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modulated proton therapy (IMPT) plans. Modeled OARs
included the parotid glands (PGs), submandibular glands
(SMGs), oral cavity (OC) and individual swallowing
muscles. For 10 evaluation patients,
PSP
was used to
derive mean predicted OAR doses generated by
M
VMAT
/M
IMPT
, which were further interpreted using NTCP
models for the following OARs: 1: PGs (stimulated flow
ratio<25% pre-treatment, Dijkema IJROBP 2010), 2: SMGs
(grade 4 toxicity, Murdoch-Kinch IJROBP 2008), 3: OC
(grade 3 mucositis, Bhide R&O 2012) and 4: Superior
pharyngeal constrictor muscle/supraglottic larynx
(sPCM/SGL, grade 2-4 dysphagia, Christianen R&O 2012).
Results
Figure 1 shows a screenshot of a DVH comparison for the
left PG using
PSP
, with the red/blue shaded regions
representing the dose predictions by M
VMAT
/M
IMPT
. On
average, mean CL/IL parotid gland doses were
21.3±3.3/33.6±4.2Gy and 14.5±6.0/28.8±5.0Gy using M
VMAT
and M
IMPT
, respectively, associated with a 5.0/7.8%
reduction in NTCP. Conversely, predicted CL SMG doses
were 33.0/31.9Gy with M
VMAT
/M
IMPT
, with a 0.3% difference
in estimated NTCP. The largest reduction using M
IMPT
was
noted for the occurrence of oral mucositis (15.6%), with
an average OC mean dose reduction of 16.9Gy. Finally, a
4.5% decrease in dysphagia was predicted using IMPT,
associated with sPCM/SGL dose reductions of 6.1/5.0Gy.
Conclusion
By including standard dosimetric and NTCP metrics,
PSP
may assist in optimal treatment selection for individual
patients. This analysis is based on a DVH line that is
predicted by RapidPlan, without requiring the actual (time
consuming) creation of treatment plans. This makes the
plan comparison process efficient and transparent. Note
that the proton versus photon comparison for HNC was
solely used as a paradigm, this study was not intended to
investigate the accuracy of RapidPlan for protons.
EP-1545 On mixed-modality radiation therapy
optimization using the column generation approach
M.A. Renaud
1
, M. Serban
2
, C. Lambert
3
, J. Seuntjens
1
1
McGill University - Downtown Campus Department of
Oncology, Physics, Montreal, Canada
2
McGill University Health Center - Glen Site, Medical
Physics, Montreal, Canada
3
McGill University Health Center - Glen Site, Radiation
Oncology, Montreal, Canada
Purpose or Objective
Despite considerable increase in the number of degrees of
freedom with recent radiotherapy optimisation
algorithms, treatments are typically delivered using only
a single modality. Mixed-modality plans such as electron-
photon provide dosimetric advantages for sites with a
superficial component. The column generation method,
which is an iterative method that finds the aperture with
the largest potential to improve the cost function at every
iteration, is well suited for mixed-modality optimisation
as the aperture generation and modality selection
problem can be solved quickly. We assess the performance
of the column generation method applied to mixed-
modality planning and investigate its behaviour under
different modality mixing schemes.
Material and Methods
The column generation method was applied to a chest wall
case (Fig. 1). Photon beamlets were created for a coplanar
distribution of beam angles every 20º around the patient.
In addition, 5 shortened-SAD (70 cm) electron beam angles
were included and beamlets were generated for energies
of 6, 9, 12, 16, 20 MeV. A photon MLC acted as the sole
collimating device for electrons.
Photon-only (IMRT), electron-only (MERT) and mixed
electron-photon (MBRT) treatment plans were created
using the same optimisation constraints. To analyse the
sensitivity of treatment plans to initial conditions, a
perturbation on the original mixed-modality treatment
plan was created by forcing the first 50 apertures to be
photon apertures before allowing other modalities.
Finally, the efficiency and plan quality of four different
modality mixing schemes was analysed by creating
treatment plans with more than a single aperture per
iteration of the column generation loop.
Results
The MBRT plan produced better target coverage and
homogeneity while preserving the normal tissue-sparing
advantages of electron therapy (fig. 2, inset), with a final
cost function between 25-30% of the values for IMRT and
MERT.
The fraction of total dose among modalities in the
treatment plan with the perturbation (fig. 2, full lines)
converged to the unperturbed treatment plan (fig. 2,
dashed lines) with identical plan quality.