ESTRO 2022 - Abstract Book

S1634

Abstract book

ESTRO 2022

Results A total of 101 CBCT images were analyzed. The average and standard deviations (Table 1) between skin marking-based and SGRT-based group were 0.026±0.500cm, 0.199±0.366cm, 0.049±0.209cm and 0.012±0.126cm, 0.002±0.301cm, - 0.003±0.307cm in x(lateral), y(longitudinal) and z(vertical) translational directions; 0.462±1.279°,-0.048±0.942°, - 0.281±0.882°, and -0.151±1.629°, 0.082±0.625°, -0.053±0.813° in Rx(pitch), Ry(roll), and Rz(rotation) rotational direction. The results showed that the setup error in longitudinal and pitch could be improved with SGRT system. The percentage of positioning accuracy within 5mm and 2° were shown in Table 2. The positioning accuracy based on SGRT was comparable to skin marking-based, and SGRT was notably beneficial in lateral, longitudinal and roll direction.

Conclusion Incorporating a good bladder management can improve the accuracy of SGRT application on cervical cancer radiotherapy patients. It is feasible, reliable, and potentially mark-free. We will continue to study for bigger sample size for more concrete conclusion.

PO-1841 Infleunces of Interfractional changes of rectum/bladder on vagina motion in endometrium radiotherapy

Y. Tsang 1 , A. Abduallah 2 , A. Stewart-Lord 3 , D. Megias 1 , H. Tharmalingam 4 , P. Hoskin 4

1 Mount Vernon Cancer Centre, Radiotherapy, Northwood, United Kingdom; 2 Mount Vernon Cancer Centre, Radiotherapy , Northwood, United Kingdom; 3 London Southbank University , Institute of Health and Social Care, London, United Kingdom; 4 Mount Vernon Cancer Centre, Clinical Oncology, Northwood, United Kingdom Purpose or Objective To investigate the influence of interfractional position and volumetric changes of the bladder and rectum on vagina displacements during postoperative external beam radiotherapy (EBRT) of endometrial cancer. Materials and Methods In a single centre retrospective cohort study, measurements of interfractional displacements and volumetric changes of bladder, rectum and vagina were carried out for 28 patients that underwent EBRT for endometria tumours. Measurements were carried out on initial planning computer tomography (CT) scans and on daily cone beam computer tomography (CBCT) scans. Rectum displacements were determined by measuring anterior-posterior (AP) rectal diameters at S5 vertebrae and 4 cm inferior to S5 level. Bladder volumetric changes were determined via measure of AP bladder diameter and superior- inferior (SI) bladder diameter. Vaginal displacements were determined by measure of difference in distances from the pubic symphysis and from the S5 vertebrae at planning CT scans and daily CBCT scans. Wilcoxon signed-rank test was used to analyse the significance in displacement of the rectum/bladder and vagina between planning CT scans and daily CBCT scans. Spearman’s rank-order correlation was then used to analyse significance of the association of changes in rectum and bladder with the displacement of the vagina from planning CT scans to daily CBCT scans. Significant interfractional changes in rectum position, bladder volume and vagina position were determined between planning CT scans and daily CBCT scans. No statistically significant correlations were found between interfractional changes of bladder and vaginal displacements. Weak relations were found (i) between increase in rectal AP diameter and increase in vaginal displacement from S5 (rs=0.14 p value<0.0.1) (ii) between increase in rectal AP diameter and increase in vaginal displacement from 4cm inferior to S5 (rs=0.22 p value<0.05) Greater interfractional changes in vaginal positions were found in patients who had larger interfractional variations in rectal volumes (Figure 1). Results 28 initial planning CT scans and 756 CBCT scans were analysed.