ESTRO 36 Abstract Book

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S447 ESTRO 36
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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. Subramanian1, S. Chilukuri1, V. Subramani2, M.
Kathirvel 1, G. Arun 1, S.T. Swamy1, K. Subramanian1, A.
Fogliata 3, L. Cozzi3
1 Yashoda Super Specialty Hospital, Radiation Oncology,
Hyderabad, India
2 All India Institute of Medical Sciences, Radiation
Oncology, New Delhi, India
3Humanitas Research Hospital and Cancer Center,
Radiation Oncology, Milan-Rozzano, Italy
Purpose or Objective
The possibility to deliver intensity modulated plans using
most of the 4-pi space, i.e. with extensive use of non-
coplanar beams and complex trajectories for the couch-
gantry-collimator system, has been explored on
stereotactic irradiation in the brain, lungs and prostate
and have shown significantly sharper dose gradients. The
applicability of 4p techniques to large target volumes with
volumetric modulated arc therapy (VMAT) treatments
remains unaddressed for head and neck cancer (HNC). The
aim of this work is to explore the feasibility and
deliverability of multi-isocentric 4-pi VMAT (4pi-VMAT)
plans in comparison with coplanar VMAT (CP-VMAT) plans
for the irradiation of HNC patients characterized by large
targets and the presence of several organs at risk.
Material and Methods
25 previously treated patients of HNC were planned to
achieve the highest dosimetric plan quality with 2 full
coplanar VMAT arcs (CP-VMAT) on 6MV from a Clinac-iX
(Varian), planned with Eclipse version 13.1, calculated
with Acuros. 4pi-VMAT plans were then generated using
same priorities and objectives, using 1 full arc and 4-6
non-coplanar arcs on 2-3 isocenters: typically 1 full arc
with couch at 0°, 2 partial arcs (length of ±210°) with
couch ~±45°, and 2 partial arcs (length of ±250°) with
couch ~±15°. Dose was prescribed on three levels: 70,
60/63, and 56 Gy on targets of median volumes of 720,
492, and 94 cm 3, respectively. The following organs at risk
(OAR) were defined and analyzed: parotids, oral cavity,
esophagus, trachea, larynx, pharyngeal constrictor
muscles, mandible, temporomandibular joint, middle ear,
spinal cord and brain stem. Pre-treatment quality
assurance was performed to assess deliverability and
accuracy of the 4pi-VMAT plans.
Results
CP-VMATand 4pi-VMAT plans achieved the same degree of
coverage for all target volumes related to near-to-
minimum and near-to-maximum doses. 4pi-VMAT plans
resulted in an improved sparing of OARs. The average
mean dose reduction to the parotids, larynx, oral cavity
and pharyngeal muscles were 3Gy, 4Gy, 5Gy and 4.3Gy
respectively. The average maximum dose reduction to the
brain stem, spinal cord and oral cavity was 6.0Gy, 3.8Gy
and 2.4Gy respectively. The average MUs were 525±78 and
548±70 for 4pi-VMAT and CP-VMAT, respectively. The
average simulated beam on time for 4pi-VMAT plans
(612±77 s) was 3.7 times higher than that of CP-VMAT
plans (167±30 s). Pre-treatment QA results showed that
plans can be reliably delivered with mean gamma
agreement index of 97.0±1.1% with 3% dose difference and
3% distance to agreement criteria.
Conclusion
4pi-VMATplans significantly decrease dose-volume metrics
for relevant OARs and results are technically feasible and
reliable from a dosimetric standpoint. Early clinical
experience has begun.
PO-0832 The impact of variable RBE and breathing
control in proton radiotherapy of breast cancer
J. Odén1,2 , K. Eriksson2, A.M. Flejmer3, A. Dasu4, I. Toma-
Dasu1,5
1Stockholm University, Department of Medical Radiation
Physics, Stockholm, Sweden
2RaySearch Laboratories, Department of Research,
Stockholm, Sweden
3Linköping University, Department of Oncology and
Department of Clinical and Experimental Medicine,
Linköping, Sweden
4The Skandion Clinic, Uppsala, Sweden
5Karolinska Institutet, Department of Oncology and
Pathology, Stockholm, Sweden