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patient. On average CyberArc decreased treatment times
by 1.76x ± 0.23x for the prostates cases and 1.62x ± 0.13x
for brain patients, not taking into consideration the gantry
speed limitations. Staying within the tolerance of the
machine speed specifications, the average time decrease
was 1.56x ± 0.19x for prostate patients and 1.39x ± 0.11x
for brain patients.
Figure 2. DVH comparison between the original CyberKnife
plan (solid line) and the corresponding CyberArc plan
(dashed line).
Conclusion
CyberArc is able to deliver plans that are dosimetrically
comparable to their CyberKnife counterparts, while
reducing treatment times considerably.
OC-0448 Near real-time automated dose restoration in
IMPT to compensate for daily tissue density variations
T. Jagt
1
, S. Breedveld
1
, S. Van de Water
1
, B. Heijmen
1
,
M. Hoogeman
1
1
Erasmus MC Cancer Institute, Radiation Oncology,
Rotterdam, The Netherlands
Purpose or Objective
Intensity-modulated proton therapy (IMPT) allows for very
localized dose deposition, but is also highly sensitive to
daily variations in tissue density along the pencil beam
paths, induced for example by variations in organ filling.
This potentially results in severe deviations between the
planned and delivered dose. To manage this, we
developed a fast dose restoration method that adapts the
treatment plan in near real-time.
Material and Methods
The dose restoration method consists of two steps: (1)
restoration of the geometrical spot positions (Bragg peaks)
by adapting the energy of each pencil beam to the new
water equivalent path length (Figure 1), and (2) re-
optimization of pencil beam weights by minimizing the
dosimetric difference with the planned dose distribution,
using a fast and exact quadratic solver.
Figure 1
Restoring spot positions. Left: The intended spot
positions. Middle: An air cavity causes a displacement and
a change in spot shape (not depicted). Right: The energy
of the pencil beam has been adapted to restore the spot
position.
The method was evaluated on 10 prostate cancer patients,
using 8-10 repeat CT scans; 1 for planning and 7-9 for
restoration. The scans were aligned based on intra-
prostatic markers. Prostate, lymph nodes and seminal
vesicles were delineated as target structures. Dose was
prescribed according to a simultaneously integrated boost
scheme assigning 74 Gy to the high-dose planning target
volume (PTV) (prostate + 4 mm) and 55 Gy to the low-dose
PTV (lymph nodes and seminal vesicles + 7 mm).
Results
While substantial dose deviations were observed in the
repeat CT scans without restoration, clinically acceptable
dose distributions were obtained after restoration (Figure
2). This resulted in PTV V
95%
≥ 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: D
mean
, V
45Gy
,
V
60Gy
, V
75Gy
and bladder parameters: D
mean
, V
45Gy
, 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. Simões
1
, H. Yang
1
, R. Patel
1
, F. Le Grange
2
, S. Beare
3
,
E. Miles
1
, B. Seddon
2
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