ESTRO 2020 Abstract book

S1124 ESTRO 2020

(p=0.001; LQ4.4,UQ7.6) were all achieved with use of DIBH. Chest wall PTV at V47.5 and V45 did not significantly differ between DIBH and FB plans. However, DIBH increased IMC coverage of V47.5Gy by 5.7% (p=0.009; LQ-7.8,UQ-0.03), V45Gy by 2.9% (p=0.003; LQ-4.3,UQ-0.4) and V42Gy by 1.3% (p=0.004; LQ-2.4,UQ-0.5). Conclusion DIBH is a feasible option allowing the reduction of OAR dose in right-sided PMRT patients when the IMC field is included. Specifically, it decreased heart, RCA and right lung irradiation, whilst maintaining optimally-dosed plans. PO-1920 Inter-fractional uterine and cervix motion during radiotherapy for cervix cancer G. Lewis 1 1 Cheltenham General Hospital, Radiotherapy, Cheltenham, United Kingdom Purpose or Objective Knowledge of inter-fraction organ motion plays an important role in accurately treating cervix cancer with IMRT. Studies have shown that the positional change of the uterus during radiotherapy for cervix cancer can be significant and can be caused by changes in bladder volume. This investigation quantified the inter-fractional movement of the uterus and cervix in patients with cervical cancer undergoing radiotherapy treatment. It also assessed the relationship between uterus and cervix positional change and bladder volume. Material and Methods 85 retrospective cone beam computed tomography (CBCT) images from 11 pre-operative cervix cancer patients who had undergone radiotherapy were fused with the planning CT scans. Patients had been given instructions to finish drinking four cups of water (approximately 800 ml) 40 minutes prior to their planning CT scan and radiotherapy treatments. The change in the uterus and cervix positions on the CBCT scans compared to the planning CT scans was quantified by measuring the distance between points of interest (POI) on the tip of the uterine fundus (Point F), the most anterior and superior aspects of the uterus (Point A and Point S) and the most posterior aspect of the cervix (Point CP). The change in uterine angle and bladder volume were also measured. The bladder was outlined on each axial slice of the planning CT and CBCT scans and the volume computed. The change in bladder volume between the planning scan and CBCT scans was then calculated. Changes in the POI position and uterine angle were correlated with bladder volume changes using linear regression. Results The range of movement of the uterus was 0.02 cm to 3.61 cm in the superior / inferior direction (mean 0.71 cm). In the anterior / posterior direction(AP) it was 0.03 cm to 2.59 cm (mean 0.72 cm). The cervix had a range of 0.01 cm to 2.26 cm (mean 0.48 cm) in the AP direction and the change in uterine angle was 0 o to 23 o (mean 6.68 o ). For all patients combined there was a significant correlation relating the change in bladder volume to the movement of the uterus and cervix with regards to all the measured points and the change in uterus angle (p<0.05). Despite all patients in this study having been informed to follow a bladder full drinking protocol, there was large variability of bladder volumes measured on the CBCT scans. Conclusion Inter-fractional uterus and cervix movement can be substantial and can vary from patient to patient. Despite the use of a full bladder drinking protocol large variations in bladder volumes between fractions can occur and this can impact on the position of the uterus and cervix. The larger the bladder volume change the larger the uterus and cervix movement from CT planning position to on

and collaborations between the trial team and centres are required. If this is undertaken RTTs can undertake the online responsibility of PoD and make optimal PoD selections. PO-1919 DIBH reduces right coronary artery and lung radiation dose in right breast cancer loco-regional RT K. Stuart 1,2,3 , E. Gough 4,5 , S. Ashworth 4,5 , T. Moodie 4 , W. Wang 2,3,4 , K. Byth 6,7 , J. Buck 8 , S. Ghattas 9 , L. Burke 10 1 Crown Princess Mary Cancer Centre, Department of Radiation Oncology- Westmead Hospital, Sydney, Australia ; 2 Westmead Breast Cancer Institute, Westmead Hospital, Westmead, Australia ; 3 The University of Sydney, Western Clinical School, Camperdown, Australia ; 4 Crown Princess Mary Cancer Centre, Department of Radiation Oncology- Westmead Hospital, Westmead, Australia ; 5 Blacktown Cancer and Haematology Centre., Blacktown Hospital, Blacktown, Australia ; 6 NHMRC Clinical Trials Centre, The University of Sydney, Sydney, Australia ; 7 Research and Education Network, Western Sydney Local Health District, Westmead, Australia ; 8 Nepean Hospital, Clinical Trials Unit, Nepean, Australia ; 9 Westmead Hospital, Cardiology, Westmead, Australia ; 10 Chris OBrien Lifehouse, Radiation Oncology, Camperdown, Australia Purpose or Objective The aim of this study was to determine whether deep inspiration breath-hold (DIBH) would reduce dose to organs at risk (OAR), in particular the right coronary artery (RCA), in women with breast cancer requiring right-sided post- mastectomy radiation therapy (PMRT) including internal mammary chain (IMC) RT. Material and Methods Consecutive patients requiring right-sided PMRT including IMC RT were retrospectively identified. Nodal delineation (IMC, SCF, axilla) was completed by two breast radiation oncologists (RO) in accordance with ESTRO guidelines and with tangential chest wall fields marked, whilst OAR contouring followed the ASTRO 2018 Consensus. Each patient was planned with AAA using a free-breathing (FB) and a DIBH data set for dosimetric comparison between the two techniques. A prescription of 50Gy in 25 fractions was integrated into a wide angle, hybrid IMRT plan using Eclipse 13.7, ensuring ≥95% coverage to PTV structures at D95, with departmental guidelines used for OAR doses. Each patient was calculated using Acuros External Beam algorithm (dose to medium) due to its accuracy in modelling the transport of radiation in heterogeneous regions. All plans underwent full quality assurance checks, including RT, RO and physics approval to guarantee satisfactory plan quality. FB and DIBH dose comparisons were analysed for heart, RCA and right lung, as were PTVs for chest wall and IMC PTV. Skewed data were summarised using medians, lower and upper quartiles (LQ,UQ), Wilcoxon tests, minimums and maximums. Results Fourteen women were scanned between January 2016 and April 2018. A median reduction of 1.7Gy (p=0.019; LQ- 0.02,UQ7.1) in maximum heart dose was recorded when comparing DIBH with FB; however, no significance difference was observed in Heart V5, V10, V15, mean heart dose or LAD (mean, max and V5). In assessing DIBH, vs. FB, the RCA mean dose had a median decrease of 0.6Gy (p=0.002; LQ0.1,UQ1.9), a 1.8Gy decrease in max dose (p=0.002; LQ0.8,UQ6.1), and 2.9% median reduction of V5Gy (p=0.016; LQ0.0,UQ37.2). The RCA data indicated no statistically significant dosimetric reduction ≥10Gy. The median right lung mean dose was 2.8Gy lower for DIBH vs. FB plans (p=0.001; LQ1.6,UQ3.6). Median reductions of 5.1% for V5Gy (p=0.004; LQ2.4,UQ6.8), 5.9% for V20Gy (p=0.001; LQ3.3,UQ7.6), and 5.6% for V30Gy