ESTRO 37 Abstract book

S1193

ESTRO 37

Table I were registered. All patients should meet the desirable constraints, but we accept as tolerable the acceptable variation in cases where an OAR is near or overlaps with the PTV. Patients must fulfill this variation for all the OAR. If a patient does not fulfill at least all the acceptable constraints, the fractionation scheme is then changed to 70Gy in 28 fractions (2.5Gy/fx) to avoid acute toxicity and late side effects in the normal tissue.

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 hypo- fractionated 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

Results The dosimetric results for the PTV and organs at risk are shown in Table II. For the PTV, the mean of MU was 461.29 [range: 387.08-683.98]; the mean number of segments was 84.90 [range: 50-121] and the mean treatment time was 66.23 seconds [range: 61.29-127.14]. In addition, approximately 72% of the patients meet all the desirable constraints. A 28% do not meet one or more parameters of the desirable constraints but always meet all the acceptable variation. In these cases a considerable portion of the bladder and/or rectum was overlapping with the PTV. Furthermore, no acute toxicity for any patient was reported during the course of the treatment.

Conclusion According to the results we can conclude that our constraints are acceptable for this fractionation scheme, but a further analysis with more data and a longer time of follow-up to evaluate late toxicity should be done.