ESTRO 38 Abstract book

S248 ESTRO 38

We considered a lung treatment plan delivered with step- and-shoot IMRT, and a lung treatment plan delivered with VMAT. We used the Pinnacle 3 treatment planning system (TPS), version 14 (Philips Medical Systems, Eindhoven, The Netherlands) and dose calculations were performed with Pinnacle’s adaptive convolution-superposition (CS) algorithm with a 2 mm grid. In all cases, fields were produced by a 6 MV flattened photon beam delivered by a Varian Clinac® iX linear accelerator equipped with a Millennium 120-multi-leaf collimator (MLC). To assess the performance of the MP512, we used a gamma index analysis (acceptance criteria: 3%/3 mm and 2%/1 mm) considering absolute dose map measurements with the detector, with films and calculations with the TPS. Results 3%/3 mm: gamma passing rates (%GP) were >99% for all plans when comparing the MP512 and films, and =100% when comparing the MP512 and TPS. 2%/1 mm: %GP were >95% for all plans when comparing the MP512 and films, and >96% when comparing the MP512 and TPS. For %GP, we found a better agreement between the MP512 and TPS than with films and TPS. This may be explained by noise artefacts created by scanning procedures and/or film handling, heterogeneities affecting film response, or the higher spatial resolution of films. Selected dose profiles measured with the MP512 and with films are in Figure 1.

London, United Kingdom ; 2 Institute of Cancer Research, CRUK and EPSRC Cancer Imaging Centre, London, United Kingdom ; 3 Rider University, Department of Chemistry and Biology, Lawrenceville, USA Purpose or Objective The Elekta MR-linac (Elekta Unity, AB, Stockholm, Sweden) system comprises a 1.5T Phillips magnet, a 7 MV Linac and a treatment table which cannot be used to correct for target miss-positioning. An MRI scan of the patient position and anatomy is used to localize the target and adapt the treatment plan. In this work, PRESAGE® 3D dosimeters are used together with a lung phantom as an end-to-end test to verify the dose distributions obtained by following this daily adaptation workflow. Material and Methods A CT scan of a dummy PRESAGE® sample (3.5 cm diameter and 5.7 cm length) placed in the central cylinder of the QUASAR™ MRI4D motion phantom was performed. The scan was transferred to Monaco (version 5.4, Elekta AB, Stockholm, Sweden) and an IMRT plan with 5 beams was calculated with a maximum dose to the target of 9.4Gy. Three irradiations were performed using different PRESAGE® samples. One sample (sample 1) was placed at the linac isocenter, verified via EPID images acquired at gantry 0° and 90°, and irradiated as per the plan. The two other samples were irradiated with the phantom away from the isocenter to test the “adapt segments” option of the MR-linac, in which the isocenter shift is accounted for by repositioning the MLC leaves to match the original plan aperture shapes. Prior to the adaptation an MRI scan of the phantom on the table was taken and registered with the planning CT. Sample 2 was irradiated with the phantom placed 3 cm away from the isocenter in the sup- inf direction, while for sample 3 the phantom was moved an additional 3 cm in the left-right direction (Figure 1).

Samples were imaged before and after irradiation with an in-house telecentric optical-CT scanner with a voxel size of ~0.2 mm 3 . PRESAGE® samples have shown a higher sensitivity to radiation at the axial edges, so a correction image, which was obtained previously by uniformly irradiating samples from the same batch, was applied here. Results PRESAGE® measured and Monaco simulated dose distributions agreed well with adapted plans (Figure 2). For a local gamma criterion of 2%, 2mm and 10% threshold, the gamma passing rate was 98.5% for sample 1, 99.7% for sample 2 and 98.4% for sample 3.

Conclusion Results suggest that the MP512 system has the required characteristics for high-resolution (2 mm) absolute dose maps measurements in fully-customizable heterogeneous phantoms in the context of IMRT/VMAT QA. The MP512 could potentially be used to verify dose calculations with a TPS in heterogeneous media. PV-0482 Dosimetric verification of Elekta MR-linac adaption workflow using 3D dosimeters F. Costa 1 , I. Hanson 1 , S. Doran 2 , J. Adamovics 3 , S. Nill 1 , U. Oelfke 1 1 The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Joint Department of Physics,