ESTRO 2021 Abstract Book

S1653

ESTRO 2021

Sixty-five breast cancer patients who underwent prone breast fixator posing from January 2019 to December 2020 were selected and divided into weight standard group (27 in group A with 18.5≤BMI<24), overweight group (23 in group B with 24≤BMI<28) and obese group (15 in group C with BMI≥28) according to the patients' BMI. CBCT images of 65 patients before each treatment were acquired using kilovoltage cone beam CT, and soft tissue alignment was used to compare the positional errors between different groups. Results A total of 1124 CBCT images were acquired, and the pose errors (absolute values were taken for error calculation) in the Vrt(cm), Lng(cm), Lat(cm), Pitch(°), Roll(°), and Rtn(°) directions for patients in groups A, B, and C were group A(0.14±0.53,-0.13±0.65,-0.04±0.44,0.59±1.63,-0.16±1.55,-0.01±1.43), group B(0.09±0.55,-0.09±0.70,-0.01±0.58,0.44±1.67,- 0.02±1.53,-0.12±1.39), and group C(0.16±0.55,-0.11±0.74,-0.02±0.60,0.18±1.83,-0.16±1.67,-0.16±1.56), respectively. With increasing BMI (group A-group B-group C), there was no significant difference in positional error in all directions except in the Pitch direction; there was a statistically significant difference in positional error between the three groups in the Pitch direction (P < 0.01), and the larger the BMI value was, the smaller the error in the Pitch direction became. Conclusion In radiotherapy with prone breast fixator placement, the effect of BMI on the positional error is not significant; but in the Pitch direction, the larger the patient's BMI value is, the smaller the positional error will be. Although prone breast positioning is effective in reducing the dose to organs at risk, such as the heart, lungs and spinal cord, positional errors are sometimes large and therefore image guidance is recommended for each radiotherapy session. PO-1940 Variation in bladder filling for cervical cancer patients undergoing radical radiotherapy C. Nelder 1 , R. Chuter 2,3 , J. Berresford 1 , R. Benson 1 , A. Clough 1 , L. McDaid 1 , L. Barraclough 1 , K. Haslett 1 , P. Hoskin 1,4 , A. Choudhury 1,5 , C.L. Eccles 1,3 1 The Christie NHS Foundation Trust, Radiotherapy, Manchester, United Kingdom; 2 The Christie NHS Foundation Trust, Radiotherapy , Manchester, United Kingdom; 3 The University of Manchester, Division of Cancer Sciences, Manchester, United Kingdom; 4 University of Manchester, Division of Cancer Sciences, Manchester, United Kingdom; 5 The University of Manchester, Division of Cancer Services, Manchester, United Kingdom Purpose or Objective The introduction of new preparation instructions for patients undergoing radiotherapy (RT) may cause challenges. These include anatomical mismatch, increased duration on the treatment couch and/or the need for a re-plan. This work quantified variation in bladder filling from planning CT (pCT) to on treatment imaging following the introduction of a An ethics approved clinical trial for women undergoing radical RT for cervical cancer required the introduction of treatment with a “comfortably full” bladder. To achieve this, women were asked to empty their bladder, drink 350mls of water and wait 30 minutes before scanning or treatment. Women undergoing pCT and planning Magnetic Resonance Imaging (MRI) (pMRI) during the first month of study were included. A single observer retrospectively registered and contoured bladder volumes (BV) on pCT, pMRI and Cone-Beam Computed Tomography (CBCT) in Monaco (Elekta, Version 5.40.01). BV variations from pCT were calculated for all fractions. Time from initial CBCT to beam-on, and annotations regarding details of the daily image registration (e.g., manual match or intervention from a senior radiographer (RTT) required) were included in the analysis. Descriptive statistics were used to determine trends. Further work is ongoing to assess the dosimetric impact of bladder filling on cervix, and rectum. Results Eight pMRI, 10 pCT (including 2 re-scans), and 200 CBCTs were evaluated. Overall patients mean BV on pCT was 157.1cm 3 (range 41.9 – 335.4) and on MRI 118.0 cm 3 (range 20.9 – 321.0) with a mean change in CBCT BV from pCT of +35% (range +164% to -24%). BVs varied from pCT ≥50% in 41% of all fractions for all patients. Figure 1 shows BVs across all fractions for patients 1 and 3. A minimum of one repeat CBCT was required on 38 fractions in 7 patients, with >2 required on 4 of 200 fractions. bladder filling protocol. Materials and Methods

7 of 8 patients spent more than 45 minutes in the department ≥1 treatment day. This did not correspond to larger BV changes on CBCT compared to pCT (Figure 2). Reasons for long appointments included the need to ask patients to get off the bed to empty or fill her bladder further, or to wait for a more senior RTT to arrive to review the images. Advice from a more senior RTT was required on 119/200 CBCTs (range 3-24 fractions per patient).