac.
Conclusion