ESTRO 36 Abstract Book

S180 ESTRO 36 2017 _______________________________________________________________________________________________

Oncology, Sutton, United Kingdom 4 Institute Curie, Radiation oncology, Paris, France 5 West German Proton Therapy Center Essen, Clinic for Particle Therapy, Essen, Germany 6 Instituto nazionale dei tumori, radiation oncology, Milano, Italy 7 Radboud university medical center, Department of Radiation Oncology, Nijmegen, The Netherlands 8 Addenbrooke's Hospital, Radiation Oncology, Cambridge, United Kingdom 9 Hygeia Hospital, Medical physics department, Athens, Greece 10 Radboud university medical center, radiation oncology, Nijmegen, The Netherlands 11 Aarhus University Hospital, radiation oncology, Aarhus, Denmark 12 Oslo University Hospital, Radiation oncology, Oslo, Norway 13 The Christie NHS Foundation Trust, Radiation oncology, Manchester, United Kingdom 14 AMC, radiation oncology, Amsterdam, The Netherlands 15 Timone hospital, radiation oncology, Marseille, France 16 Hygeia Hospital, MEidcal Physics, Athens, Greece 17 Santa Chiara Hospital, Proton therapy Center, Trento, Italy 18 Aarhus University Hospital, Medical Physics, Aarhus, Denmark 19 University Medical Center Utrecht, Radiation Oncology, Utrecht, The Netherlands Purpose or Objective The craniospinal irradiation (CSI) is challenging due to the long target volume and the need of field junctions. The conventional 3D-CRT technique (two lateral opposed cranial fields matched to a posterior field) is still widely adopted. Modern techniques (MT) like IMRT, VMAT, Tomotherapy and proton pencil beam (PBS) are used in a limited number of centres. A multicentre dosimetric analysis of five techniques for CSI is performed using the same patient, set of delineations and dose prescription. We aimed to address two questions: Is the use of 3D-CRT still justifiable in the modern radiotherapy era? Is one technique superior? Material and Methods One 14 year-old patient with medulloblastoma underwent a CT-simulation in supine position. The CTV and OARs were delineated in one centre. A margin for PTV was added to CTV: 5 mm around the brain and spinal levels C1-L2, 8 mm for levels L3-S3. Fifteen SIOP-E linked institutes, applying 3D-CRT, IMRT, VMAT, Tomotherapy, or PBS (three centres per technique), were asked to return the best plan applicable for their technique: high weighting for PTV coverage (at least 95% of PTV should receive 95% of the prescribed dose) and low weighting for OAR sparing. Plans for a prescription dose of 36 Gy were compared within and between techniques, using a number of dose metrics: Paddick conformity (range 0-1, with 1 being highly conformal), and heterogeneity (range 0-1, with 1 being highly heterogeneous) indices for brain and spine PTVs, OAR mean doses and non-PTV integral doses. Results Conformity- (range 0.75-0.90) and homogeneity (range 0.06-0.08) indices of brain PTV were similar among all techniques. However for the spinal PTV inferior indices (CI: 0.30 vs 0.61 HI: 0.18 vs 0.08) are observed for 3D-CRT with respect to modern techniques (Figure 1). Compared to more advanced photon techniques, 3D-CRT increased mean dose to the heart (13Gy vs 8Gy), thyroid (28Gy vs 15Gy), and pancreas (17Gy vs 12Gy) but decreased dose to both kidneys (4Gy vs 6Gy) and lungs (6Gy vs 8Gy) (Figure 2). PBS reduced the mean dose to the OARs compared to all photon techniques: a decrease of more than 10Gy was found for parotid glands, thyroid and pancreas; between 5-10Gy for lenses, submandibular glands, larynx, heart,

We conducted the neutron measurement under the collaboration with National Institute of Standards and Technology (NIST). We employed Bubble detectors (BTI, Canada) to measure the neutron dose and energy spectrum with good spatial resolution. The detectors provide six energy thresholds from 10 keV to 10 MeV allowing to validate dose and the neutron energy spectrum. To simulate neutron scatter, a polyethylene cylindrical phantom was milled and the bubble detectors were placed inside. The phantom was then irradiated with a Californium-252 neutron source to simulate the secondary neutrons. We also simulated the experiment in TOPAS to compute the neutron dose and energy spectrum for comparison (Figure 1). Results

The measured spectrum was unfolded and shows to be in good agreement with the simulation. On average, the percent difference in the spectrum was less than 31% (Graph 1) and the percent difference of dose was under 23%. The agreement was best at the neutron energies 10 keV – 100 keV (19 %) and worst at 2.5-10 MeV (91 %). Better statistics are needed for the higher energy spectrum region. We plan to conduct the measurement three times to minimize statistical errors and plan to extend the validation to anthropomorphic physical phantoms. Conclusion We validated the dose and energy spectrum of scattered neutrons computed from TOPAS Monte Carlo code by the measurements using Bubble Detector. We plan to utilize TOPAS dose calculation system coupled with patient- specific proton therapy data for normal dose calculations to support epidemiological studies of proton therapy patients. OC-0345 Comparing cranio spinal irradiation planning for photon and proton techniques at 15 European centers E. Seravalli 1 , M. Bosman 2 , G. Smyth 3 , C. Alapetite 4 , M. Christiaens 5 , L. Gandola 6 , B. Hoeben 7 , G. Horan 8 , E. Koutsouveli 9 , M. Kusters 10 , Y. Lassen 11 , S. Losa 4 , H. Magelssen 12 , T. Marchant 13 , H. Mandeville 3 , F. Oldenburger 14 , L. Padovani 15 , C. Paraskevopoulou 16 , B. Rombi 17 , J. Visser 14 , G. Whitfield 13 , M. Schwarz 17 , A. Vestergaard 18 , G.O. Janssens 19 1 UMC Utrecht, Department of Radiation Oncology, Utrecht, The Netherlands 2 University Medical Center Utrecht, Radiotherapy, Utrecht, The Netherlands 3 The Royal Marsden NHS Foundation Trust, Radiation Proffered Papers: Treatment planning applications