ESTRO 2022 - Abstract Book

S1550

Abstract book

ESTRO 2022

Fig 1 - examples of fully registered CT images (gray colormap) and corresponding functional data (NF heat maps) for all patients and times τ .

Fig 2 - for all patients, (top) NF for each lung lobe, (bottom) FR for selected dose ranges. The location of the tumor is identified with an

asterisk.

Conclusion The proposed methodology is suitable to follow the evolution of lung function after treatment. The NF and FR can be used as metrics to identify regions with low functionality and poor response to radiation. Further work will apply this methodology to a large cohort study of >50 patients and up to 24 months follow-up, with the long-term objective to limit and predict possible complications following lung radiotherapy.

PO-1749 Evaluating diffusion weighted signal change on the MR-Linac during bladder cancer radiotherapy

M. Ingle 1 , M. Blackledge 2 , A. Wetscherek 3 , R. Huddart 4 , S. Lalondrelle 5 , S. Bhide 5 , S. Hafeez 1

1 The Institute of Cancer Research/The Royal Marsden Hospital, Radiotherapy and Imaging, London, United Kingdom; 2 The Institute of Cancer Research , Radiotherapy and Imaging, London, United Kingdom; 3 The Institute of Cancer Research, Radiotherapy and Imaging, London, United Kingdom; 4 The Institute of Cancer Research/Royal Marsden Hospital, Radiotherapy and Imaging, London, United Kingdom; 5 The Royal Marsden Hospital/The Institute of Cancer Research, Radiotherapy and Imaging, London, United Kingdom Purpose or Objective Hybrid MRI and linear accelerator platforms such as the MR-Linac (Unity, Elekta AB, Stockholm Sweden) enable functional imaging tumour assessment during treatment. A decrease in diffusion-weighted imaging (DWI) signal and increase in apparent diffusion coefficient (ADC) is associated with complete response (CR) on diagnostic MRI. We present results of a preliminary study determining feasibility of measuring DWI signal change in bladder cancer patients treated on MR-Linac. Materials and Methods Patients with T2a-T4, N0-N2, M0-M1 bladder cancer suitable for radiotherapy on MR-Linac were prospectively recruited to a single institutional study. Radiotherapy was delivered to the whole bladder using 30-36Gy/5-6 weekly fractions. DWI with b-values of 0, 50, 100, 250 and 1000 s/mm 2 was acquired. ADC maps were created using all b-values (ADC all ) and 100, 250 and 500 s/mm 2 (ADC b100 ). Qualitative analysis of images was performed to determine best visibility of signal in tumour. DWI signal in tumour was manually contoured and mean ADC all and ADC b100 values were calculated using an in-house tool. Results Between May 2019 and April 2021, 5 patients underwent radiotherapy on the MR-Linac for bladder cancer with DWI acquisition. Tumour signal visibility is best seen in b500 image when bladder is full (fig 1). However, during treatment when bladder is not filled tumour signal is difficult to distinguish from surrounding structures. In this situation, restriction of tumour on ADC map was better visualised (fig 2). Thus, for ADC calculation GTV is manually contoured on b500 image (GTV b500 ) and modified using restriction on ADC map. 4/5 patients demonstrated a non-statistically significant trend to increasing mean ADC all between first to final fraction (0.70 x 10 -3 mm/s 2 to 0.80 x 10 -3 mm/s 2 ; p=0.1875) (fig 2). One patient demonstrated decreasing tumour ADC mean but DWI signal in tumour was difficult to ascertain. In this patient pelvic nodal involvement was visible which demonstrated increasing ADC all mean (0.53 x 10 -3 mm/s 2 to 0.63 x 10 -3 mm/s 2 ) following treatment. This suggests possibility that ADC calculations are not reliable if DWI signal is not clear. All patients achieved MR assessed CR at 3-month follow-up. ADC all mean values are 7.1- 30.5% lower than ADC b100 values. Tumour mean ADC all and ADC b100 for all patients at first fraction was 0.70 x10 -3 mm 2 /s and 0.82 x10 -3 mm 2 /s (p=0.0625) respectively. The difference in values is likely due to inclusion of perfusion factors present at low b-values