ESTRO 36 Abstract Book

S953 ESTRO 36 2017 _______________________________________________________________________________________________

modalities and choice of associated cases need to take into consideration the utility for clinical trials. Results The Level III audit is an end-to-end test using a humanoid thorax phantom (CIRS, Norfolk, VA). The custom phantom has a central insert for either conformal modality with two farmer chambers, or for IMRT and VMAT with seven CC13 ion chambers as the primary detectors. The IMRT/VMAT central insert includes a film holder for supplementary measurements. The custom phantom includes removable lungs that are replaced with solid water inserts to investigate the effect of inhomogeneity on IMRT and VMAT deliveries. Figure 1 shows the custom phantom. The CC13 chambers are connected to the TomoTherapy ® TomoElectrometer, an 8 channel reference class electrometer for simultaneous measurement on all chambers for each audit case. The IMRT and VMAT planning cases were designed for addition to the current Level III audit. Clinical plans were prepared based on the AAPM Publication TG119 [1] and adapted for use in the ACDS audit program. Table 1 shows an example of how the audit outcomes are reported. Each modality is scored separately, and assigned a Pass (Optimal Level), Pass (Action Level), or Out of Tolerance outcome. Field trials on the IMRT, VMAT and FFF modalities began in September 2016. In 2017 the new modalities scoring criteria will be finalised and the new modalities will go live, and field trials on a SABR modality are scheduled to begin. Table 1 Example of modality scoring in the ACDS Level III audit.

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

Figure 1 Images of the custom CIRS phantom for the new ACDS Level III audit, showing the removable lungs and removable central insert. 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