ESTRO 38 Abstract book

S54 ESTRO 38

a mix of GTV tracking and the ITV concept. For the ITV planned patients no GTV volume was recorded. Results Figure 1a shows that the minimum dose in the PTV (D98%) is not for all centres higher than a BED 10Gy of 100 Gy (for a BED 10Gy of 100 Gy in 3 fractions, a total dose of 42 Gy is needed while for 4 fractions a total dose of 47.5 Gy is needed). Figure 1b shows that all centres had a GTV/ITVmean dose higher than a BED 10Gy of 100 Gy. The intra-centre variation in dose parameters was much smaller than the inter-centre variation.

Conclusion We demonstrated that CMs can be used effectively to predict intrinsic uncertainties in the TPS. These results can be used to indicate the probability of dosimetric errors for a given plan. PV-103 Linking ACROP guidelines to ICRU91: a multicentre study in lung SBRT on prescription and reporting E. De Jong 1 , M. Guckenberger 2 , N. Andratschke 2 , K. Dieckmann 3 , M.S. Hoogeman 4 , M. Milder 4 , D. Sloth Moller 5 , T. Bisballe Nyeng 5 , S. Tanadini-Lang 2 , E. Lartigau 6 , T. Lacornerie 6 , A. Mendez Romero 4 , W. Verbakel 7 , D. Verellen 8 , G. De Kerf 8 , C.W. Hurkmans 1 1 Catharina Ziekenhuis, Department of Radiation Oncology, Eindhoven, The Netherlands; 2 University Hospital Zurich, Department of Radiation Oncology, Zurich, Switzerland; 3 Medical University of Vienna, Department of Radiation Oncology, Vienna, Austria ; 4 Erasmus MC Cancer Institute, Department of Radiation Oncology, Rotterdam, The Netherlands; 5 Aarhus University Hospital, Department of Medical Physics, Aarhus, Denmark ; 6 Centre Oscar Lambret, Department of Radiation Oncology, Lille, France ; 7 VU University Medical Center, Department of Radiation Oncology, Amsterdam, The Netherlands; 8 GZA St-Augustinus Iridium Cancer Network, Department of Radiation Oncology, Antwerp, Belgium Purpose or Objective In 2017 the ESTRO Advisory Committee on Radiation Oncology Practice (ACROP) consensus guideline on implementation and practice of SBRT for peripherally located early stage NSCLC was published, recommending to prescribe 3 x 15 Gy to the PTV as this results in a BED 10Gy >100Gy. Later that year the ICRU published report 91 about prescribing, recording and reporting of stereotactic treatments with small photon beams advocating to also report the mean GTV dose. The purpose of this multicentre study is to establish the link between the ACROP and ICRU-91 recommendations. Material and Methods From each, of the eight participating centres, 15 consecutive clinical treatment plans of patients with peripherally located early stage NSCLC were selected. Treatment plans were generated following the institutional protocol, centres A and B prescribed 3 x 13.5 Gy, centre C 4 x 12 Gy, centres D and E 3 x 15 Gy, centre F 3 x 17 Gy and centres G and H 3 x 18 Gy. Subsequently, dose parameters of the target volumes were reported as recommended by ICRU91 and also converted to BED 10Gy . The centres employed various techniques for motion management. Four centres used an ITV concept while two centres used the mid-ventilation concept, one centre a mix of the ITV and mid-ventilation concept and one centre

Conclusion Three out of 8 ACROP centres did not follow the ACROP guideline in having a BED 10Gy > 100 Gy. This, together with the large variation in dose characteristics between centres raises the question whether or not further harmonization is warranted. PV-104 Out of field dose for three imaging modalities in case of image guided prostate cancer radiotherapy C. Le Deroff 1 , R. Lefeuvre 1 , J. Bouvier 1 , R. De Crevoisier 1,2 , C. Lafond 1,2 1 Centre Eugène Marquis, Radiotherapy, Rennes, France; 2 Univ Rennes- CLCC Eugène Marquis- Inserm- LTSI-UMR 1099, Radiotherapy, Rennes, France Purpose or Objective In prostate image guided radiotherapy (IGRT), a recent randomized trial reported an occurrence of other malignancies two times higher in case of a daily imaging