ESTRO 36 Abstract Book

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S959 ESTRO 36
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uncertainty margins by quantifying bone set-up variation,
and, by inference, OAR position.
The aim of this work is to optimise the current imaging
practice on a Truebeam Linac STx(2.5) for adult brain
tumour patients by investigating the relationship between
CBCT parameters, patient dose and image quality for both
bone and soft tissue.
Material and Methods
In our institution we currently image this cohort with daily
kV imaging and weekly CBCT. A daily ‘shift to zero’ is
applied for set-up variations of less than 3mm with
residual error evaluated on a weekly basis via a standard
Full-Fan, partial arc CBCT template (Table 1-Template 1)
The CBCT image quality was evaluated by comparing three
progressive dose-reduction trial imaging templates +/-
length reduction (Table1-Templates 2, 3, 4) with the
standard CBCT template.
Initial tests were carried on Catphan® and Rando ®
phantoms to assess image quality and scan artefact
limitations. Three consecutive skull-base meningioma
patients were then imaged with the four templates on
sequential weekly imaging sessions during treatment.
Each CBCT was reviewed by an expert group of a clinician,
two radiographers and two physicists to evaluate, by
consensus, the discernibility of the optic nerves,
ventricles, cranial bones, temporalis muscle, and the
external contour. In addition, the fidelity of the co-
registration to the skull-base anatomy (ROI) was assessed.
A standard threshold was applied throughout the
investigation.
Results
A total of 15 CBCT images were acquired and reviewed.
All structures were visible for each template except for
the ventricles which were assessed as indistinct with
templates 3 and 4 (Figure 1).
The fidelity of registration was satisfactory for each
template.
Conclusion
A length-reduction template can be utilised in brain
tumours providing the skull-base is included in the CBCT
ROI. In cases where the PTV is distant to the skull base,
length reduction should be used with care.
A concomitant dose reduction is also feasible unless the
discernibility of the ventricles is essential, such as when
changes in ventricular volume/position are of concern.
In our institution we will adopt a new imaging strategy for
brain tumour patients employing template 4 for all benign
skull based tumours and retaining template 1 (standard)
for all other lesions.
Further similar work will be carried out to optimise the
CBCT protocols in other cohorts of patients (e.g.
paediatrics).
EP-1743 Evaluation of proton grid therapy in
challenging clinical cases
T. Henry 1, A. Valdman2, A. Siegbahn1
1Stockholm University, Department of Medical Physics,
Stockholm, Sweden
2Karolinska Institutet, Department of Oncology and
Pathology, Stockholm, Sweden
Purpose or Objective
For several decades unidirectional photon-grid therapy
has been a useful tool in radiation oncology. Its main
advantage is to limit the normal tissue toxicity when
irradiating the patients with bulky tumors. In this work
we use proton grid therapy (PGT). PGT delivered with a
crossfiring technique has been used instead of a
unidirectional approach. The physical properties of
proton beams allow for the protection of risk organs
posterior to the target while the crossfiring technique
enables a larger separation between the beams, thus
better preserving the normal tissue. Here we evaluate the
possibility to use PGT as a therapeutic option in certain
clinical situations. For example, due to the ability of
interlaced proton-beam grids to significantly spare normal
tissue, this technique may be useful in re-irradiation cases
not otherwise eligible for radiotherapy treatment because
of too high doses to organs at risk.
Material and Methods
CT data from patients previously treated with
conventional photon therapy at Karolinska Hospital,
Stockholm, were reused in order to create PGT treatment
plans with the TPS Eclipse (Varian Medical Systems).
Patients that could benefit re-irradiations or palliative
care were selected. The aim was to deliver a high and
nearly homogeneous target dose, while keeping the grid
pattern of the dose distribution, made of peak and valley
doses, as close to the target as possible. A low grid dose,
with low peak and valley doses, was also preferable to
better protect the normal tissue. The dosimetric
characteristics of those plans were then evaluated, with a
focus on the overall homogeneity of the target dose, as
well as dose profiles outside of the target (i.e. evaluation
of the grid dose distribution through peak and valley doses
analysis).
Results
All the studied cases presented dose distributions for
which the grid pattern was preserved until the direct
neighborhood of the targets. When normalizing the
minimum target dose to 100%, the valley doses reached
around 5%, while the peak doses were approximately 60-
70%, depending on the grid geometry used. Inside the
targets, a good dose homogeneity could be achieved (σ=
±10 %). The volumes of organs at risk irradiated with high
doses remained small and limited spatially to the dose
peaks of the grids.
Conclusion
PGT produces a combination of nearly homogeneous and
high target dose. The grid pattern can be preserved in the
normal tissue, from the skin to the direct vicinity of the
target, preventing extensive damage to the organs at risk.
The PGT approach could present a therapeutic possibility
in difficult clinical situations where conventional
radiotherapy would fail to provide any suitable option for
the patients.
EP-1744 Failure modes and effects analysis of Total
Skin Electron Irradiation (TSEI) technique
B. Ibanez-Rosello 1, J.A. Bautista-Ballesteros1, J.
Bonaque1, J. Perez-Calatayud1,2, A. Gonzalez-Sanchis3, J.
Lopez-Torrecilla3, L. Brualla-Gonzalez4, M.T. Garcia-
Hernandez4, A. Vicedo-Gonzalez4, D. Granero4, A.
Serrano4, B. Borderia4, J. Rosello4,5