ESTRO 36 Abstract Book

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S236 ESTRO 36 2017
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2). This resulted in PTV V95% ≥ 98% and V 107% ≤ 2% for all
scans. For the bladder, the differences between the
restored and intended treatment plans were below 2 Gy
and 2%-point. The rectum differences were below 2 Gy
and 2%-point for 90% of the scans. In the remaining scans
the rectum was filled with air and partly overlapped with
the PTV, resulting in unavoidably higher rectum doses.
Figure 2 Boxplots showing differences in dosimetric
parameters between the distorted and intended (left) and
re-optimized and intended dose distributions (right) for all
80 scans. Left to right, rectum parameters: Dmean, V45Gy ,
V60Gy , V75Gy and bladder parameters: Dmean, V45Gy, V 65Gy.
The mean time needed for energy adapta tion was 5.4
seconds (3.5-10.6). The re-optimization time was on
average below 5 seconds (maximum 9.0). T he most time
consuming and currently limiting operation was
calculating the dose distribution matrix (average 4.3
minutes (2.4-9.6)), performed once betw een the two
steps.
Conclusion
The impact of density variations on the penci l beam path
in IMPT can be reduced by performing an automated dose
restoration consisting of a water equivalent path length
correction of the pencil beams, followed by a re-
optimization of the pencil beam weights.
Proffered Papers: Planning and quality assurance
OC-0449 A novel and objective plan evaluation for
limb sarcomas IMRT in the IMRiS phase II trial
R. Simões1, H. Yang1, R. Patel1, F. Le Grange2, S. Beare3,
E. Miles1, B. Seddon2
1 Mount Vernon Cancer Centre, National Radiotherapy
Trials Quality Assurance RTTQA Group, London, United
Kingdom
2 University College Hospital, Sarcoma Unit, London,
United Kingdom
3 University College of London, Cancer Research UK &
University College London Cancer Trials Centre, London,
United Kingdom
Purpose or Objective
IMRiS (Clinicaltrials.gov id:NCT02520128) is a multicentre
phase II trial of intensity modulated radiotherapy (IMRT)
in soft tissue and bone sarcomas. IMRT was implemented
in the UK for limb soft tissue sarcomas (STS) in the context
of this trial, which opened to recruitment in March 2016.
As limb STS volumes are very variable, there are several
ways of optimising the plans. It is often difficult to assess
plan quality without understanding fully if the presented
plan has been well optimised. We describe novel metrics
used to evaluate IMRT plan quality for limb STS.
Material and Methods
A case of liposarcoma of the left thigh was available to the
29 IMRiS participating centres. The prescription was 50Gy
in 25 fractions. The clinical target volumes and the
relevant organs at risk (OAR) were pre-contoured. The
planning target volume (PTV) was derived at each centre
(5-10 mm). All plans were checked, PTV conformality (PTV
V95%/PTV Vtotal) and PTV compromise (OAR V95%/OAR&PTV
VTotal overlap) indexes were also calculated. The relevant OAR
for this case were the femoral head and neck (FHN) and
the femur in treatment field (FTF). The IMRT dose fall-off
gradient for FHN (FHN V95%/ FHN V80%) was also assessed.
Normal tissues and the joint were not analysed, as their
tolerances were easily met for this specific case.
Results
19 centres completed 20 IMRT plans. The plan quality of
9/20(45%) submissions was suboptimal and had to be
repeated. The results (see table) include the resubmitted
cases (total 29 plans). The case was particularly
challenging near FHN and FTF, due to an overlap of OARs
with the PTV. Depending on PTV margins, overlapping FHN
volumes varied from 13.8% (for PTV margins of 5mm) to
33.0% (for PTV margins of 10mm). FTF overlapping volume
with PTV ranged from 24.7% to 51.1%.Plans were very
conformal to PTV; however, the PTV conformality index
was not useful for areas where PTV overlapped with OAR.
We therefore calculated a compromise index for the PTV
areas overlapping with FHN and FTF, which support the
visual assessment of plans. The graph below highlights
plans in which V80% was suboptimal in relation to the V95%
(in total 5 plans had a suboptimal IMRT fall-off dose
gradient).
Conclusion
Limb STS tumours are a heterogeneous group of tumours
with significant variation in PTV shape and size. Evaluating
the plans for a newly implemented technique can be
challenging, particularly when determining if a plan is
optimal. We developed an objective assessment method
that is applicable to all limb STS. The first planning results
show that 45% of plans had either compromised PTV
coverage in favour of meeting OAR dose constraints, or
had not created a steep enough dose gradient near the
OAR. We attribute this to a change in the planning
technique paradigm, as many of the centres were using
IMRT for limb STS for the first time.
OC-0450 Geometric variation of the axillary lymph
node region in locoregional breast/chest wall
irradiation.
K.N. Goudschaal 1, N. Bijker 1, A. Bel 1, N. Van Wieringen1,
M. Kamphuis1