ESTRO 38 Abstract book

S461 ESTRO 38

differences in dose to OARs and no significant differences in PTV homogeneity or conformity were observed.

(n=5). Following PBT, cerebral vessel volume (CVV) decreased with time (p=0.02), and this effect was more notable in younger patients undergoing RT (interaction variable, p<0.01). CVV was also found to be positively associated with language-related TCB domains [composite cognitive function (p<0.01), crystallized function (p=0.02), and picture vocabulary tests (p=0.05)], which all decreased from baseline during follow up (p<0.01 for each test) in these patients. On dosimetric analysis, total brain volume receiving 10 Gy (V10) was found to be the strongest predictor of TCB testing with worse outcomes found with higher values of V10. Conclusion Based on this prospective study, use of RT is associated with decreases in performance in neurocognitive testing, and this may be mediated through declines in cerebral vessel volume. Ultimately, limiting dose to total brain volume and maintaining a low V10 may help to mitigate these changes, though further follow-up and studies are needed. PO-0874 The feasibility of MR-Linac treatment planning in childhood abdominal Neuroblastoma M. Llewelyn 1 , G. Smyth 2 , N. Lavan 1 , S. Nill 2 , U. Oelfke 2 , H. Mandeville 1 1 The Royal Marsden NHS Foundation Trust, Clinical Oncology, London, United Kingdom ; 2 The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust- Sutton- London- UK, Joint Department of Physics, London, United Kingdom Purpose or Objective Radiotherapy plays an important role in the management of paediatric abdominal malignancies. Treatment planning techniques are often highly complex due to large target volumes, the close proximity of OARs, and the challenge of ensuring uniform future vertebral bone growth. The Elekta Unity MR-Linac (Elekta AB, Stockholm, Sweden) enables highly detailed real-time imaging while avoiding the concomitant doses of daily kV imaging. However, the dosimetric effects of a magnetic field on treatment plan quality for paediatric abdominal radiotherapy are unknown. This study aims to explore the feasibility of IMRT planning with the presence of a magnetic field for abdominal malignancies. Dosimetric comparisons of target volumes and OARs between conventional linac and MR-Linac based treatment plans are made. Material and Methods Ten postoperative paediatric patients with abdominal Neuroblastoma were identified and two nine-field IMRT plans were produced for each child. One plan was produced for the MR-Linac, accounting for the effects of the 1.5T magnetic field, and another for a conventional linac (Monaco V5.19, Versa HD, Elekta AB, Stockholm, Sweden). All plans were produced to deliver a median PTV dose of 21 Gy in 14 fractions and were planned to observe clinical dose constraints. Dose calculation was performed using a Monte Carlo dose engine and a calculation uncertainty of 2%, grid size 3x3x3mm. Doses to PTV, adjacent vertebral bodies and OARs were compared and Homogeneity Index (HI) and Conformity Number (CN) were measured for each technique. Results All plans met required dose objectives and were clinically acceptable; Mean PTV volume was 259.1 cm 3 (range 143- 498 cm 3 ). There were no significant differences in PTV near maximum (D 2% ) or near minimum (D 98% ) doses between MR-Linac and conventional linac. For those tumour sites where there was a component of lung irradiation (n=7), MR-Linac treatment plans achieved a small but significant reduction in the volume of lung receiving 15 Gy (p=0.03) compared to the conventional linac. There were no other statistically significant

Conclusion MR-Linac treatment planning for upper abdominal malignancies in children is feasible and produces clinically acceptable plans. Achieved dose distributions were comparable to a conventional linac, despite large treatment volumes and the high complexity of plans. Combining plan quality with more accurate visualisation of abdominal soft tissues and improved delineation of target volumes, the MR-Linac has the potential to reduce setup errors and further decrease normal tissue radiation exposure/ integral dose compared with radiotherapy on a conventional linac. PO-0875 Development of pituitary deficits after radiotherapy in pediatric patients after long follow-up. C. Satragno 1 , E. Tornari 1 , S. Barra 2 , F. Giannelli 2 , L. Belgioia 1 , M. Giaccardi 3 , N. Di Iorgi 3 , M.L. Garre 4 , R. Haupt 5 , R. Corvò 1 1 University of Genoa and San Martino Hospital, Health Science Department DISSAL and Radiation Oncology, Genoa, Italy ; 2 San Martino Hospital, Radiation Oncology, Genova, Italy ; 3 Istituto Giannina Gaslini, Department of Pediatrics, Genoa, Italy ; 4 Istituto Giannina Gaslini, Department of Neurooncology, Genoa, Italy ; 5 Istituto Giannina Gaslini, Epidemiology and Biostatistics Unit, Genoa, Italy Purpose or Objective To evaluate the risk of developing pituitary deficits in pediatric patients with central nervous system (CNS) tumors treated by radiotherapy. Material and Methods We retrospectively analyzed patients with brain tumors, younger than 16 years, with a minimum follow up of 2 years after the end of radiotherapy. In this study, we evaluated GH, ACTH and TSH deficiency and if endocrine deficit correlate with pituitary gland and hypothalamous dose. Results Between 1996-2015, 88 consecutive patients have been identified. Among them, we had to exclude 20 patients because they presented hormone deficiency before starting RT, and 4 more because they had the