ESTRO 36 Abstract Book

S284 ESTRO 36 2017 _______________________________________________________________________________________________

radiotherapy delivery, three treatment plans were made by each institution on each static 3D-CT and on the 4D-CT using the 15mmd animated by the 20bpm/15A signal. Prior to phantom measurements, a BOA was performed in water under reference conditions for the institution chosen energy. The plans were measured twice using EBT3 films and a 0.04cc ionization chamber. The films analysis was done in RIT113. Gamma analyses were performed using film dose as reference, a normalisation at the centre of the sphere, a dose threshold at 20%Dmax and 3%dose/3mm deviation as agreement criteria. Results Volume deviations (VD) ( % true volume) were respectively for the 15mmd and the three motions tested: +10%(+/- 7%), + 1% (+/-17%), +12%(+/-12%) and for the 25mmd: +6%(+/-7%), +4%(+/-7%). . VD were found higher at the end of inspiration than at the end of expiration 8% (+/-26%) insp and 1% (+/-3%) exp. The range of motion was underestimated in all cases: -0,15cm (+/- 0,07cm), the breathing pattern 10bpm presented the largest error - 0,2cm(+/- 0,2cm) compared to the breathing pattern 20bpm -0.09cm(+/- 0,08cm). Regarding the dosimetric evaluation, the output dose mean deviation was 0.57% (+/- 1.42%) across institutions, agreement between chamber and point-planned doses were respectively for the 15mmd and the 25mmd static 98.9%(+/-1.3%), and 99.9%(+/- 2.8%). Agreement with the planned dose (centre of PTV taken as ref. point) for the 15mmd in motion was 98.6% (+/- 0.86%). The film gamma mean pass rates were 70% for 15mmd static, 59% for 15mmd dynamic and 74% for 25mmd static. Conclusion QA of SBRT on moving targets are not yet practice routine, film dosimetry in 4D-conditions are challenging due to the absence of a consortium on where to register the films to the planned dose. Moreover we lack of consistent data to define acceptability thresholds. These results are a starting point, with more dataset we hope to correlate 4DCT and dosimetric data to propose relevant evaluation criteria. OC-0540 A national cranial stereotactic radiosurgery end-to-end dosimetry audit A. Dimitriadis 1,2 , R.A.S. Thomas 2 , A.L. Palmer 3 , D. Eaton 4 , J. Lee 4 , R. Patel 4 , I. Silvestre Patallo 2 , A. Nisbet 5 , C.H. Clark 2 1 University of Surrey, Department of Physics, Surrey, United Kingdom 2 National Physical Laboratory, Radiation Dosimetry, Teddington, United Kingdom 3 Portsmouth Hospitals NHS Trust, Medical Physics, Portsmouth, United Kingdom 4 NCRI, Radiotherapy Trials Group, London, United Kingdom 5 University of Surrey, Physics, Guildford, United Kingdom Purpose or Objective To assess the geometric and dosimetric accuracy of stereotactic radiosurgery (SRS) in the UK for linac-based (LB), TomoTherapy (TT), Cyberknife (CK) and Gamma 26 SRS centres were visited and 28 treatment plans were assessed (16 LB, 7 GK, 4 CK, 1 TT). The audit methodology employed an anthropomorphic head phantom with realistic tissue densities with one irregularly-shaped target (PTV), modelled on a brain metastasis, located centrally in the brain and in close proximity to the brainstem (OAR). The phantom was immobilised, scanned, planned and treated following the local protocol. Previously characterised near-water equivalent dosimeters were placed inside the phantom (EBT-XD film and alanine pellets) to measure absolute dose, both inside the PTV and OAR (Figure 1), and compare with TPS Knife (GK) radiosurgery. Material and Methods

predictions. Film measurements were digitised with triple-channel-correction and compared to TPS dose planes on FilmQA Pro using γ-analysis for a range of global and local criteria.

Figure 1: Schematic representation of detector positions. Results Figure 2 shows the alanine measurements inside the PTV. LB showed the largest range in percentage difference to the TPS of 5.2% (-1.3% to +3.9%) with a mean of +0.5%. CK had a range of 2.6% (+1.4% to +4%), with the highest mean difference in comparison to the other platforms (+2.5%). GK showed the smallest range at 2.4% (-0.8% to +1.5%) being comparable to that of CK, with the smallest mean percentage difference (+0.4%) comparable to that of LB. Similar trends were observed in the OAR with alanine measurements showing a range from -1% to +3.6% (mean= +1.3%), 0% to +1.9% (mean= +0.9%) and -1.1% to +0.9% (mean= +0.1%), for LB, CK and GK respectively. The film measurements showed comparable passing rates between axial and sagittal films, regardless of the platform used. As expected, higher passing rates were observed for Global-γ criteria. For 3%-2 mm Local-γ, all except two films showed passing rates above 75%. For 5%- 1 mm Global-γ, all except 2 films showed passing rates above 90%.

Figure 2: Results of PTV alanine pellets (dotted lines:1σ, dashed lines:2σ). Conclusion This audit enabled the comparison of all participating centres in terms of the accuracy achieved during the delivery. The techniques used differed in many aspects. The LB group showed the largest variations in agreement to the TPS, related to more heterogeneous practices within the group, compared to small variations seen in CK, and more consistent practices seen in GK. Good overall agreement with the TPS was observed with only 3 centres falling above two standard deviations of the mean (2 centres in the target measurements and 1 in the OAR). Film measurements showed comparable γ-passing rates for all centres assessed with small differences between platform groups. The results suggest that good agreement with the predicted dose distributions is achievable by all treatment modalities but highlight the need for standardisation in SRS practices. OC-0541 Automated treatment planning for prospective QA in the TRENDY randomized trial on liver- SBRT for HCC