ESTRO 37 Abstract book

S1137

ESTRO 37

Conclusion The EPID-based commissioning of Adaptivo© is an encouraging and original approach with a fast clinical implementation. The automatic export and dose calculation make the procedure low-time consuming. This daily-dose monitoring may become a near future clinical routine process. Despite these perspectives, one should consider the absence of elastic deformation evaluation as a possible weakness for the results’ reliability. EP-2071 Determination of the flattening filter free beam penumbra of the MR-Linac without a reference beam W. Van den Wollenberg 1 , A.J.A.J. Van de Schoot 1 , J. Kaas 1 , T. Perik 1 , D. Roberts 2 , T.M. Janssen 1 , J.J. Sonke 1 , M.F. Fast 1 1 Netherlands Cancer Institute, Radiation Oncology, Amsterdam, The Netherlands 2 Elekta Ltd., Linac House, Crawley, United Kingdom Purpose or Objective The MR-Linac (Elekta Unity, Elekta AB, Stockholm, Sweden) combines a 7 MV linac with a 1.5 T MRI scanner. The machine design and the presence of a magnetic field substantially influence the beam profile and in particular its penumbra. Quantifying the shape of this penumbra is important in order to assess the ability to achieve dose conformity and to derive appropriate PTV margins for treatment planning. For a conventional linac the flattening filter free (FFF) penumbra is typically defined as the spatial distance between the 80% and 20% dose fall-off points after scaling the beam profile to a flattening filter (FF) reference beam [Pönisch et al., Med. Phys., 2006]. However, this parametrization is inadequate for the MR-Linac due to the lack of an FF reference beam profile. We therefore propose a novel general beam penumbra parametrization. Material and Methods The novel furthest-point method involves tracing a line from the center-axis of a beam profile to the 20% dose point (figure 1). The shoulder point is defined as the point on the profile furthest from this line. The profile is then scaled so that the shoulder point has a dose equal to 95% of the original center dose. Afterwards the spatial distance between the 80% and 20% dose fall-off points can be used. To ensure compatibility of the conventional and proposed penumbra definitions both were tested for profiles of the Elekta Agility MLC (6MV and 10MV, FF and FFF) for various field sizes. Next, the beam penumbrae of the MR-Linac were quantified. Profiles were obtained both by direct measurements in a water tank at a depth of 10 cm and by Monte Carlo (MC) simulation using the Monaco (research) treatment planning system. Results The conventional and new penumbra definitions agree for the Agility to within 0.2 mm for all investigated beams. For the MR-Linac the inline (leaf) penumbra varies between 4-8 mm in water depending on the field size (table 1) and was found to be comparable to the Agility 6MV FFF. The electron-return effect introduces a crossline (diaphragm) penumbra asymmetry of up to 1.5 mm. For the MC simulations, the MR-Linac penumbra varies between 5-10 mm in ICRU-44 lung tissue depending on the profile orientation and field size (table 1). Here the penumbra asymmetry in crossline direction is up to 2.8 mm. To test the new definition’s robustness to experimental and MC noise the 20% dose point was varied between 40- 10%, and the location of the shoulder point by 1 mm. Both had only a <0.2 mm effect on the resulting penumbra size. In addition, because the shoulder point in an FF profile lies around the 95% dose point, when

of ‘normal’ or ‘coarse’ do not meet the defined gamma criterion. On chest phantom, all registration errors larger than 1 mm appeared at superior-inferior axis, which cannot be avoided with the smallest AP and RI. On pelvic phantom, craniocaudal errors are much smaller than chest, however, AP of ‘coarse’ presents larger registration errors which can be reduced from 2.90mm to 0.22mm by registration technique of ‘full image’. Conclusion AP of ‘coarse’ with RI of 6mm is recommended in adaptive radiotherapy (ART) planning to provide craniocaudal longer and faster MVCT scan, while registration technique of 'full image' should be used to avoid large residual error. Considering the trade-off between IGRT and ART, AP of ‘normal’ with RI of 2mm was highly recommended in daily practice. EP-2070 Commissioning of Adaptivo© for adaptive radiation therapy: first retrospective results. M. Antoine 1 , S. Tolsa 1 , P. Sargos 1 , A. Petit 1 , J. Caron 1 , E. Blais 1 , G. Kantor 1,2 , A. Cugny 1 1 Institut Bergonié, Department of Radiotherapy, Bordeaux Cedex, France 2 Univ. Bordeaux, F-33000, Bordeaux, France Purpose or Objective The clinical use of Adaptive radiation therapy (ART) solution is a promising field of research for both physicist and radiation oncologist. Adaptivo© is an offline strategy- based solution which could be useful to evaluate the quality of the radiotherapy sequence. Based on Cone- Beam-CT (CBCT) dose calculation and elastic-propagated structures, it provides daily and cumulative (back- projection of the dose in the CT space) Dose-Volume Histogram (DVH). First, we describe the different steps required for the commissioning. Second, we present our retrospective results obtained from two pelvic cases. Material and Methods The commissioning first step involved an automatic Adaptivo© template creation of twenty two different beams. Each of these beams was delivered onto a previously calibrated Electronic portal imaging device (EPID). Once the beam model generated, acquired and calculated profiles were compared. Secondly, an end-to- end treatment case was realized. A basic treatment plan based on a 15 cm CT-scan of water slabs was made with 8 beams equally oriented around the phantom. An Image Values to Density Table (IVDT) was established on the pelvis mode with a Catphan600© phantom. Then, each predicted and acquired dose profiles were compared. For the retrospective study, one prostate and one gynecological case were investigated with both daily and cumulative DVH approaches. For the two cases, the maximum PTV D 95% daily variations (PTV-D 95% -variation), the maximum bladder D 50% daily variations (bladder-D 50% - variation) and), the maximum rectum D 50% variations (rectum-D 50% -variation) daily variations were compared to the initial CT-planning. Results For the commissioning process, acquired and calculated profiles were superposed (<0.5% difference) on the central part of the beam (0 to 5 cm) whereas up to 5% differences were observed on the off-axis part (above 5 cm). For the end-to end case, each of the 8 beams had a superposition within less than 1% of difference. Concerning the prostate case, the bladder-D 50% -variation and the rectum-D 50% -variation were ≤ 8% while PTV-D 95% - variation was ≤ 3%. For the cumulative DVH, PTV D 95% and rectum D 50% variations were ≤ 3% while bladder D 50% differences reached 5%. Concerning the gynecological case, rectum-D 50% -variation and bladder-D 50% -variation reached 9% and 5% respectively. For the cumulative DVH, PTV D 95% , bladder D 50% and rectum D 50% differences were ≤ 3%.