ESTRO 36 Abstract Book

S442 ESTRO 36 2017 _______________________________________________________________________________________________

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.

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 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 proton pencil beam. Material and Methods

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