ESTRO 36 Abstract Book

S182 ESTRO 36 _______________________________________________________________________________________________

3 The Royal Marsden NHS Foundation Trust, Radiation 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, lungs, intestine and stomach; smaller than 5Gy for scalp and kidneys (Figure 2). Moreover, protons provide the smallest non-PTV integral doses (V1Gy: 53% 3D-CRT, 69% photons MT, 15% PBS; V5Gy: 23% 3D-CRT, 43% photons MT, 12% PBS). A considerable variation in PTV and OAR dosimetry was observed within a certain technique.

Conclusion Modern radiotherapy techniques demonstrate superior conformity and homogeneity, and reduced mean dose the OARs compared to 3D-CRT. PBS produced the case with the lowest mean dose for each OAR and integral doses. However, the variability among centres using the same technique means it is not possible to clearly identify the best technique from this data. Efforts should be made to improve inter-centre consistency for each technique. OC-0346 Multicentre audit of SBRT oligometastases plan quality J. Lee 1 , R. Patel 1 , C. Dean 2 , G. Webster 3 , D.J. Eaton 1 1 Mount Vernon Cancer Centre, National Radiotherapy Trials QA RTTQA Group, Northwood, United Kingdom 2 Barts Health NHS Trust, Radiotherapy Physics, London, United Kingdom 3 Worcestershire Oncology Centre, Radiotherapy Physics, Worcester, United Kingdom Purpose or Objective SBRT for oligometastases is currently being used to treat patients at 17 centres in England, as part of the NHS England “Commissioning through Evaluation” programme. The national trials QA group conducted QA for the programme, which included establishing appropriate clinical plan quality metrics for auditing submitted SBRT plans. The purpose of the audit was to inform future guidance on plan quality metric tolerances and help centres determine whether a given plan is optimal. Material and Methods Plans included were either benchmark plans using pre- delineated CT images planned by all cen tres prior to patient recruitment; or plans of initial patients reviewed prior to treatment. VODCA software (Medical Software Solutions) was used for independent plan review. Lung plans were analysed separately due to the inherent