ESTRO 2020 Abstract Book

S897 ESTRO 2020

the CBCT. Original structures are deformed and transferred to the aCT. The dose is recalculated on the cCTs (cCT-plans) and on the aCTs (aCT-plans) based on the CBCTs that timely matches the cCTs. Calculations are performed with +/-3.5% range uncertainty. Clinical plans are robustly optimized with 0cm and +/-0.4cm isocenter shifts in addition to +/-3.5% range uncertainty. A coverage of V95%(CTV1) = 99% in the worst-case scenario is required for the clinical plan. The aCT-plans and the cCT-plans are compared with respect to changes in target coverage and dose to organs at risk (OARs). Results Calculations have been performed at each of the 5 weekly controls (C1-C5) for the first two patients with HNC who have completed their course of treatment, see fig. 1. Differences in CTV1 (66-68Gy) coverage with respect to clinical plans are shown in the top while dose to spinal cord and brainstem are shown in the middle and the bottom, respectively. Patient 1 (left) was re-scanned and -planned after C4 (top left). The aCT-plan did not immediately indicate a need for re-scan in this case with V95%(CTV1) = 99.5%. However, the cCT-plan showed that V95%(CTV1) = 98.5%. The 3D dose distributions (fig. 2), revealed the risk of target compromise in both the aCT-plan (top) and the cCT plan (bottom). The sub-optimal distribution was caused by a part of the immobilization device, marked with arrow in fig. 2, which led to re-planning. Critical OAR doses were similar for Patient 1 and the re-scan after C4 is clearly seen at C5. Patient 2 (right) was re-planned before C2 and re-scanned after C2. In this case both aCT- and cCT-plans indicated a need for re-planning at C2 (top right). The cCT-plan showed an increase in dose to spinal cord at C3 and brainstem at C4. However, the doses remained far below organ tolerances.

Conclusion Results are promising for the first two HNC patients who have completed their proton therapy course. The frequency of cCTs may be downscaled and substituted with aCTs any time during the treatment course for evaluation of target coverage, robustness and dose to OARs. PO-1638 Treatment delivery uncertainties in rectal cancer radiotherapy – evidence-based margin estimates A. Appelt 1,2 , M. Beasley 1,2 , M. Teo 2 , F. Slevin 1,2 , R. Muirhead 3 , D. Sebag-Montefiore 1 1 Radiotherapy Research Group- Leeds Institute of Medical Research at St James's, University of Leeds, Leeds, United Kingdom ; 2 Leeds Cancer Centre, St James’s University Hospital, Leeds, United Kingdom ; 3 Department of Oncology, Oxford University Hospitals NHS Foundation Trust, Oxford, United Kingdom Purpose or Objective Multiple studies have evaluated individual sources of uncertainty associated with rectal cancer radiotherapy delivery. Only limited practical guidance has been available, however, to guide local centre-specific choice of treatment margins. We reviewed the literature, and combined relevant data to estimate PTV margins for long- course chemoradiotherapy (LCRT), for various image guidance strategies. Material and Methods Sources of uncertainty in rectal cancer radiotherapy delivery were identified through literature review. As per ICRU 83, these were divided into systematic and random uncertainties. Some uncertainties were considered general for (pelvic) radiotherapy, such as mechanical factors for the treatment delivery platform, variation in image match on bony structures, and residual uncorrected rotation. Others were specific to rectal cancer radiotherapy, for example interobserver delineation variation and inter-/intrafraction soft tissue shifts relative to bony structures. Values required for margin calculation, as per the van Herk approach (van Herk et al, 2000), were extracted from published studies (including information on beam penumbra and impact of target deformation). All