Abstract Book

S1073

ESTRO 37

produce plans of comparative quality, with the aim of gaining efficiency savings through potential automation and parity with standard planning procedures within the department. Material and Methods VMAT plans were optimised using a 1.5 arc technique for each target volume. This consisted of a 360 o coplanar arc plus a partial arc, close to sagittal but avoiding exit through the entire patient. This technique was applied to a cohort of ten cases previously treated using the dynamic conformal arc iPlan technique. As per local protocol prescriptions were dependent on PTV volume and planned to the 80% isodose. Plan comparison was performed using a range of plan metrics including PTV coverage (D99%), Paddick’s conformity index (PI) [2], gradient index (GI), defined as V50%/V100%, and dose to the normal brain tissue (V12Gy). For consistency these metrics were analysed using the Matlab based package CERR [3] independent of both planning system Results Preliminary results, (Table 1), show that the VMAT technique is capable of producing plans of similar quality to the dynamic Conformal Arc technique in terms of conformality and dose fall-off. The dose homogeneity across the PTV, however, is poorer. The VMAT plans resulted in inferior gradient indices for prescriptions of 21Gy (related to the smallest target volumes), but the reverse is true for other prescriptions.

specification differs significantly from conventional arm- gantry linacs. The department has also three beam- matched Clinacs on-site with 120-leaf Millennium MLC. Halcyon requires Aria v15.1. Its beam model in Eclipse is reconfigured and cannot be changed by the user; the Halcyon is tuned at the factory to conform the beam model and the machine can be tuned on site if necessary. Halcyon has a single 6MV FFF beam energy, with a double-stacked MLC giving a 28cmx28cm maximum field size. Each stack’s leaves project 1cm at the isocentre, and the stacks are offset by 0.5cm giving a potential resolution of 0.5cm: in Eclipse v15.1, only the lower layer of leaves is used for optimisation/field shaping. The rotational gantry speed is up to 2 revs/min to deliver VMAT treatments, and up to 4 revs/min between static- gantry IMRT fields. Halcyon v1.0 has MVCBCT and orthogonal pairs for MV imaging, and daily imaging is mandatory. Eclipse calculates the dose from this imaging and takes it into consideration when optimising, i.e. it is part of the treatment dose. Twenty pelvic cases were planned for intended treatment on Halcyon, with Clinac 10MV plans as back-ups. Eclipse v15.1 AAA algorithm was used to calculate dose for all plans. VMAT plans had two or three full arcs; IMRT plans had five or seven fields, using sliding-window technique. All plans were normalised with prescription dose to median volume. MV CBCTs were obtained on Halcyon and for these the imaging dose was calculated in the plan. Plans were analysed according to department protocols, using DVHs to ensure that OARs and target coverage achieved clinical criteria. Results All plans passed clinical criteria specified in our protocols and were appropriate for treatment. Halcyon plans had up to 25% more MUs than the 10MV Clinac plans. This is to be expected for 6MV compared with 10MV, but also the 1cm-wide Halcyon leaves may not be optimising as efficiently as the Clinac’s central 0.5cm leaves. In the patient scan, the total dose was up to 29% higher in Halcyon plans, but note that this includes imaging dose, which accounts for up to 11.3% of the dose and is not currently calculated in Clinac plans. Conclusion The Halcyon can be used to treat pelvic sites that are currently treated on a conventional 10MV linac, with comparable and clinically-acceptable plan quality. EP-1974 Multi-scenario robustness evaluation; transition to a ‘proton proof’ alternative to PTV evaluation E.W. Korevaar 1 , D. Scandurra 1 , M. Gelderman 1 , R.G. Kierkels 1 , A.C. Knopf 1 , S. Both 1 , M. Unipan 2 , M.G.C. Eenink 3 , J.A. Langendijk 1 1 University of Groningen- University Medical Center Groningen, Department of Radiation Oncology, Groningen, The Netherlands 2 Zuid-Oost Nederland Protonen Therapie Centrum, Maastricht, The Netherlands 3 Holland Proton Therapy Center, Delft, The Netherlands Purpose or Objective When dosimetrists, physicians and physicists review the dose distribution of an intended treatment, PTV coverage is checked to ensure that the CTV receives adequate dose despite patient setup errors. The underlying assumption that the dose distribution is invariant to errors is not generally true in photon treatments and becomes problematic in intensity modulated proton treatments where range errors should be taken into account as well. This limitation is solved in a multi-scenario simulation by explicitly calculating doses for various (typically > 10) error scenarios. Clinical introduction of this method to replace PTV based plan evaluation, however, introduces challenges of how to evaluate multiple dose distributions and what criteria to apply for consistency with (historic)

Conclusion Further development of the VMAT optimisation approach is required to balance the trade-off between dose homogeneity and conformality. References [1] R. Soffietti, P. Cornu, J. Delattre, R. Grant, F. Graus, W. Grisold, J. Heimans, J. Hildebrand, P. Hoskin, M. Kalijo, P. Krauseneck, C. Marosi, T. Siegal and C. Vecht, European Journal of Neurology , 13 : 674-681, 2006. [2] I. Paddick, J Neurosurg (Suppl 3) 93 :219–222, 2000 [3] J. Deasy, A. Blanco and V. Clark, Medical Physics, vol. 30, no. 5, pp. 979-985, 2003. EP-1973 Treatment planning for a 6MV FFF Halcyon linac as a replacement for a 10MV-capable Clinac D. Withers 1 , G. Kidane 1 , D. Farmakidis 1 , L. Crees 1 1 Queen's Hospital, Radiotherapy Physics, Romford, United Kingdom Purpose or Objective To establish that a Halcyon 6MV FFF linac can be used for IMRT and VMAT pelvic cases to replace a 10MV conven- tional linac. Material and Methods The Halcyon is Varian’s new “ring-design” linac that has features facilitating rapid patient throughput and a more comfortable patient experience. The Halcyon’s technical