ESTRO 36 Abstract Book

S377 ESTRO 36 _______________________________________________________________________________________________

validation) were 0.63, 0.68 and 0.66 respectively. Based on this model, 2 groups risk were identified. The 3-year OS, DFS and LRC were 88% (95CI: 67.4 – 100%), 78.7 % (63.6 – 97.3%) and 83.6% (95CI: 70.1 – 99.7%) for low-risk group vs 45.5% (95CI: 23.8 – 86.8%), 33.3% (95CI: 15 – 74.2%) and 38.1% (95CI: 17.9 – 81.1%) for high-risk group (p<0.01) (Figure 1 and 2). Conclusion TLG of the primary tumor and the distance between lymph node and the primary tumor, weighted by the MTV of lymph nodes, were correlated with LRC, DFS and OS. These parameters seem to have a higher predictive value than classical prognostics parameters, and may be useful to tailoring the therapeutic approach in this type of cancer. PO-0719 The use of ultrasound bladder scanning in cervical IMRT to reduce variability of uterine motion S. Otter 1 , M. Hussein 1 , S. Why 1 , A. Franklin 1 , A. Stewart 1 1 Royal Surrey County Hospital, St Luke's Cancer Centre- Oncology, Guildford, United Kingdom Purpose or Objective IMRT is being increasingly used over 3D conformal radiotherapy in the treatment of locally advanced cervical cancer. However, the uterus and cervix are mobile structures whose positions vary depending on bladder and rectal filling. The purpose of this study was to assess whether the use of a portable US bladder scanning protocol prior to each fraction of IMRT can reduce bladder filling variability and therefore ultimately reduce cervical Patients with locally advanced cervical cancer treated with IMRT received daily CBCT imaging. The bladder was retrospectively contoured on each CBCT. Ten patients were treated using an US bladder scanning protocol (BVI 9400 bladder scan, Verathon). They had a daily pre- treatment bladder scan (BS) and were imaged with CBCT and then irradiated when the bladder volume fell in a pre- determined individualized range based on planning CT scans with empty and full bladder. If the bladder volume was below the target range, patients were rescanned after 15 minutes, if it was above the range, they emptied their bladder and recommenced bladder filling. Nine patients were treated with a standard bladder filling protocol which involved drinking 450ml water in 5 minutes then treatment after 30 minutes. Results were analysed with a Bland-Altman Plot to assess correlation between bladder volume on US and on CBCT. Mann Whitney 1-tailed test was used to assess whether the bladder scanning protocol improved bladder filling consistency (the percentage of radiotherapy fractions where the bladder volume on CBCT fell within the pre-determined range). Results CBCTs and BS readings were available for 246 fractions of radiotherapy in 10 patients in the BS cohort. 249 CBCTs were available in 9 patients in the pre-BS cohort. The mean difference between BS readings and bladder volume on CBCT was -53.8cc, however the 95% CI band was wide (38.6cc - 146.3cc) and there were a lot of outliers suggesting a lack of statistical significance. This may be related to a combination of: interoperator variability, limited probe size, and differences in the time between the BS readings and the CBCT. The percentage of fractions that had bladder volumes within range were significantly higher in the BS cohort (mean 70.5%) than the pre-BS cohort (mean 38.5%, p=0.024). Two patients were excluded from this analysis in the BS cohort as they had empty bladder volumes greater than full bladder volumes on planning CT. / uterine movement. Material and Methods

Conclusion The introduction of a bladder scanning protocol has significantly improved the consistency of bladder filling for each fraction of radiotherapy compared to a pre- bladder scan cohort. This may enable a reduction in the CTV-PTV margin in the future which would reduce the volume of irradiated tissue and therefore hopefully reduce toxicity. PO-0720 CTV change during adaptive EBRT for cervix cancer: is mid-treatment plan adaptation required? D.D. Vignarajah 1 , M.C. Marbán 2 , K.T. Bath 3 , P.J. Hoskin 1 1 Mount Vernon Cancer Centre, Clinical Oncology, Northwood, United Kingdom 2 Hospital Universitario Doctor Negrin, Radiation Oncology, Las Palmas de Gran Canaria, Spain 3 Mount Vernon Cancer Centre, Radiotherapy Physics, Northwood, United Kingdom Purpose or Objective Evaluation of clinical target volume (CTV) motion and volumetric changes during cervical cancer external beam radiotherapy (EBRT), to determine the value of mid- treatment plan adaptation compared to pretreatment plan adaptive radiotherapy. Material and Methods Patient Population Patients undergoing pretreatment adaptive EBRT for cervix cancer were eligible. Consistent bladder and bowel preparation was used as per departmental protocol. Analysis Daily cone beam CT (CBCT) images were reviewed, with the CTV and organs at risk (OARs) delineated on each image set based on GEC-ESTRO guidelines. Organ motion was assessed in 4 planes (anteriorly, posteriorly, superiorly and inferiorly) at the same anatomical points on each CBCT. Daily CTV volumes were combined to form a weekly internal target volume (ITV), with volume and motion assessed on a weekly basis. Based on the changes noted, new PTV margins were added to the