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transmitted to the linac gating interface via fibre-optic
cables.
Results
Kinect v2 was able to acquire all breath holds from each
patient successfully. Extracted traces from each patient
depth file were sent to the motion platform. In the gating
experiments (see Fig. 2), a Kinect v2 was able to track the
phantom motion with a root mean square error of between
0.6 mm and 1.3 mm. The latency of our in-house gating
software was found to range between 30 and 100 ms. In
all cases, the gated radiation delivery dose agreed with
the baseline dose measurement without gating to better
than
0.4%.
Figure 2: Top: A patient breathing trace programmed into
the motion platform, monitored for gating by a Kinect v2
. Bottom: corresponding beam state transmitted to linac.
Conclusion
The Kinect v2 provides a cost-effective method of
monitoring patients during VBH, and gating the delivery of
radiation to only the peak inhale phase. This is a
markerless, convenient alternative to manual monitoring.
EP-1625 Comprehensive prospective evaluation tool
for treatments of thoracic tumours with scanned
protons
C. Ribeiro
1
, A. Meijers
1
, G. Janssens
2
, J. Widder
1
, J.
Langendijk
1
, E. Korevaar
1
, A. Knopf
1
1
University Medical Center Groningen UMCG,
Department of Radiation Oncology, Groningen, The
Netherlands
2
Ion Beam Applications IBA, Advanced Technology Group,
Louvain-la-Neuve, Belgium
Purpose or Objective
Due to the high sensitivity of Pencil Beam Scanning (PBS)
to water equivalent thickness (WET) variations,
differences between the planned and delivered dose to
the CTV (robustness) are of great concern, especially for
the treatment of moving targets located in the thorax.
Effects that influence the robustness of plans created for
patients with moving targets are: machine uncertainties,
setup and range errors and the interplay effect, which
occurs due to the interference of the time structure of
treatment delivery and target motion. The aim of this
study is the development and application of a tool that
realistically evaluates PBS deliveries to patients with
moving targets prior to the actual treatment.
Material and Methods
A robustly optimized plan with a nominal dose of 60 Gy to
the CTV was created using our treatment planning system
for an exemplary lung cancer patient (non-small cell lung
cancer (NSCLC) stage III). We considered the delivery of
this nominal plan over 8 fractions, which has been shown
representative for the clinical delivery over 30 fractions.
Our tool simulates
(1)
machine uncertainties (spot
position, dose, and energy errors),
(2)
setup and range
errors (by shifting the patient and 3% scaling the CT
intensity values in order to create 14 scenarios
representing 14 possible treatment courses),
(3)
breathing
motion (by performing 4D dose accumulation in the
planning 4DCT and in repeated 4DCTs),
(4)
interplay effect
(incorporating the time structure of delivery by splitting
the nominal plan in 10 different sub-plans with the help of
the scanning control system ScanAlgo), and
(5)
a
combination of all previously mentioned effects
(1)
-
(4)
.
To evaluate robustness, the V95 of the CTV was analysed.
In case of presence of multiple scenarios (
(2)
and
(5)
) the
V95 of the voxel-wise minimum dose distribution of the
CTV (minimum dose obtained from all the scenarios in
each voxel of this structure) was determined.
Results
V95 values for the simulation scenarios
(1)
-
(5)
are present
in Table 1. The V95 for the CTV dropped from 100%
(nominal case) to 90.11% when all effects were considered
in combination (simulation
(5)
). Figure 1 shows dose
distributions of the nominal plan and the voxel-wise
minimum obtained for simulation
(5)
. Furthermore, DVH
curves of the nominal plan, all treatment scenarios
resulting from the realistic combination of effects and the
corresponding voxel-wise minimum dose are shown.
Conclusion
We developed a realistic and comprehensive tool for a
prospective robustness analysis of PBS treatment plans for
patients with moving targets. The power of this tool was
demonstrated in one exemplary lung cancer patient,
showing the significant impact of the combination of PBS
delivery effects for target coverage. In clinical practice
this tool will help to make decisions concerning the
necessity to employ further motion mitigation techniques.