Abstract Book

S1052

ESTRO 37

4X-4X as compared 6X-6X, 4X-6X for left lens, right lens and small bowel (p>0.05). In addition, decrease of NTID, volume receiving dose >5Gy and >10Gy were found in 6X- 6X as compared 4X-4X and 4X-6X (p>0.05). However, 6X- 6X showed lesser MU of 8.92% compared to 4X-4X and 15.11% to 4X-6X (p<0.05). Whereas, delivery time was lesser in 6X-6X as compared to 4X-4X and 6X-6X (p<0.05). Conclusion For CSI VMAT plans, 6MV found to be more suitable photon energy as compare to 4MV and 4MV&6MV in terms of better plan quality, higher dose rate, shorter treatment time and lesser MU. EP-1937 Dosimetric impact of stabilizing spinal implants in proton therapy: titanium vs. carbon systems B.S. Müller 1 , Y.M. Ryang 2 , F. Ringel 3 , M. Oechsner 1 , H. Specht 1 , B. Meyer 2 , S.E. Combs 1,4 , J.J. Wilkens 1 1 Klinikum rechts der Isar- Technical University of Munich, Department of Radiation Oncology, Munich, Germany 2 Klinikum rechts der Isar- Technical University of Munich, Department of Neurosurgery, Munich, Germany 3 University of Mainz, Department of Neurosurgery, Mainz, Germany 4 Institute of Innovative Radiotherapy, Helmholtz Zentrum München, Neuherberg, Germany Purpose or Objective Over the last years carbon pedicle screw systems were introduced for spinal stabilization, which promise to reduce dosimetric uncertainties in radiotherapy (RT) compared to metallic implants. We investigated the impact of uncertainties in assigned relative stopping powers (SP) of spinal stabilizing implants on dosimetric quality for intensity modulated proton therapy (IMPT). In particular we compare titanium to carbon systems. Material and Methods The retrospective planning study is based on computed tomography (CT) images of ten patients, who had undergone surgery for spinal stabilization prior to RT treatment, and were previously treated with photon RT at our institute. Five patients had titanium (TI), five carbon (CA) systems. Carbon pedicle systems (Icotec, Altstätten, Switzerland) comprised radiolucent non- metallic screws, titanium coated regions in the pedicle area and titanium rods. To simulate uncertainties in the assigned Hounsfield units (HU) of titanium, corresponding regions were delineated in all CTs and assigned with varying SPs. For each patient, one IMPT plan of three fields was optimized onto the planning CT with a SP of 3.2 (prescribed dose to planning target volume (PTV): 30 Gy à 3 Gy). These plans were recalculated onto the identical CT with reduced SP (SP=3.0) and increased SP (SP=3.4). Dose distributions were analyzed by PTV coverage, homogeneity and several dose volume histogram (DVH) values. Additionally, CT artifacts were contoured and resulting volumes evaluated. RT planning was performed in matRad (www.matrad.org). Results Volumes of CT artifacts and metallic components were smaller in CTs of CA compared to TI systems, with mean volumes of V CA (artifacts)=5.9±1.2 cm 3 vs. V TI (artifacts)=18.7±7.4 cm 3 and V CA (metal)=10.1±0.2 cm 3 vs. V TI (metal)=24.4±5.4 cm 3 . The initial plan qualities of both implant types were comparable, with a slightly superior PTV coverage and homogeneity in CA plans. The impact of varying SPs on DVH criteria was larger for TI. For a SP increase of 0.2, i.e. SP=3.4, the average minimum dose received by 1 cm 3 decreased by ΔD min (PTV)=5.9±2.9% for TI and ΔD min (PTV)=1.5±0.5% for CA (see figure, with box plots showing the median minimum dose, its interquartile range (box) and most extreme values). Recalculated doses of both implant types resulted in more heterogeneous target coverage,

Conclusion The plans of cyberknife with incise MLC is known to require long beam-on times and not show the best conformity, but achieves the shortest dose gradient range than other delivery techniques. EP-1936 A study on craniospinal irradiation in VMAT treatment use of different photon energy beams P. Mohandass 1 , D. Khanna 2 , N.K. Bhalla 1 , D. Manigandan 3 , C. Saravanan 1 , A. Puri 1 1 Fortis Hospital, Radiation Oncology, Mohali, India 2 Karunya University, Department of Physics - School of Science and Humanities, Coimbatore, India 3 Medanta The Medicity Hospital, Radiation Oncology, Gurgoan, India Purpose or Objective To study plan quality and deliverability on craniospinal irradiation (CSI) use of different photon energy in volumetric modulated arc therapy (VMAT). Material and Methods Three CSI patients treated with 23.4Gy/13fractions followed by boost dose were chosen for this study. All the plans were generated using VMAT technique for Elekta Synergy™ linear accelerator with 1cm leaf width at isocenter. In each case, 6MV (6X-6X) photon beam along with dual partial arcs were used for cranium (50°– 180°&180°–310°), upper spine (125°–180°& 180°–235°) and lower spine (110°–180°&180°–250°). Keeping all other parameters constant, by changing only photon energy, plans were re-planned for 4MV (4X-4X) and 4MV&6MV (4X-6X). For consistency, plans were generated by the same planner using Monaco™V5.1 treatment planning system (TPS). For plan evaluation, PTV covered by 95% of prescribed dose, conformity index (CI) and homogeneity index (HI) were analyzed. For organ at risk (OAR), mean dose and dose volume received by left lung, right lung, left eye, right eye, left parotid, right parotid, left kidney, right kidney, heart and liver were analyzed. Moreover, max dose to left lens, right lens and small bowel were analyzed. In addition, normal tissue (i.e., patient volume minus PTV) volume receiving dose >5Gy&>10Gy, normal tissue integral dose (NTID) (liter-Gray), total monitor unit (MU) and delivery time were compared. Results The CI for PTV was 1.0977±0.0165, 1.1063±0.0319 and 1.1083±0.1159 for 4X-4X, 6X-6X and 6X-4X respectively (p>0.05). Similarly, for HI was 0.0948±0.0078, 0.0895±0.0115 and 0.0941±0.0397 (p>0.05). The PTV coverage by 95% of prescribed dose slightly decreased in 4X-6X as compared to 4X-4X & 6X-6X (p>0.05). Moreover, the max dose was higher in 4X-6X as compared to 4X-4X & 6X-6X (p>0.05). The mean dose and dose volume received by left eye, right eye, left parotid, right parotid, left kidney, right kidney, heart and liver were showed less dose for 4X-4X as compared to 6X-6X, 4X-6X (p>0.05). Similarly, the slight increase of max dose was observed in