ESTRO 2022 - Abstract Book

S1497

Abstract book

ESTRO 2022

A. Prado Barragán 1 , D. Zucca 1 , M.Á. De la Casa 1 , P. García 2 , J. García 2 , L. Alonso 1 , J. Martí 1 , O. Hernando 3 , P. Fernández- Letón 4 , C. Rubio 4 1 Hospital Universitario HM Sanchinarro. HM Hospitales, Medical Physics and Radiation Protection, Madrid, Spain; 2 Hospital Universitario HM Puerta del Sur. HM Hospitales, Medical Physics and Radiation Protection, Madrid, Spain; 3 Hospital Universitario HM Puerta del Sur. HM Hospitales, Radiation Oncology, Madrid, Spain; 4 Hospital Universitario HM Sanchinarro. HM Hospitales, Medical Physics and Radiation Protection , Madrid, Spain Purpose or Objective In this work the intra-fraction (IF) motion was estimated through intra-fraction CBCT (Elekta Medical) when DIBH was performed using Catalyst (CRad) or Active Breathing Coordinator (Elekta Medical) in liver SBRT treatments. Materials and Methods 160 patients with liver metastases were considered. In 55 of them Catalyst was utilized and for the other 105 ABC was employed. A total of 232 (479) IF CBCTs were recorded for Catalyst (ABC). For every treatment session an IF CBCT was synchronized with the first treatment arc. The full treatment was composed of two arcs. The patient position was corrected before the second arc was imparted. Using these corrections the IF motion for each patient and treatment session was estimated. Median values and standard deviations were computed for each spatial component (SI, LR and AP). Furthermore, the IF offset percentage lower than 5mm (O <5 ) and the IF offset percentage greater than 8mm (O >8 ) were also calculated. Moreover, systematic and random errors were obtained for each DIBH systems employed. To elucidate whether there were significant differences in the IF offset distribution widths between Catalyst and ABC IF motion data a Fisher-Snedecor F test was performed with a 0.05 significance level. Results Table 1 shows medians, standard deviations and systematic and random errors computed for SI, LR and AP. O <5 and O >8 values were recorded in table 2. Both tables were arranged by spatial component and DIBH system used. Distribution widths were statistically distinct, as p-values computed through Fisher F-test were much lower than 0.001. Systematic and random errors were higher in Catalyst treatments for all spatial components. For both DIBH systems systematic and random errors were higher for SI direction. At least 81% (90%) of the IF offsets were lower than 5mm and no more than 11% (4%) were higher than 8mm in Catalyst (ABC) treatments. Lower O <5 and higher O >8 values were found for SI in both DIBH systems.

Table 1: Medians, standard deviations and systematic ( Σ error ) and random ( σ error ) errors arranged by DIBH system and spatial component.

Table 2: Percentage of IF offsets greater than 8mm (O >8mm ) and lower than 5mm (O <5mm ) arranged by DIBH system and spatial component. Conclusion DIBH reduces IF motion in liver SBRT but does not eliminate it completely, as some residual motion is still present due to the imperfectness of breath-hold and internal liver motion. Although both systems are perfectly suitable to perform DIBH in liver SBRT, ABC is more precise than Catalyst. These differences might be associated with the distinct behavior of the respiratory cycle in the gated area. When utilizing ABC the air flux is stopped and a completely flat respiratory pattern is obtained. However, Catalyst forces the patient to hold its breath and to maintain the cycle inside a gating window. Inside that window the respiratory cycle moves and so does the patient. Further investigation should be perform to unravel IF liver motion during SBRT treatments.

PO-1697 Assessment of the impact of CBCT-guided online adaptation on dose distribution in cervical cancer

C. Shelley 1 , M. Bolt 2 , R. Hollingdale 2 , M. Rashid 2 , S. Reinlo 2 , N. Fazel 2 , E. Adams 2 , A. Stewart 2 , C. South 2

1 Royal County Surrey Hospital, Radiotherapy, Guildford, United Kingdom; 2 Royal Surrey County Hospital, Radiotherapy, Guildford, United Kingdom Purpose or Objective Online adaptive radiotherapy (oART) in cervical cancer has the potential to reduce dose to organs at risk (OAR) whilst improving clinical target volume (CTV) coverage. However, uncertainty remains over the optimal application. The Varian Ethos emulator allows simulation of oART treatments using previously acquired CBCT images. This simulation study evaluates the dosimetric impact of oART in cervical cancer using the Ethos Therapy system.

Materials and Methods