ESTRO 36 Abstract Book

S447 ESTRO 36 _______________________________________________________________________________________________

Oncology, New Delhi, India 3 Humanitas 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. Odén 1,2 , K. Eriksson 2 , A.M. Flejmer 3 , A. Dasu 4 , I. Toma- Dasu 1,5 1 Stockholm University, Department of Medical Radiation Physics, Stockholm, Sweden 2 RaySearch Laboratories, Department of Research, Stockholm, Sweden 3 Linköping University, Department of Oncology and Department of Clinical and Experimental Medicine, Linköping, Sweden 4 The Skandion Clinic, Uppsala, Sweden 5 Karolinska Institutet, Department of Oncology and Pathology, Stockholm, Sweden

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