ESTRO 37 Abstract book

S1058

ESTRO 37

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.

The prescribed dose in all cases was 25 Gy delivered in 10 days. All patients underwent both a CT and a MRI in order to delineate the different organs. Contouring of the brain and the OAR was made following the RTOG indications. A 5 mm PRV was defined for the hippocampus. Finally, the PTV was created adding a 5 mm margin to the brain and subtracting the hippocampal PRV. For inverse planning optimization, Dose Volume Optimi- zation of Varian Medical Systems was used. Each treatment consisted of a total of 11 fields with 4 couch angles: 0, 45, 90 and 315 degrees. Results Target coverage and dose received by the hippocampus in each treatment are shown in Table 2.

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).

Comparing the previous results with the specifications given by reference clinical trials, it can be seen that all treatments satisfied the minimum goal of D95 ≥ 95%. Also, all the values of D 2 were below the most demanding limit of 107%. On the other hand, D 98 results didn’t comply with the optimal treatment specifications given in Table 1, although they were well above the acceptable treatment ones. Finally, doses received by hippocampus and the rest of OAR were under the limits proposed by reference clinical trials. Conclusion Our experience shows that it is possible to achieve a good PTV coverage while reducing the dose received by the hippocampus, compared to classical PCI treatments. Taking into account the good preliminary results obtained in clinical trials, the future of this planning technique looks promising. EP-1946 Effect of small field (SF) output factor (OF) measurements on FSBRT VMAT plans M.D. Falco 1 , S. Clemente 2 , E. Cagni 3 , V. Landoni 4 , S. Russo 5 , C. Talamonti 6 , A. Alparone 7 , A. Vinciguerra 1 , C. Fiandra 8 1 University "G. D’Annunzio”- SS. Annunziata Hospital, Radiation Oncology Department, Chieti, Italy 2 AOU "Federico II", Medical Physics Department, Napoli, Italy 3 AUSL-IRCCS, Medical Physics Department, Reggio Emilia, Italy 4 Istituto Nazionale Tumori Regina Elena, Medical Physics Department, Roma, Italy 5 Azienda USL Toscana Centro, Medical Physics Department, Firenze, Italy 6 AOU Careggi, Biomedical Experimental and Clinical Science "Mario Serio" Department, Firenze, Italy 7 Tecnologie Avanzate, Medical Physics Unit, Torino, Italy 8 Università di Torino, Radiation Oncology Department, Torino, Italy