ESTRO 2020 Abstract Book

S214 ESTRO 2020

adjuvant therapy received and number of involved LNs. Patients with 1-5 involved LNs had 3-year OS of 82.4% with combined chemoradiation (CRT) and 74.2% with single modality therapy, while patients with 6+ involved LNs had a larger difference of 66.8% with CRT and 48.9% with single modality therapy (p-interaction=0.013). Conclusion We used both conventional regression and machine- learning based methods to demonstrate that number of involved LNs is a stronger prognostic factor compared to PA node involvement. Additionally, number of positive LNs was predictive of the survival benefit of combined modality adjuvant therapy. Our findings strongly support more robust incorporation of lymph node burden into endometrial cancer staging systems, pending further validation in other datasets. PH-0401 What are the main causes of interfraction motion of the uterine fundus and cervix? A. Cree 1 , E. Vasquez Osorio 2 , G. Price 1 , M. Van Herk 2 , P. Hoskin 2 , A. Choudhury 2 , A. McWilliam 2 1 The Christie NHS Foundation Trust, Radiothearpy Related Research, Manchester, United Kingdom ; 2 The University of Manchester, Division of Cancer Sciences, Manchester, United Kingdom Purpose or Objective Interfraction motion of uterus and cervix can be large and often exceeds applied CTV-PTV margins. There is variation between patients and different parts of the uterus move differently. Most strategies accounting for this are based on bladder filling. However, we hypothesise that there are other causes of uterine motion. Our study aims to provide a qualitative assessment of causes of motion of the uterine fundus and cervix in a large cohort of patients. Material and Methods Anonymised scans were retrospectively obtained for 83 patients who received radical radiotherapy for cervical cancer, with imaging at 3 time points (Fig. 1); diagnostic MRI scan (1), planning CT, ~2 weeks later (2) and final week MRI scan, ~6 weeks later (3). Scans were registered on bony anatomy to the diagnostic MRI for each patient. The uterus was contoured by a single observer for all 249 scans on a single sagittal slice identified as mid of the uterus on scan 1. Motion at the cervix and uterine fundus was evaluated between scans 1-2 (S1-2) and 1-3 (S1-3). The main cause of motion and direction of motion was recorded based on visual interpretation.

with 30 cases (36%) and in S1-3 it was tumour regression, also with 30 cases (36%). Bladder filling differences only accounted for cervix motion in 5 cases (6%) in S1-2 and 1 case (1%) in S1-3. Main drivers of motion at the fundus were bladder filling with 23 cases (28%) in S1-2 and 18 cases (22%) in S1-3. However, motion was also related to rectal changes in 13 cases (16%) in S1-2 and S1-3, to bowel changes in 21 cases (25 %) in S1-2 and 12 cases (15%) in S1- 3, and to tumour regression in 16 cases (19%) in S1-3. At the cervix, in S1-2 , there was a superior/inferior component of motion in 20 cases (24%), mainly related to rectal changes. In S1-3, there was a superior/inferior component of motion in 38 cases (45%), mainly related to tumour regression.

Conclusion The main causes of cervical motion in our cohort are changes in rectal filling and tumour regression, with bladder filling playing a limited role. Motion at the uterine fundus is affected by bladder filling but other factors also have an important role. Rectal motion can lead to changes in the superior/inferior position of the cervix, which should be considered if developing an ITV. Our study suggests that current radiotherapy motion management strategies based on bladder filling may not account for the most important causes of cervix motion. Alternative approaches such as online adaption may be beneficial.

PH-0402 Cervical cancer prognostic factors following complete metabolic response after chemoradiation

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PH-0403 Correlation early morbidity with external beam dose volume parameters in loc. advanced cervix cancer Y. Seppenwoolde 1 , K. Majercakova 2 , E. Dörr 3 , M. Buschmann 3 , A. Sturdza 3 , R. Pötter 3 , D. Georg 3 1 Erasmus MC Cancer Center, Radiotherapy, Rotterdam, The Netherlands ; 2 Hospital de la Santa Creu i Sant Pau, Department of Radiation Oncology-, Barcelona, Spain ; 3 Medical University of Vienna, Department of Radiotherapy, Vienna, Austria Purpose or Objective Predicting external beam radiotherapy (EBRT) related morbidity based on dose-volume parameters (DVHs) remains an unresolved issue in definitive radiochemotherapy (RCT) for patients with locally

Results In S1-2, large motion (>1cm) was seen in 44 cases (53%) at the cervix level and in 65 (78%) at the fundus level. In S1- 3, this was 57 (69%) at the cervix level and 64 (77%) at the fundus level. Large motion at the cervix rarely occurred without large motion at the fundus: in S1-2 , 3 cases (4%) and S1-3 , 9 cases (11%). The main causes and direction of motion are summarised in figure 2. For the cervix, the most common cause of motion in S1-2 was rectal change