ESTRO 2020 Abstract book

S958 ESTRO 2020

and intra-fractional changes of rectal gas in cervical cancer patients. Material and Methods Six cervical cancer patients underwent four scanning sessions (~20 minutes each). Within each session, T2-w MRIs were taken at seven time-points (TP, mostly ~3 minutes apart). The rectum on each image was delineated by a clinical oncologist. Slices where rectal delineations were present across all TPs in a scan session were selected for analysis. The length of the rectum was normalised, going from 0 (most cranial) to 1 (most caudal). The area of gas within the rectum delineations was identified on each slice using thresholding and recorded (A TPX ) {X = 1,…,7} (fig. 1). The (A TPX ) for averaged over all TPs was used to calculate the local inter-fractional local group Mean (M), systematic (Σ) and random (σ) error. To determine the intra-fractional changes, the difference in gas area between TP2, TP3, … TP7 vs. TP1 was found on each slice (ΔA TPX ) and used to calculate the local M, Σ, σ and σ f, but now for intra-fractional motion.

scenarios. As expected, the DVH parameter differences with both set-up uncertainties and organ deformations were always larger than where each uncertainty was considered separately. Considering only organ deformations resulted in the smallest ranges, except for (Σ, σ) = (0, 1) mm, and similar distributions for (Σ, σ) = (1, 1) mm considering only set-up errors. For simulations considering set-up uncertainties, the range of the DVH parameter differences increases most as Σ increases and to a lesser extent as σ increases for both the training and validation data sets. The overall distributions are similar for both the 13 training patients and the 7 validation patients.

Results Figure 2 shows the local statistics (M, Σ, σ (and σf)) for inter (panel A) and intra (panel B) fractional changes across all patients. Both inter and intra-fractional changes are larger in the cranial aspect of the rectum, and gas appears more stable at the caudal end. Further, inter- fractional changes (~2cm 2 /slice) are larger than intra- fractional changes (~1cm 2 /slice). The intra-fractional changes (~1cm 2 /slice) are small compared with the absolute area of gas in each slice, which is up to 10cm 2 /slice in the patient in figure 1. This indicates that gas remains relatively stable during a 20 minute period.

Conclusion We evaluated the effect of time-varying organ deformation and set-up uncertainties on clinical head and neck cancer plans. Our data suggests that the effect of organ deformation may be modelled by considering (Σ, σ) = (1, 1) mm for this dataset. We found that for small set-up uncertainties, (Σ, σ) = (0, 1) mm, organ deformations are the dominant contribution to DVH parameter differences for the parotids and the brainstem. This highlights the need to correct for organ deformations in modern image-guided radiotherapy workflows, where patient set-up errors are being considerably reduced. PO-1651 Inter and intra-fractional stability of rectal gas in cervical cancer patients J. Shortall 1,2 , E. Vasquez Osorio 1,2 , A. Cree 1,2 , M. Dubec 2 , R. Chuter 2 , A. McWilliam 1,2 , K. Kirkby 1,2 , R. Mackay 1,2 , M. Van Herk 1,2 1 The University of Manchester, Department of Cancer Sciences, Manchester, United Kingdom ; 2 The Christie NHS Foundation Trust, Medical Physics and Engineering, Manchester, United Kingdom Purpose or Objective Due to the Electron Return Effect during Magnetic Resonance guided Radiotherapy (MRgRT), rectal gas during pelvic treatments can potentially result in hotspots in the rectal wall. Determining the clinical impact of this effect on rectal toxicity requires estimation of inter and intra- fractional stability of rectal gas. We investigate local inter

Conclusion We are the first to statistically evaluate stability of rectal gas in cervical patients. Inter-fractional gas changes are larger than intra-fractional gas changes. I.e. gas is