ESTRO 2021 Abstract Book

S1572

ESTRO 2021

including where segments of OARs were re-irradiated. Gross tumour volume node (GTVn) median TCP was 95.7% (94.4-96), 90.7% (87.1-91.2) and 78.6% (75.8-81.1) for 40, 35 and 30 Gy SIB plans respectively, where an a/b ratio of 1.5 was assumed. SacralPlex median NTCP was 43.2% (0.7-61.2), 12.1% (0.6-29.7) and 2.5% (0.5-5.1) for 40, 35 and 30 Gy SIB plans respectively. NTCP for Bowel_Small was <0.3% and zero for other OARs for all three plan types. In Figure A-C, axial, coronal and sagittal representations of the isodose distribution (in absolute values of Gy) for a 30 Gy SIB plan to a pre-sacral nodal recurrence can be seen. GTVn (red), PTVn (green), PTV_Elective (blue), Bowel_Small (yellow), Colon/Colon_Sigmoid (orange) and SacralPlex (brown) are shown. The fall-off in dose superior to the post-operative radiotherapy isodose distribution (52.5 Gy in 20 fractions) in Figure D can be visualised.

Conclusion Ultra hypofractionated ENI planning for pelvic nodal relapsed PCa post radical prostatectomy/post-operative radiotherapy appears feasible with encouraging estimates of nodal TCP and low estimates of NTCP, especially where a low a/b ratio is assumed and a 30 Gy SIB is delivered. This solution should be further evaluated within a clinical trial and compared against SABR to the involved node(s) alone.

PO-1844 Adaptive dose escalation for non-small cell lung cancer patients treated with VMAT N.S. Tambe 1 , I.M. Pires 2 , C. Moore 1 , A. Wieczorek 3 , S. Upadhyay 3 , A.W. Beavis 1

1 Hull University Teaching Hospitals NHS Trust, Radiation Physics, Cottingham, United Kingdom; 2 University of Hull, Faculty of Health Sciences, Hull, United Kingdom; 3 Hull University Teaching Hospitals NHS Trust, Clinical Oncology, Cottingham, United Kingdom Purpose or Objective Higher radiation doses could improve local control and overall survival of lung cancer patients. However, increasing dose to the target volume without exceeding organs at risk (OAR) dose constraints could be challenging especially for patients with advanced-stage inoperable disease. Increasing OAR doses could increase toxicities and adversely affect the patient’s quality of life and overall survival. It is therefore important to investigate methods to increase the dose to target volume without exceeding OAR dose constraints. Materials and Methods Retrospective data for eleven patients were used to investigate the continuous adaptive dose-escalation method. Gross tumour volume (GTV) was contoured on synthetic computerised tomography (sCT) images produced using the Velocity adaptive radiotherapy software. The fractions where GTV size decreased compared to the GTV size at fraction 0 (before starting radiotherapy) or at fraction_n (i.e., the fraction on which dose was escalated during treatment) were considered for inhomogeneous dose escalation. Dose to the adapted GTV plus 0.5 cm margin (for setup) was increased until OAR doses started to increase compared to the original clinical plan. Also, planning target volume (PTV) coverage was maintained for all the plans. Doses were also escalated on fraction 0 (before starting radiotherapy) using the same method. Adaptive dose- escalated plans were combined to estimate the total dose to GTV D 99 , PTV D 99 , and OAR doses and compared with the doses achieved in the original clinical plans. In addition, tumour control probability (TCP) for clinical, fraction 0 and total plans were calculated and compared. Furthermore, two knowledge based planning (KBP) models were developed to predict GTV D 99 (dose to 99% of GTV) whilst keeping the OAR doses and PTV coverage similar to the original clinical plans. Both models were verified using data outside the models and prediction accuracy were assessed. Results The results show that our continuous adaptive dose-escalation method can significantly increase the dose to GTV compared to the original clinical plan without exceeding OAR doses or compromising PTV coverage. Adaptive dose-escalation increased average dose to GTV D 99 by 15.1 Gy and 6.4 Gy compared to the clinical plan and the fraction 0 plans. GTV TCP increased by 156.4% and 10.3 % compared to the clinical and fraction 0 plans (see Table 1). The knowledge-based planning models developed were verified using an independent dataset; the prediction accuracy of models 1 and 2 were -0.6 % and - 2.0 % respectively.