ESTRO 2020 Abstract book

S282 ESTRO 2020

To date, 3 male patients with MIBC deemed unfit for daily radical radiotherapy, have completed treatment in the ongoing PERMIT trial (Prospective Evaluation of Radiotherapy Using Magnetic Resonance Image Guided Treatment, NCT03727698) receiving hypofractionated radiotherapy. The patients underwent a radiotherapy (RT) planning CT with an empty bladder. RT was planned using Monaco version 5.40 (Elekta) to a dose of 36Gy in 6 once weekly fractions to the whole bladder (CTV) using 7-field IMRT. The PTV was defined as CTV plus an anisotropic margin of 1.5cm superiorly/anteriorly, 1cm posteriorly and 0.5cm laterally/inferiorly. The PTV was covered by 95% of prescription dose with OAR dose constraints as per institutional guidelines. For each fraction, a daily T2W session MRI (sMRI) was acquired followed by an ‘adapt to shape’ (ATS) workflow. Under this workflow, the CTV is re-contoured and a new treatment plan optimised using the daily anatomy. Pre ‘beam on’ verification (vMRI) and post-treatment MRI (pMRI) were acquired to enable position verification and CTV dose coverage assessment respectively. CTV coverage assessment was performed by offline contouring of the CTV and OARs on the vMRIs and pMRIs and re-calculating the estimated dose delivered using Monaco. Plan conformity index (CI RTOG ), the proportion of total volume receiving >95% dose compared to CTV was used as a surrogate for the dose received by normal-tissue such that the higher the CI the more normal tissue receives >95% and a CI RTOG = 1 means no normal tissue received this dose. Results In total 18 fractions were delivered. Table 1 summarises the time taken for key adaptive workflow stages.

Conclusion MR-guided online adaptive radiotherapy is feasible with acceptable tolerability even in patients unfit for radical daily radiotherapy. Current PTV margins maintain acceptable intra-fraction coverage of the CTV. Recruitment to PERMIT is ongoing.

Proffered Papers: Proffered papers 26: New technology

OC-0470 Investigation of a low-Z sintered diamond target for 2.5 MV imaging J. Borsavage 1 , D.A. Cherpak MCCPM 1,2,3 , D.T. Monajemi MCCPM 1,2,3 , D.J. Robar FCCPM 1,2,3 1 Dalhousie University, Department of Physics and Atmospheric Science, Halifax, Canada ; 2 Dalhousie University, Department of Radiation Oncology, Halifax, Canada ; 3 Nova Scotia Health Authority, Department of Medical Physics, Halifax, Canada Purpose or Objective A 2.5 MV 'low-X' imaging mode is available on the Truebeam (Varian Medical Systems, Palo Alto, CA) platform for beam's eye view imaging. Compared to therapeutic photon beams, this mode offers improved contrast-to-noise ratio (CNR) due to the softer energy spectrum. Previous research indicates that the 2.5 MV beam can be further optimized by using a low-Z sintered diamond target, which should reduce the self-absorption of photons in the diagnostic energy range. This work describes the first installation of a sintered diamond target A 5 mm-thick sintered diamond target was machined into the Truebeam target arm replacing the copper low-X imaging target and installed (see Figure 1). The new 2.5 MV sintered diamond target beam (low-Z beam) was characterized dosimetrically in terms of inline, crossline, and percent depth dose (PDD) profiles for various square fields. CNR versus dose was evaluated for four tissue equivalent materials, including cortical bone, CB2-30%, breast and brain, using in-house thin (4 cm) and thick (20 cm) phantoms for the 2.5 MV low-X and low-Z beams and 6 MV (see Figure 2). Imaging dose was measured for all three beams at phantom mid-separation and corrected for detector energy sensitivity using Monte-Carlo derived perturbation factors for both phantoms using the EGSnrc egs_chamber user code. Results The depth of maximum dose and the PDD at 10 cm for the 2.5 MV low-Z beam were 4.8 mm and 50.15%, compared to 5.8 mm and 52.85% for the low-X beam. Compared to the in the Truebeam unit. Material and Methods

Patient/treatment parameters are detailed in Table 2. Mean intra-fraction CTV change (surrogate for bladder filling) ranged between 9-60cc. Post treatment CTV coverage (>95% CTV receiving >95% dose) was maintained for 17/18 fractions; for 1/18 fraction dose was 94.6%. In 17/18 fractions, the delivered plan resulted in estimated dose to OARs within the mandatory dose constraints (based on the anatomy seen on vMRI and pMRI). For non- catheterised patients, the mean CI RTOG improved between the session MRI and post-treatment MRI due to the impact of bladder filling. All patients completed treatment as planned. The treatment was well tolerated with no unexpected acute toxicity seen at one month.