ESTRO 2021 Abstract Book

S661

ESTRO 2021

identification of radioresistance and detection of early relapse, enabling personalised treatment. Materials and Methods In this study, patients are part of a trial led by the Institute of Cancer Research testing effectiveness of intra- tumoural H 2 O 2 as a radiosensitiser. 2D SRUS and 3D DCE/DW-MRI of the breast tumours are acquired pre-RT and 2w, 6m and 12m post-RT (36Gy in twice weekly fractions of 6Gy). Super-resolution maps of tumour microvasculature were generated from high frame-rate contrast enhanced US acquisition by detecting intravenously injected, spatially isolated Sonovue contrast agents. 3D ADC (apparent diffusion coefficient) maps were generated from DW-MRI for measurement of cellularity and 3D AUC 90 (area under 1/T1 curve for first 90 sec of contrast uptake) maps were generated from DCE-MRI for measurement of vascular permeability. Results Preliminary analysis comparing pre- and 2w post-RT images in 4 patients confirmed that breast tumour microvasculature can be imaged using SRUS (Fig 1) with remarkable clarity. A reduction in microvascular density was visually detected 2w post-RT in 3 out of 4 patients. There was an increase in mean ADC after RT for all patients apart from one (p-value=0.03, Fig 2). Lower ADC values reflect higher cellularity; therefore, our results suggest reduced tumour cellularity is detectable 2w post-RT. Overall no significant difference was detected in mean AUC 90 values 2w post-RT (p-value=0.31, Fig 2), suggesting it may be too early to detect changes in vascular permeability using this measurement. The patients showing a reduction in microvasculature correspond to those with large increases in ADC (Fig 2, green & cyan plots).

Fig 1. B-mode images of tumour tissue (left) and corresponding SRUS images of tumour microvasculature (right) pre- (top) and 2w post-RT (bottom).

Fig 2. ADC (left) and AUC 90

(right) values pre- and 2w post-RT. Colours correspond to the same patient. Fig 1

corresponds to the same patient as the green plots. Conclusion

We have successfully demonstrated application of SRUS to study breast tumour microvasculature with high resolution. Initial pilot data shows that radiation induced changes can be detected as early as 2w post-RT, suggesting that this method, along with mean ADC, can be used to detect early response/resistance to RT and possibly early relapse. Further work will include correlating quantitative measurements from SRUS with MRI parameters. 1. Christensen-Jeffries et al. (2020) Ultrasound Med Biol 46 (4):865-891

PD-0827 Development of target-optimized CBCT imaging to reduce imaging dose and improve image quality N. Lackner 1 , B. van der Heyden 1 , I.M. Messner 2 , P. Steininger 2 , A. Zechner 3 , A. Ableitinger 3 , M. Stock 3 , G.