ESTRO 2021 Abstract Book

S589

ESTRO 2021

Purpose or Objective Cone beam CT (CBCT) is used in paediatric image-guided radiotherapy (IGRT) for patient setup and internal anatomy assessment. Adult CBCT protocols can lead to excessively high doses in children, increasing the risk of radiation-induced malignancies. However, reducing imaging dose increases quantum noise, degrading image quality. Unlike phantom images, patient CBCTs also include “anatomical noise”, in particular motion artefacts degrading image quality independently of dose (Figure 1). This study determines such noise contributions in paediatric CBCT, recommending practical imaging protocols based on the imaging need and defining thresholds above which increasing dose yields no improvement in image quality. Materials and Methods 60 CBCTs including the thorax and/or abdomen/pelvis from 7 paediatric patients (aged 6-13 years, left-right diameter ranging 19.6-27.4cm) were acquired at a range of doses and used to accurately simulate lower dose scans. In total, 192 scans with doses ranging 0.5-12.8mGy per scan were analysed. Total noise (including anatomical noise) was measured in the liver, heart and kidneys in the patient scans and compared to noise measured in corresponding regions of an ATOM 10 year old paediatric phantom to establish quantum noise contributions. Anatomical, quantum and total noise were modelled as a function of dose using a non-linear least squares model. Contrast-to-noise ratio (CNR) was measured between fat and muscle. Soft tissue registration was performed on the kidneys, directly comparing the registration accuracy of each simulated low dose scan to the “gold standard” registration result of its respective high dose scan. Results Quantum noise contributed <20% to total noise in all cases, suggesting that anatomical noise is the largest determinant of image quality in the abdominal/pelvic region, even at very low doses. Soft tissue visibility was maintained at all doses (Figure 1). CNR exceeded 3 (Rose Criterion for visibility) in over 90% of cases at or above 1mGy, and 57% of cases at 0.5mGy. Soft tissue registration accuracy was good for doses >1mGy, with a median (10-90% percentile range) vector registration discrepancy of 0.98 (0.21-2.69) mm overall for both kidneys. Based on these results, we recommend paediatric ultra-low dose protocols of 0.125-0.50mGy for bony anatomy setup only, 0.5-1mGy for improved soft tissue visibility, and ~1mGy for soft tissue registration (Table 1).

Conclusion Anatomical noise dominates quantum noise in paediatric CBCT. Appropriate soft tissue contrast and registration accuracy can be achieved for doses as low as 1mGy, while ultra-low doses below this are suitable for setup on bony anatomy. Increasing dose far above 1mGy has no benefit in improving image quality or soft tissue registration accuracy due to the presence of anatomical noise.

Poster discussions: Poster discussion 6: Normal tissues and Immune-radiobiology

PD-0758 Fraction Size Sensitivity (α/β ratios) For Late Genitourinary Toxicities After Prostate EBRT D. Brand 1 , S. Brüningk 2 , A. Wilkins 1 , O. Naismith 3 , A. Gao 4 , I. Syndikus 5 , D. Dearnaley 1 , N. van As 4 , E. Hall 6 , S. Gulliford 7 , A. Tree 4 1 The Institute of Cancer Research, Division of Radiotherapy and Imaging, London, United Kingdom; 2 ETH Zurich, Department of Biosystems Science and Engineering, Basel, Switzerland; 3 The Royal Marsden NHS Foundation Trust, Radiotherapy Trials QA Group (RTTQA), London, United Kingdom; 4 The Royal Marsden NHS Foundation Trust, Urology Unit, London, United Kingdom; 5 Clatterbridge Cancer Centre, Radiotherapy Department, Wirral, United Kingdom; 6 The Institute of Cancer Research, Clinical Trials and Statistics Unit, London, United Kingdom; 7 University College London, Department of Medical Physics and Biomedical