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ESTRO 36 2017
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PO-0830 Quantification of density and tissue changes
in pencil beam scanning proton treatment.
F. Van den Heuvel
1
, F. Fiorini
1
, B. George
1
1
University of Oxford, CRUK/MRC Oxford Institute for
Radiation Oncology, Oxford, United Kingdom
Purpose or Objective
Proton pencil beam scanning (PBS) is becoming the
methodology of choice to deliver proton therapy in many
cases. Several authors have reported discrepancies
between the dose distributions generated by commercial
planning systems, using analytical models, compared to
those using stochastic methods. The differences are
greatest in areas with extensive tissue inhomogeneities.
In analytically based commercial planning systems,
inhomogeneities are taken into account using a water
equivalent path length (WEPL) scaling. In this work we
quantify and investigate the impact of different densities
and tissue on the dose deposition characteristics of a
proton pencil beam.
Material and Methods
A single pencil beam with nominal energy 226 MeV from
an IBA-facility is modeled in homogenous cubic 40x40x40
cm3 phantom using FLUKA. The pencil beam’s dose
deposition is uniquely characterised using a stable
distribution parameterisation, yielding three parameters α
(halo or tail describing parameter), γ (scale parameter)
and ID (integral dose) as a function of depth in the
phantom. Changes of the parameters with changing
densities are investigated and the WEPL technique is
assessed. In addition, the behaviour of the parameters in
a selection of relevant tissues is evaluated.
In addition we investigated different specific media having
different atomic properties and show that an effective
density representation is can be used for these.
Results
The parameters α (Fig 1) describing the scattered
radiation and ID (not shown) clearly scale with the density
of the material. The scaling parameter γ shows a more
complicated behaviour. Indeed, this work shows that an
effective density can be calculated which has the form of
ρ_eff = 1-(1-ρ)/2
Figure 2 shows the difference between both curves. Note
that the maximum of the curves follows the WEPL rule as
they are linked to the position of the bragg peak.
Conclusion
Simple WEPL scaling used in analytical dose calculations
may not correctly model the physical properties of a
proton pencil beam. A more complex scaling framework
that separates the halo and scale parameters could
provide a more accurate representation of dose deposition
from a proton pencil beam. In further work (not shown)
we also show that tissue specific (i.e. stopping power
differences) properties can be handled by using effective
densities.
PO-0831 Multi isocentric 4-pi volumetric modulated arc
therapy approach for head and neck cancer
S. Subramanian
1
, S. Chilukuri
1
, V. Subramani
2
, M.
Kathirvel
1
, G. Arun
1
, S.T. Swamy
1
, K. Subramanian
1
, A.
Fogliata
3
, L. Cozzi
3
1
Yashoda Super Specialty Hospital, Radiation Oncology,
Hyderabad, India
2
All India Institute of Medical Sciences, Radiation