ESTRO 38 Abstract book

S621 ESTRO 38

Minute, BPM), two kinds of amplitudes (± 1 cm, ± 2 cm) with reference to the sinusoidal wave, and the breathing waveform of the respiratory synchronizer. The delay time from the gating signal until the proton beam is generated and cut off was measured by our own measurement Flatness and Symmetry were confirmed to be within ± 2% in all conditions, within the range of ± 3% with respect to the field size and Dose, and clinically no problem. On the other hand, with regard to Penumbra, BPM tends to increase as BPM gets bigger, especially with amplitude of ± 2 cm, it was necessary to consider both AbchesET and ANZAI for clinical use. Regarding the evaluation of the dose distribution, the gamma pass rate tends to decrease as the Gate width increases and as BPM increases, particularly when the amplitude is ± 2 cm, it shows a remarkable drop in the pass rate. Regarding measurement of delay time, the delay time of AbchesET is 36.7 ± 27.2 msec, 46.8 ± 28.7 msec with beam ON and OFF, the delay time of ANZAI is 48.6 ± 24.8 msec, 57.2 ± 25.2 msec with beam ON and OFF respectively there were. In beam ON and OFF, the delay time tends to be less for AbchesET, and clinical use is considered to be no problem. Conclusion Although respiratory synchronous irradiation of proton beam wobbler method using AbchesET has some use restrictions, it was confirmed that it can be sufficiently used clinically. We conclude that respiration synchronized irradiation of proton beam wobbler method with higher accuracy is achieved by using AbchesET. PO-1119 STrategies to mantain bladder and rectum volumes do not reduce the GTV movement for rectal cancer rt N. Espinosa 1 , A. Coral 1 , M. Lizondo 2 , J. Balart 3 , S. Bermejo 3 1 Hospital de la Santa Creu i Sant Pau, Servei de Radiofísica i Radioprotecció, Barcelona, Spain ; 2 Institut de Recerca Hospital de la Santa Creu i Sant Pau, Servei de Radiofísica i Radioprotecció, Barcelona, Spain ; 3 Hospital de la Santa Creu i Sant Pau, Servei d'Oncologia Radioteràpica, Barcelona, Spain Purpose or Objective The aim of our study is to determine whether the variability of rectal volume disminishes when bladder and rectum filling preparation instructions are followed by patients who undertake preoperative rectal IMRT. Material and Methods Thirty patients that received preoperative IMRT (45 Gy in 5 weeks, 1.8 Gy/day) for a rectal cancer between May 2016 and May 2017 were evaluated. Patients were treated in supine position. Fifteen patients without (group A) and 15 with bladder and rectum filling preparation instructions (group B) were evaluated. Group B patients were required to urinate and drink 330 mL of water 1 hour before planning CT and each fractional treatment. In addition, they were required to empty the rectum just before planning and treatment with probiotics help. Radiation was delivered using a 2100CD Clinac linear accelerator equipped with on-board Imager System (Varian). A no- action level offline setup protocol was employed by taking and averaging first 5r images and at least a cone beam CT (CBCT) was weekly undertaken. Two staff members delineated the GTV on each slice of CBCT (weekly_GTV). As well, CBCTs were co-registered (Eclipse, Varian) with its respective planning CT to determine whether any weekly_GTV was 1.5 cm or more beyond the planningGTV according to the Figure 1. The results were reported as percentages (%), and the chi-square test (SPSS) was used to examine differences between groups. system. Results

Results In total, 72 CBCT per group were analyzed. It was found that the weekly_GTVs meet the criteria of the Figure 1 in 23 CBCTs (31.9%) of the patients in the group A and in 19 CBCTs (26.4%) of the group B; p-value = 0,58. Per each patient, weekly_GTVs were found ≥ 1.5 cm beyond planningGTV in 28% times in the group A and 22.7 % in the B; p-value = 0.39 (Figure 2). However, variations in the group B were lower than group A in which there were patients that every weekly_GTV were ≥ 1.5 cm beyond, whereas this scenery was not found in group B (Figure 2). Weekly_GTVs meet the criteria 4 of the Figure 1, in which GTV bring outside PTV, in 9 CBCTs (12.5 %) in the group A and in 6 (8.3 %) in B.