ESTRO 2020 Abstract book

S1101 ESTRO 2020

χ2 = 151.231, and the p-value = 0.345. K-W test statistics are T = 4.504 and p = 0.479. Therefore, research hypotheses should be rejected. Due to the large difference in the number of measurements in individual groups, weaker tests have been used, which could result in the lack of relationship between the CIED dose and the irradiated area, because the relations were quite high, but with a small amount of data, critical values were unusually high. χ2 test statistics for independence between the CIED dose from the entire RT course and the irradiation technique is χ2 = 88.212, and the p-value = 0.444. K-W test statistics are T = 1.887 and p = 0.596. Conclusion Retrospective statistical analysis of CIED doses during the first fraction showed that: The dose per CIED received throughout the radiotherapy course will not be dependent on the total PTV dose. The CIED dose may depend on the irradiation area (brain, head and neck, thorax and breast), but a larger group of people should be examined to be considered statistically significant. The irradiation technique (3DCRT, IMRT, VMAT, 4D(Gating)) should not affect the CIED dose. PO‐1879 A novel and objective plan evaluation tool for dose escalation in NSCLC within the ADCSCaN trial A.R. Lopes Simões 1 , R. Mir 1 , C. Lawless 2 , A. Shaw 2 , C. Peedell 3 , T. Pope 4 , J. Lester 5 , D. Landau 6 , C. Faivre- Finn 7 , H. Matthew 8 1 National Radiotherapy Trials Quality Assurance RTTQA group, Mount Vernon Cancer Centre, Middlesex, United Kingdom ; 2 Cancer Clinical Trials Unit Scotland, The Beatson West of Scotland Cancer Centre, Glasgow, United Kingdom ; 3 James Cook University Hospital, Radiotherapy, Middlesbrough, United Kingdom ; 4 Clatterbridge Cancer Centre, Radiotherapy, Liverpool, United Kingdom ; 5 Singleton Hospital, South West Wales Cancer Centre, Swansea, United Kingdom ; 6 Guys and St Thomas Cancer Centre- London, Radiotherapy, London, United Kingdom ; 7 The Christie Hospital, Radiotherapy, Manchester, United Kingdom ; 8 Weston Park Hospital, Radiotherapy, Sheffield, United Kingdom Purpose or Objective ADSCaN (Accelerated, Dose-escalated, Sequential Chemo- Radiotherapy in Non-small cell lung Cancer (NSCLC); ISRCTN47674500) is a randomised phase II clinical trial comparing four experimental radiotherapy (RT) regimes for NSCLC (CHART-ED, IDEAL, I-START and ISOTOXIC IMRT) to a standard RT regime of 55Gy in 20 fractions. RT planning for IDEAL, I-START and Isotoxic IMRT includes novel dose escalation and intensification schemes which require a change in planning technique. The aim of this study was to identify and to validate novel metrics to assess whether an RT plan is optimal. Material and Methods A NSCLC benchmark case was sent to ADSCaN centres. Centres submitted a plan for at least one of the experimental arms. GTV, CTV and OARs were pre- contoured. The PTV was derived at each centre as per the protocol; 5mm isotropic margin was used by centres performing per fraction cone-beam CT, and 7mm axially and 9mm cranio-caudally for those who did not. Table 1 shows the ADSCaN research arm requirements for RT planning. PTV conformity (V 95%isodose /PTV V total ) and OAR compromise (OAR V 95%isodose /OAR&PTV V Total overlap ) indices were calculated and validated for each dose escalation arm from the 22 submitted plans. This analysis focuses on the 3 research arms that employ isotoxic dose escalation methods; therefore CHART-ED arm was not included in this analysis.

Results The OAR limiting dose escalation for IDEAL and I-START plans was the oesophagus, whereas the mediastinal envelope limits prescription dose for ISOTOXIC IMRT. Table 2 shows the dose achieved to PTV, oesophagus, mediastinal envelope, PTV conformity and OAR compromise indices. The remaining OAR doses although assessed, are not reported in this table as they were easily achieved for this case. All plans were assessed as per the requirements described (table 2) and were considered sub- optimal if not achieving the criteria for at least two of the following: PTV coverage, OAR dose-volume constraints, PTV conformity and OAR compromise indices. All plans were also visually assessed in all slices.

Conclusion 45.5% of the submitted plans were sub-optimal as plans had compromised PTV coverage in favour of meeting OAR dose constraints, overdosed OARs or did not escalated the prescription dose as high is achievable. We attribute the high number of sub-optimal plans to the learning curve in the new individualised planning techniques included in ADSCaN. Assessing plans that use novel dose escalation can be challenging, particularly in determining whether a plan is optimal. This is a novel and objective evaluation that can be applicable to any dose escalated lung RT plan. PO‐1880 Cardiac exposure in left‐sided breast cancer radiotherapy: which substructure should be contoured? Z. NaimI 1 , R. Moujahed 1 , M. Ben Rejeb 1 , S. Ghorbel 1 , J. Yahyaoui 1 , A. Hamdoun 1 , M. Bohli 1 , L. Kochbati 1 1 Abderrahmen Mami Hospital, Radiation Oncology, Ariana, Tunisia Purpose or Objective The aim of our study was to analyse radiation dose distribution to cardiac chambers and coronary arteries in left-sided breast cancer radiotherapy and to clarify