Abstract Book

S1056

ESTRO 37

OARs). All plans were created by the same physicist using RaySearch System planning (Raysearch laboratories, Stockholm, Sweeden, v5.0) and validated by an experienced radiation oncologist. Doses to target volume and 11 OARs were extracted from the HDV and treatment plans quality indices were derived. Results The mean integral dose was much lower with IMPT compared with VMAT (p<0.001). Planned target volume coverage (V95%) was similar. Conformity (CI) and homogeneity indices (HI) were better with protons for most cases (p<0.001). Mean and maximum doses to organs at risk were significantly lower with protons (p<0.001). In cases of posterior fossa tumor, with OARs in close vicinity to the PTV, a significant gain with protons was observed on dose for the brainstem, on V59 Gy, mean and D2% near maximum doses. Conclusion Dose escalation in pediatric ependymoma is feasible with both techniques but there is a potential significant gain on dose to OARs, including brainstem, with protons. One perspective for this study is LET based optimization. The other one is to stratify the population that could undergo such dose increase based on residue presence [3], advanced imaging factors [5, 6] and molecular biology [7] References 1 - Ducassou et al, ESTRO 2014 2 - Tensaouti, et al. Radiother Oncol. 2017 3 - Massimino et al, Neuro Oncol. 2016 4 - MacDonald, et al.Int J Radiat Oncol Biol Phys. 2008 5 -Tensaouti et al, BJR 2016 EP-1944 Brain stereotactic Radiotherapy : four versus three table rotation position R. Garcia 1 , G. Francois 1 , E. Jaegle 1 , V. Bodez 1 , C. Khamphan 1 , M. Alayrach 1 , A. Badey 1 , P. Martinez 1 1 Institut Sainte Catherine, Physics, Avignon, France Purpose or Objective The implementation of stereotactic treatments with the help of VMAT irradiation were configured in the way to be close to the initial practice based on cones. To optimize the dose distribution, four table rotation positions were fixed in the dose planning process. Even if the dose distributions were not comparable, VMAT offered the benefit of accessing the entire sphere around the cranium and new advantages appeared covering multiple targets with the MC. However the four table position brought to a long session duration. The purpose was to consider removing one table position and validate that the dose distributions remained acceptable. Material and Methods The brain targets are irradiated with the use of VMAT on four table rotations : 0°, 45°, 270° and 315°. The stereotactic brain plans of 15 patients were re- planned, removing the 270° table position and using the same inverse planning parameters. The prescribed total dose was between 24 Gy and 33 Gy. The number of targets were 1 to 3 with volumes between 2 and 41 cc. The collimator rotations were adapted to optimize the MLC effect on the modulation. The comparisons were established with the use of indexes : Homogeneity, Conformity, healthy tissue coverage and target coverage. Other parameters were collected : MU amount, CTV Dmax, PTV Dmax, Brainstem Dmax, Braim Dmean and V10 Gy. The session duration was evaluated for both configurations. Results The dose distributions were compared and found with similar clinical acceptability. 6 - Tensaouti et al, ESTRO 2016 7 – Pajtler et al, cancer cell, 2015

This first analyze was confirmed with the different indexes and parameters. HI, CI, HCO and TCO showed a low effect as the other parameters listed in the joined tabular. The session duration initially between 40 and 50 mn decreased until 30mn. Removal of the 270 ° rotation decreases the spread of low isodoses in the head-feet direction to create a redistributed stretch in the volume of the arc paths of the other 3 angulations. In the case of several target volumes, according to their positions in space, the dose distribution between them varies. This effect depends on the isocenter position, the arc path and the collimator rotations. Unfavorable arcs are those whose image of the BEV includes two target volumes.

Conclusion An ambitious planning configuration for brain stereotaxy has been modified to reduce the session duration. Removing the 270° rotation had no significant dosimetric effect. The new configuration based on three table position is acceptable. The indexes and parameters used show a low effect. The future will be to use new inverse planning softwares which take into account the arcs length and the table positions with variable combinations. EP-1945 Hippocampal avoidance in Prophylactic Cranial Irradiation D. Martínez 1 , L. Bragado 1 , V. Raposo 1 , A. Fernández 1 , M. Rico 2 , N. Fuentemilla 1 , F. Mañeru 1 , S. Pellejero 1 , F. Caudepón 1 , S. Miquélez 1 , A. Rubio 1 1 Complejo Hospitalario de Navarra, Servicio de Radiofiísica y Protección Radiológica, Pamplona, Spain 2 Complejo Hospitalario de Navarra, Servicio de Oncología Radioterápica, Pamplona, Spain Purpose or Objective The loss of neurocognitive functions is the main side effect of prophylactic cranial irradiation (PCI). A number of clinical trials suggest that reducing the dose received by the hippocampus could mitigate this type of toxicity. Hence, the possibility to develop a treatment capable of doing so is being studied by several work groups around the world. In this context, our institution has submitted a number of treatment cases to a national clinical trial about this subject. The purpose of our work is to show and analyze our experience and results. Material and Methods Since October 2015, 8 hippocampal avoidance PCI treatments were performed with LINAC-based IMRT. The treatment planning parameters used to evaluate the treatment plans are given in Table 1 (taken from reference clinical trials).