ESTRO 2020 Abstract book

S1036 ESTRO 2020

sites and use of in vivo dosimetry. Responses were collated and analysed for consistency and outliers identified. Results All centres completed the survey. On average, 360 patients are treated with TBI nationally per year, with 12 centres treating fewer than 20 patients annually. Patient set-up is summarised in Table 1. Dose prescription is centre umbilicus in 12 centres, 1 centre prescribes to the level of the axillae and 2 to lung D max . 2 centres plan on CT, 2 use CT data to calculate effective depth at multiple points and the rest perform manual calculations. Physical measurement and pre-irradiation test doses are used in these cases. Use of compensation and accepted dose inhomogeneity are summarised on Table 2. Dose rates are 15-30cGy/min and energies of 6/10MV are used at most centres. Brain and testes are boosted in 10 centres and all but 1 use in vivo dosimetry.

Results The bolus support design is stable on the Unity couch and there was no detrimental effect on image quality when present. The bolus is visible in MR images. In B1 maps, the disturbance to the B1 field was smaller than the deviation seen between QA measurements, hence the conductivity of the bolus was not considered sufficient to cause significant RF shielding. No direct heating was observed during the continuous scanning. Volunteers have tolerated the bolus well, with no reports of claustrophobia. Conclusion The use of a bolus curtain is a practical and safe method of reducing the ESE dose in out of field areas, such as patients’ head and arms in partial breast treatments. No patient customisation is needed, and it sits independent of the Unity coil or coil holder. For all treatment areas that are at risk from high ESE doses, routine use of the bolus curtain removes the need for extended superior/inferior imaging to localise and quantify the ESE dose. PO-1767 A national survey of total body irradiation to define best practice for the ALL-RIC trial P. Diez 1 , R. Mir 1 , C. Stacey 2 , E. Thomas 3 , E. Miles 1 , A. Hodgkinson 4 , D.I. Marks 5 1 National RTTQA Group- Mount Vernon Cancer Centre, Radiotherapy Physics, Northwood, United Kingdom ; 2 University College London Hospital, Radiotherapy Physics, London, United Kingdom ; 3 Leeds Cancer Centre, Radiotherapy, Leeds, United Kingdom ; 4 Cancer Research UK Clinical Trials Unit- University of Birmingham, IMPACT Hub, Birmingham, United Kingdom ; 5 Bristol Haematology and Oncology Centre, Haematology, Bristol, United Kingdom Purpose or Objective ALL-RIC (NCT03821610) is a randomised, multicentre phase II trial comparing reduced dose total body irradiation (TBI) and cyclophosphamide with a standard arm of fludarabine & melphalan reduced intensity conditioning in adults with acute lymphoblastic leukaemia. This is the first UK multicentre transplant trial comparing TBI treatment with chemotherapy only. A national survey was conducted to provide a baseline of current clinical practice in treatment planning and delivery of TBI. The results were used to quantify variations and, subsequently, inform the development of radiotherapy and quality assurance (RTQA) guidelines to promote consistency within the trial. Material and Methods A 40 question online survey was made available to 19 centres participating in the ALL-RIC trial. Topics included: numbers treated, set-up, compensation and shielding, beam data and plan calculation, dose rate and energy, dose prescription and homogeneity, boosting of sanctuary

Conclusion There is substantial variation in techniques for planning and delivery of TBI. This heterogeneity is universally accepted in this setting but consistency is essential when efficacy of TBI is the primary endpoint of a trial. The dose prescribed in ALL-RIC (8Gy) is lower than previously used and its combination with cyclophosphamide is relatively new. These and the moderately low numbers treated at most centres make RTQA guidelines a necessity for the trial. A pragmatic approach has been adopted accepting current TBI techniques to allow multi-institutional trial participation, to recruit 242 patients in 4 years. However, important standardisation has been introduced to ensure best practice: dose must be prescribed to centre umbilicus to guarantee consistent dose delivery across departments, and in vivo dosimetry is mandated to confirm this dose. This will also ensure lung doses are reported to assess TBI related symptomatic pulmonary toxicity, a trial secondary endpoint. A dosimetry audit programme using an anatomical full body phantom is under development to quantify the dosimetric impact of the variability observed, and its results will inform future trials. PO-1768 A pre-treatment quality assurance survey on patients treated with the new Accuray Radixact platform M. Fusella 1 , A. Scaggion 1 , N. Pivato 1 , M.A. Rossato 1 , A. Roggio 1 , M. Paiusco 1 1 Istituto Oncologico Veneto, Medical Physics, Padova, Italy Purpose or Objective Pre-treatment patient specific quality assurance is a necessary task to ensure accurate dose delivery. When a new machine became operational all the clinically approved plans must undergo a dosimetric verification.