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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.

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 sites and use of in vivo dosimetry. Responses were collated and analysed for consistency and outliers identified.

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.

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.