ESTRO 2020 Abstract Book

S1071 ESTRO 2020

and superior aspects of the uterus (Point A and Point S) and the most posterior aspect of the cervix (Point CP). The change in uterine angle and bladder volume were also measured. The bladder was outlined on each axial slice of the planning CT and CBCT scans and the volume computed. The change in bladder volume between the planning scan and CBCT scans was then calculated. Changes in the POI position and uterine angle were correlated with bladder volume changes using linear regression. Results The range of movement of the uterus was 0.02 cm to 3.61 cm in the superior / inferior direction (mean 0.71 cm). In the anterior / posterior direction(AP) it was 0.03 cm to 2.59 cm (mean 0.72 cm). The cervix had a range of 0.01 cm to 2.26 cm (mean 0.48 cm) in the AP direction and the change in uterine angle was 0 o to 23 o (mean 6.68 o ). For all patients combined there was a significant correlation relating the change in bladder volume to the movement of the uterus and cervix with regards to all the measured points and the change in uterus angle (p<0.05). Despite all patients in this study having been informed to follow a bladder full drinking protocol, there was large variability of bladder volumes measured on the CBCT scans. Conclusion Inter-fractional uterus and cervix movement can be substantial and can vary from patient to patient. Despite the use of a full bladder drinking protocol large variations in bladder volumes between fractions can occur and this can impact on the position of the uterus and cervix. The larger the bladder volume change the larger the uterus and cervix movement from CT planning position to on treatment CBCT position was observed. Ensuring patient compliance of the full bladder drinking protocol and daily ultrasound imaging before each fraction of radiotherapy is suggested to minimise changes in bladder volume and therefore reduce uterus and cervix movement. PO-1921 Comparison of dynamic tumor volumes in 4D- CT with two types of respiratory motion sensor PO-1922 Dosimetric replanification benefit in exceeded neurological structures doses during IMRT for NPC M. Wafa 1 , H. Daoud 1 , F. Najla 1 , S. Tarak 1 , S. Wicem 1 , F. Leila 1 , D. Jamel 1 1 Hbib Bourguiba Hospital, radiotherapy, sfax, Tunisia Purpose or Objective To quantify the dose change observed at mid-treatment in neurological structures: brainstem (BS) and spinal cord (SC)and to evaluate the replanification dosimetric impact during intensity-modulated radiotherapy (IMRT) for nasopharyngeal carcinomas (NPC). Material and Methods Our study collected 20 patients undergoing IMRT treatment for NPC. For each patient, a second dosimetric (CT2) scan was performed at 38 Gy and was fused with the initial planning scanner (CT1). Manual contouring of the BS and the SC has been performed on the CT2. We collected maximum doses (Dmax) at BS and SC on both scanners. The decision to do a second plan treatment was made in case of exceeding the dose constraints in CT2. Results For the BS, we found an increase in Dmax in 75% of patients and exceeded constraints in only one case. The average was 0.75 ± 2.1Gy (p = 0.06). For SC, the increase in Dmax was statistically significant (0.85 ± 0.38 Gy,p = 0.05. This increase was observed in 75% of cases with an overload in one case. For the first case, after replanification, the maximum BS dose decreased from 62Gy to 55.93 Gy. For Abstract withdrawn

the second case, the maximum dose in the SC was reduced from 46 Gy to 42.3 Gy. These changes were made without altering the coverage at target volumes. Conclusion An increase in neurological structures’ doses was found at 38 Gy for patients undergoing IMRT for NPC. The generation of a new treatment plan at mid of course radiotherapy reduced the organs at risk doses. This dosimetric gain should be confirmed and validated by prospective trials. PO-1923 Intrafractional uncertainty of the heart position in DIBH and free breathing radiotherapy S. Fortin Jørgensen 1 , W. Ottoson 2 , C.F. Behrens 2 , G.F. Persson 3 , S.N. Bekke 2 1 Københavns Professionshøjskole, Radiography, Copenhagen, Denmark ; 2 Herlev Hospital- University of Copenhagen, Radiotherapy Research Unit, Herlev, Denmark ; 3 Herlev Hospital- University of Copenhagen, Department of Oncology, Herlev, Denmark Purpose or Objective Deep-Inspiration Breath-Hold (DIBH) radiotherapy is standard treatment for left sided breast cancer patients in order to minimize dose to the heart and lung. However, knowledge of the position uncertainty of the heart during radiotherapy in DIBH and free breathing (FB) is lacking. Current study estimates this uncertainty using data from patients treated with lung stereotactic radiotherapy (SBRT). Material and Methods This retrospective study includes 21 consecutive DIBH plans (16 patients, 5 with dual targets) and 12 randomly chosen FB plans delivered between April 2018 to March 2019. Treatments were delivered in 3 or 5 fractions with volumetric modulated arc therapy (VMAT) using 2-4 half arcs and patients were immobilized using an upper body vacuum bag, Vacfix. Daily cone beam CTs (CBCTs) were acquired before and in the middle of the treatment for target position verification. In order to estimate the intrafractional uncertainties, each CBCT was automatically registered to the planning CT using the intensity range [-1000; 250] with match volume of interest around the heart. Manual adjustments were made when necessary. The intrafractional uncertainties were assessed by calculating the match differences between the CBCT registrations on the heart, and statistics were applied (t- test and F-test). Results The motion of the heart ranged from -10.1 mm to 7.7 mm in DIBH, and from -3.5 mm to 5.1 mm in FB (Fig. 1). The variation of the heart motion was significantly increased in all directions for the patients treated in DIBH compared to the patients treated in FB ( p < 0.05). However, there was only a significantly different mean value of the motion between DIBH and FB in the longitudinal direction (DIBH: - 1.1 mm; FB: 0.2 mm, p = 0,01).

Conclusion In the patient cohort of the current study there is an increased uncertainty of the heart position during treatment for patients treated in DIBH compared to FB.