Abstract Book

S1190

ESTRO 37

1 CNAO Foundation, Medical Physics Unit, Pavia, Italy 2 Heidelberg Ion Beam Therapy Center - HIT, Medical Physics Unit, Heidelberg, Germany 3 CNAO Foundation, Clinical Radiotherapy Unit, Pavia, Italy 4 Galliera Hospital, Ocular Oncology Center, Genoa, Italy 5 CNAO Foundation, Clinical Directorate, Pavia, Italy 6 CNAO Foundation, Scientific Directorate, Pavia, Italy 7 European Institute of Oncology - IEO, Scientific Directorate, Milan, Italy Purpose or Objective Only few centers worldwide (around 10) treat intraocular tumors with proton therapy, all of them with a dedicated beamline. Our Centre is a synchrotron-based hadrontherapy facility equipped with fixed beamlines. Proton and carbon ion treatments are delivered with pencil beam scanning modality, with range in water from 3 to 32 cm. Recently, our general-purpose proton beamline was adapted to treat also ocular diseases. This work describes the design and the main dosimetric properties of this new proton eyeline. Material and Methods A 3 cm water-equivalent range shifter (RS) was placed along the proton beamline to shift the minimum beam penetration at shallower depths. Monte Carlo (MC) FLUKA code simulations were performed to optimize the position of the RS and patient-specific collimator, in order to achieve sharp lateral dose gradients. Lateral dose profiles were then measured with radiochromic EBT3 films to evaluate the dose homogeneity and lateral penumbra width at several depths. Different beam scanning patterns were tested. Fine adjustment of beam range was achieved using thin PMMA additional sheets. Eye- dedicated beam settings were implemented in the ocular energy range to decrease the treatment delivery (and gazing) time. The set of low energy proton beams was selected to have 1 mm Bragg Peak separation from 0 to 31 mm water-equivalent depth. Depth-dose distributions (DDDs) were measured with the Peakfinder system. To obtain uniform dose distributions, i.e. Spread-Out Bragg Peaks (SOBPs), the relative weights of each DDD were optimized simulating different beam penetrations and modulations. Absorbed doses were measured in water with an advanced Markus chamber. Neutron dose at the contralateral eye was also measured with passive bubble dosimeters. Results MC simulations and experimental results confirmed that maximizing the air gap between RS and collimator reduces the lateral dose penumbra of the collimated beam and increases the field transversal dose homogeneity. Therefore, RS and brass collimator were placed at about 98 cm (behind beam monitors) and 6 cm from the isocenter, respectively. The lateral penumbra ranged between 1.0 and 1.8 mm, in agreement with other proton therapy eye centers. Figure 1 shows examples of transversal dose profiles measured with EBT3 films at two different depths, compared against MC simulations. The distal fall-off of the DDDs ranged between 1.0 and 1.6 mm, comparable to the ones of most existing facilities. The measured SOBP doses were in very good agreement with MC simulations, as shown in Figure 2. The mean neutron dose at the contralateral eye was 68.8 ± 10.2 µSv/Gy. Beam delivery time, for 52 GyE prescribed dose in 4 fractions, was around 3 minutes per session.

Conclusion Our adapted proton beamline satisfied the requirements for safe and proper treatment of intraocular tumors. The first ocular treatment was delivered in August 2016. So far, around 50 patients have been treated, using the Varian Eclipse proton ocular TPS. EP-2156 A 60 Gy in 20 fractions scheme: a dosimetric analysis for VMAT prostate cancer E.M. Ambroa Rey 1 , R. Gómez Pardos 1 , D. Navarro Jiménez 1 , A. Ramirez Muñoz 1 , J. García-Miguel 1 , N. Feltes Benitez 1 , M. Colomer Truyols 1 1 Consorci Sanitari de Terrassa, Radiation Oncology, Terrassa, Spain Purpose or Objective In recent years, the use of moderate hypofractionated radiotherapy for prostate cancer has increased significantly. Several studies demonstrate that hypofractionated radiotherapy of 60Gy in 20 fractions is non-inferior to conventional radiotherapy fractionation for localized prostate cancer without significant changes in late toxicity. However, there is no consensus regarding the dose volume constraints. The purpose of this study was to analyze the dosimetric results for the organs at risk (OAR) and decide if the dose-volume constraints used in our institution are appropriate for this type of treatment. Material and Methods 70 patients older than 75 years old and with low or intermediate risk prostate cancer were selected for this study. The hypofractionated scheme was 60Gy in 20 fractions (3Gy/fx). VMAT plans calculations were carried out using the Monaco TPS version 5.10 based on a single arc arrangement. The dose-volume constraints were calculated using the linear-quadratic equation, with an alpha/beta value of 1.5 for the prostate and 3 for the OAR, and based on the previous restrictions of 2Gy fractionation. The Linear Quadratic model is reasonably predictive of dose- response relations in the dose per fraction range of 2 to 15 Gy. Of course it goes without saying that no mechanistic model describing dose-time patterns can be fully correct. For every plan, the following data for the PTV was recorded: V95% (%), D50% (Gy), D98% (Gy), D2% (Gy), monitor units (MU), number of segments and beam on time. For the organs at risk the constraints shown in