ESTRO 36 Abstract Book

S967 ESTRO 36 _______________________________________________________________________________________________

Conclusion The ACDS is developing a comprehensive suite of audit modalities aimed at ensuring patient safety across a range of clinical practice. The new Level III (end-to-end) audit joins the integrated range of multi-modality audits provided by the ACDS including the Level Ib audit (on-site linac output) covering reference beams, FFF and small fields and the Level II audit (slab phantom combined with array) audit covering conformal, IMRT, VMAT and FFF treatments plans. EP-1756 Treatment planning and dosimetric validation of bone oligomet SABR treatments on TomoTherapy C. Thomas 1 , T. Burrows 1 , R. Lynn 1 , N. Milesi 1 , S. Petty 2 , M. Stenson 1 , K. Blythe 1 , T. Greener 1 1 Guys and St Thomas NHS Foundation Trust, Medical Physics, London, United Kingdom 2 Guys and St Thomas NHS Foundation Trust, Radiotherapy Department, London, United Kingdom Purpose or Objective To establish whether the TomoTherapy helical delivery system can accurately deliver high dose per fraction SABR treatments to bone oligo-metastases within the NHS England Commissioning through Evaluation SABR program. Material and Methods TomoTherapy Volo treatment planning system was used to generate example SABR treatment doses of 10Gy and 15Gy per fraction to a cylindrical PTV within a CT dataset of the Delta-4 phantom. These treatments were delivered to the Delta-4 phantom. Treatment plans for clinical oligo- metastases in bone, with prescription doses of 27Gy/3# and 30Gy/5# were generated and delivered to Delta-4 phantom, ionisation chamber and Gafchromic film. Clinical treatment fractions were delivered in 2 half- fractions in order to allow a mid-fraction imaging scan to assess intra-fraction motion. Results Volo treatment planning system signalled when the treatment planning objectives were not deliverable and suggested modified treatment planning parameters. The test cases measured on Delta-4 passed local gamma analysis at 3%/3mm with >95% pass rate. Paddick and CIRTOG conformity indices were improved with the use of TomoTherapy compared with VMAT solution for the first clinical case, and dose gradient between target and critical structures was improved. For the first clinical case measured on Delta-4, 100% of sampled detectors passed within 3%/3mm gamma analysis and 98.5% passed within 2%/2mm. Initial transverse EBT2 Gafchromic measurement of the first clinical case showed satisfactory qualitative agreement with treatment plan. Subsequent EBT3 GAFchromic film measurements resulted in 97.5% gamma passrate at 3%/3mm and mean dose deviation on representative dose profiles of less than 2.2%. Average intra-fraction motion between half fractions was 0.68mm in X, 0.64mm in Y and 0.84mm in Z with standard deviation of 0.62mm, 0.48mm and 0.79mm respectively. Conclusion GSTFT is the first centre with QA approval under the NHS England CtE programme to treat bone oligometastatic cases using the TomoTherapy treatment planning and delivery system. Volo and Hi-Art systems are capable of generating and accurately delivering homogeneous dose of up to 15Gy per fraction in phantom studies. Clinically approved treatment plans for bone oligomet cases delivering up to 9Gy per fraction have been generated and accurately delivered to diodes, ionisation chamber and Gafchromic film. Intra-fraction motion was small and has permitted the reduction of PTV margin from 4mm to 3mm.

EP-1757 QA of MLC Elekta Agility for Static fields F. Tato de las Cuevas 1 1 Hosp. Univ. de Canarias, Medical Physics Dept., Santa Cruz de Tenerife, Spain Purpose or Objective QA of MLC is one of the main points of any LINAC QA program. Agility MLC (Elekta) have different properties than most of the more common MLCs, like less interleaf transmission. The objective is to perform the Agility MLC QA in static mode using the electronic portal imaging device (EPID) and make this process as fast and accurate as possible. Material and Methods The LINAC is an Elekta Synergy with Agility MLC and 6 MV photons. A software is developed in MATLAB with some remarkable points: 1. Elekta iCOMCAT software was employed to generate and send the strip-test with multiple segments as a unique treatment, as is much faster than creating and irradiating a beam for each segment. With the software of Elekta iView is difficult to acquire a complete image of each full segment as this is not fast enough, so fluency corrections of these segments were performed, in order to avoid erroneous pixel values (PV) in the way: a) In a 23x23 open field is acquired a horizontal profile and measure the % PV (in the center position of each future segment), this % is related to the PV of the position of a reference segment. b) Measure the mean PV in the center of each strip-test segment, and obtain the % PV related to the reference segment PV. c) Rescale the image of each segment in order to obtain the % PV (respect the reference segment). Finally make the sum of all images.

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Segments of 2 x 20 cm (cross-plane x in-plane) to form series of strip-test images with gaps overlapping from 1.2 to 3 mm are acquired for taking the MLC reference after calibration. The strip-test need bigger gap spread than other MLC in order to detect the gap position correctly, because of the lower penumbra. To correct the collimator angle is used the filtered back projection method, because is very tricky to use the interleaf leakage, as this MLC have much lower interleaf transmission than other MLC, like Millenium (from Varian). To localize the radiation center (RC) of the EPID is used a LINAC tray with centered radiopaque mark. Four 20x20 fields are obtained with this tray at 4 collimator angles. RC is determined for gantry 0º detecting the mark position in each image and obtaining the mean. A vector displacement is created to obtain RC with one image at 0º collimator. Tray images for various gantry angles at 0º collimator are acquired, so that with just one tray image is enough to detect RC exactly. This method is faster than using field edges, where at least 2 images at different collimator angles must be acquired for each gantry angle.

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Measurements of leaf positions using light projection are made. Also are obtained strip-test with films and analyzed with RIT software.