Purpose/Objective: We present a Fluka Monte Carlo (MC) simulation study for proton treatments based on Varian Eclipse proton plans for left breast cancers including the internal mammary chain. Doses calculated in TPSs are usually accurate within the primary field, but less accurate outside. With the MC treatment verification we can not only find possible inaccuracies in the delivered dose to the regions of main interest (PTV and OAR like heart and left lung) mainly due to body inhomogeneities close to the PTV (i.e., ribs and lung), but also find the accurate dose deposited in other healthy tissues not included in the TPS optimisation (contra lateral lung, contra lateral breast). Doses deposited by other particles not generated in the TPS, but usually created during a real treatment, such as neutrons and light ions, are also accounded for in the MC. All of this allows us to more accurately determine toxicity to healthy tissues and risk of secondary malignancies. Materials and Methods: Before starting any comparison between the MC and the TPS, the virtual CAP GENERAL machine used in the proton Eclipse TPS had to be accurately characterised. In this context studies were performed using virtual water phantoms to determine the beam characteristics (energy, energy spread, spatial spread) and so to prove the equivalence of Eclipse plans in Fluka. Eighteen different patient plans for breast treatments were then performed using Eclipse and the information about the used beams included in the TPS RTPLAN files were extracted and converted to plans in Fluka. This allowed us to perform dose comparisons between plans and simulations in the entire upper body of the patient. Results: In this work we developed a method of performing treatment verification of Eclipse breast plans with the Monte Carlo Fluka and show that dose differences up to 4% in the PTV and up to 70% in other healthy tissues (i.e., contra lateral breast) can be observed when the TPS plans are compared with Fluka simulations of the same plans. Conclusions: While the Varian TPS for conventional radiotherapy is widely used, the proton Eclipse TPS is only used in a few facilities, resulting in a dearth of comparison studies. In particular breast treatments with protons are in the early stages of investigation and not many groups so far have demonstrated the accuracy of TPSs with respect to this treatment site. With this study we demonstrate that treatment verification with Monte Carlo simulations is essential to better determine the dose deposited in the entire patient body, and in particular to the healthy tissues. EP-1459 Dosimetric evaluation of arc-based modulated electron radiation therapy A. Joosten, D. Henzen, W. Volken, D. Frei, K. Lossl, P. Manser, M.K. Fix Inselspital Bern University, Division of Medical Radiation Physics and Department of Radiation Oncology, Bern, Switzerland