ESTRO 36 Abstract Book

S846 ESTRO 36 2017 _______________________________________________________________________________________________

Results The optical fixation system was well tolerated by all patients and the stable position of the eye was obtained. The system can be used during planning and treatment without any interference with the photon beam, doses to the eye’s fixation system ranged from 0.03 – 3.71 Gy (median 0.97 Gy, mean 1.15 Gy, ± SD 1.11). Figure 1 showed the treatment plan of one patient treated on the CK-based SRT using the eye’s fixation system. Average treatment parameters of 15 total orbital’s lesions (Table 1). Figure 1. CK-based SRT treatment plan displayed in the axial plane of the patients with a medial orbital tumor in the right eye and multiple beam trajectories.

Table 1. Average treatment parameters of 15 total orbital’s lesions.

EP-1591 Evaluation of noninvasive eye fixation system on Cyberknife radiotherapy of orbital’s tumors A. Skrobala 1,2 , M. Adamczyk 1 , P. Janiga 3 , A. Jodda 1 , J. Litoborska 1 , E. Czajka 3 , J. Dura 3 , P. Milecki 2,3 1 Greater Poland Cancer Centre, Medical Physics Department, Poznan, Poland 2 University of Medical Science, Department of Electroradiology, Poznan, Poland 3 Greater Poland Cancer Centre, Ist Radiotherapy Department-, Poznan, Poland Purpose or Objective Purpose of this study was to evaluate noninvasive eye fixation system on CyberKnife-based stereotactic radiotherapy (CK-based SRT) by the planning and treatment parameters in patients group suffering from orbital’s tumors: ocular melanoma and orbital metastases. Treatment plans for patients with orbital’s tumor undergoing CK-based SRT are often challenging due to the close proximity of optic apparatuses. Material and Methods Feasibility of the eye fixation system was evaluated in the present study using data of 15 patients treated by 1 do 14 with median 5 fractions. The treated PTV volume ranged from 0.73-37.59 cm 3 (median 2.83 cm 3 , mean 7.15 cm 3 , ± SD 11.98). The prescription dose ranged from 20-50 Gy with median value 50 Gy prescribed to the 80% isodose. The eye fixation system was the polymetric device which was attached to the mask system in cranio-caudal orientation and placed in front of the diseased eye (visualization of the system at figure 1 left). The patient was looking at the black point placed at the arm of the eye’s fixation system. The proper position of eye during the imaging and treatment was fixed and the position of the system can be easily adjusted and fixed to the suitable position for any patients. The position of the system was defined by CT and used during treatment delivery. CK- based SRT of orbital’s tumors were evaluated by PTV coverage, conformity index (CI), total number of monitor units (MU), number of beams and the treatment time and the maximum dose to ipsilateral optic nerve were likewise to assess delivery efficiency.

Conclusion The results of this study and good clinical outcomes demonstrated the feasibility of the eye fixation devices for CK-based SRT without the need of implementing fiducial markers. Two years follow up reported that the salvage therapy (enucleation) was unneeded in all cases. CK-based SRT appeared to be a relatively safe technique in treating orbital’s lesions within or near optic apparatuses. CK-based SRT using fixation system provides excellent tumor coverage without delivering excessive dose to critical anterior structure and provided that the dose to the optic apparatuses were limited. The long-term safety of this treatment remains to be confirmed. EP-1592 Higher biological dose to heart and lung in IMPT of medulloblastoma patients due to increased LET E. Rørvik 1 , L.F. Fjæra 1 , C.H. Stokkevåg 2 , S. Thörnqvist 1,2 , K.S. Ytre-Hauge 1 1 University of Bergen, Department of Physics and Technology, Bergen, Norway 2 Haukeland University Hospital, Department of Oncology and Medical Physics, Bergen, Norway Purpose or Objective For medulloblastoma patients, proton therapy offers increased dose conformity compared to conventional radiation therapy. There is however growing concern about the clinical consequences of the enhanced linear energy transfer (LET) delivered to the normal tissue surrounding the tumor volume. In these patients, the biological dose received to the lung and heart may be higher than reflected by treatment plans based on the clinical practice of using a constant relative biological effectiveness (RBE) of 1.1. The aim of this study was therefore to investigate the biological dose to the heart and lung following craniospinal Intensity-modulated proton therapy (IMPT) using LET-based models for the RBE of protons. Material and Methods An intensity modulated proton therapy (IMPT) treatment plan was generated in a commercial treatment planning system(Eclipse, Varian Medical Systems) applying two cranial fields and two spinal fields. The dose prescription