ESTRO 2020 Abstract book

S232 ESTRO 2020

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.

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 A. Lin 1 , F. Dehdashti 2 , L.S. Massad 3 , P.H. Thaker 3 , M.A. Powell 3 , D.G. Mutch 3 , J.K. Schwarz 1 , S. Markovina 1 , B.A. Siegel 2 , P.W. Grigsby 1 1 Washington University in Saint Louis, Radiation Oncology, St. Louis, USA ; 2 Washington University in Saint Louis, Division of Nuclear Medicine in Radiology, St. Louis, USA ; 3 Washington University in Saint Louis, Division of Gynecologic Oncology, St. Louis, USA Purpose or Objective To determine the risk of recurrence in cervical cancer patients treated with definitive chemoradiation who had a complete metabolic response (CMR) on early post- treatment 18 F-fluorodeoxyglucose positron emission Patients who completed curative chemoradiation from 1998-2018 for FIGO 2018 stage IB1-IVA cervical cancer and had a CMR on post-treatment FDG-PET (a median of 3 months, range 1-5 months after treatment completion) were included. All patients had an initial staging FDG- PET. Patients were treated with external beam radiation to the pelvis to 50.4Gy and received either LDR or HDR brachytherapy to the cervix. Para-aortic nodal regions up to the renal veins were treated if para-aortic nodes were involved on staging FDG-PET. Concurrent weekly cisplatin was prescribed to all patients. Cox proportional hazards models were done to determine factors associated with pelvic failure, distant failure, and freedom from any recurrence (FFR). Kaplan-Meier estimates of FFR of patient subgroups were compared with log-rank tests. Results tomography (FDG-PET). Material and Methods

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 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.