ESTRO 2021 Abstract Book

S609

ESTRO 2021

intensive. Identification of the patients that will be subject to motion during external beam radiotherapy (EBRT) treatment may help to select candidates that will benefit the most from individualized approaches. This study aimed to determine the frequency of uterus motion in the locally advanced cervical cancer (LACC) patients’ population during EBRT, evaluate potential prognostic factors and quantify their predictive power. Materials and Methods In 48 LACC patients treated with 45Gy in 25 fractions, anatomical characteristics were extracted from planning CTs, in particular: Gross Tumor Volume (GTV), full bladder volume, displacement of the uterine fundus between full and empty bladder CTs and uterus length. A total of 1200 Cone-Beam CTs were acquired for daily image-guidance. These were utilized to measure the fundus motion between planning and treatment positions. The patients were categorized as either ‘large’ movers if fundus displacement ≥4.5cm for at least one fraction or ‘moderate’ movers if ≥2.5cm and <4.5cm. Fraction numbers of first motion were extracted for analysis. The frequency of motion in the population was calculated and relationships with the extracted variables investigated. In addition, the predictive power of the variables considered and their combinations was evaluated using logistic regression and Area Under the Curve (AUC). Results During their EBRT course, 54% of patients were subject to a fundus displacement ≥2.5cm at least once and 19% ≥4.5cm. Of the patients that experienced large motion, 55% did it in the first 3 days of RT, the other 45% between fractions 9-20. As shown in Fig.1, all large movers presented a uterus between 7-11cm and most had a bladder volume on planning CT ≥240cm 3 . Performing patient-selection based on fundus motion between full and empty bladder CTs ≥2.5cm, which is standard of care in some institutions, would have led to adapting treatment for 3 large movers appropriately and 4 incorrectly, while 6 would have been missed. As shown in Fig.2, uterus length and GTV volumes combined had the highest predictive power (AUCtest=0.84). Full Bladder Volume was the best single predictor (AUCtest=0.72).

Conclusion While moderate motion of the fundus is very common in LACC patients undergoing EBRT, one patient in five is subject to large motion likely to cause dose degradation and would benefit from treatment personalization. The magnitude of fundus motion is patient-specific and related to pre-treatment anatomical characteristics of the patients such as uterus length as well as large bladders at time of dose planning. While motion prediction solely based on pre-treatment characteristics has potential to improve patient selection for personalized treatments, it remains imperfect. This stresses the importance of motion monitoring on daily imaging. PD-0778 Dosimetric impact of CBCT-frequency in adjuvant breast cancer radiotherapy Y. Junker 1 , K.J. Borm 1 , M. Düsberg 1 , S.E. Combs 1 1 Technical University Munich, Medical School, Klinikum rechts der Isar, Department of Radiation Oncology, Munich, Germany

Purpose or Objective To analyze dose distribution in adjuvant breast cancer radiotherapy with simultaneous integrated boost (SIB)