ESTRO 36 Abstract Book

S443 ESTRO 36 2017 _______________________________________________________________________________________________

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

Purpose or Objective Proton therapy, with or without breathing control techniques, may be used to reduce the cardiopulmonary burden in breast cancer radiotherapy. However, such studies typically assume a constant RBE of 1.1 for protons. This study aims to assess the impact of using a variable RBE in breast proton radiotherapy and to evaluate the sensitivity to respiratory motion when no breathing control is applied. Material and Methods Tangential photon IMRT and 3-fields proton IMPT plans for breast radiotherapy were generated for twelve patients, both on a free-breathing (FB) CT and on a CT using breath- hold-at-inhalation (BHI). 2 Gy(RBE) per fraction in 25 fractions were planned to the whole breast. The physical proton dose was optimized assuming a constant RBE of 1.1. Besides the constant RBE of 1.1, the variable RBE-model by Wedenberg et al. (2013), assuming an α/β of 3.5 Gy for the CTV/PTV and 3 Gy for the OARs, was used for plan evaluation. Subsequently, the FB plans were recalculated on the CT images of the two extreme phases (inhale and exhale) to evaluate the sensitivity of a treatment delivery without breathing control. Results All photon and constant RBE proton plans met the clinical goals with similar target coverage. The target conformity and homogeneity of the proton plans were superior to the photon plans. The plan quality was generally independent on whether the FB or BHI CT-scan was used. However, if the heart was close to the target, the BHI plan lowered the dose to the left anterior descending (LAD) artery in most cases. Applying the variable RBE-model resulted in an average of the mean RBE of 1.18 for the PTV and also increased the heterogeneity. The predicted RBE values in the OARs were also substantially higher than 1.1. However, due to the low physical doses, this is expected to have a minor impact. The dosimetric parameters for the BHI plans are shown in Table 1. The recalculation of the FB plans on the extreme phases generally resulted in minor differences for the CTV coverage and OAR doses for the proton plans. Small CTV volumes may, however, receive a slightly lower dose for the recalculated photon FB plans. The ranges of dosimetric parameters for the FB plan for one patient are shown in Table 2.