ESTRO 2021 Abstract Book

S1571

ESTRO 2021

Conclusion SPArc rot

could effectively mitigate the dose uncertainties in the adjunct series OARs where max dose may play a critical role in the tissue toxicity. This new algorithm could be implemented through the multi-CT robust optimization framework using the existing commercial TPS.

PO-1843 Ultra hypofractionated extended nodal irradiation using VMAT for pelvic nodal prostate cancer F. Slevin 1 , C. Thompson 1 , R. Speight 1 , L. Murray 2 , J. Lilley 1 , A. Henry 2 1 Leeds Teaching Hospitals NHS Trust, Leeds Cancer Centre, Leeds, United Kingdom; 2 University of Leeds, Leeds Cancer Centre, Leeds, United Kingdom Purpose or Objective Prostate cancer (PCa) may recur after primary treatment but no standard of care exists for patients with pelvic nodal relapse. Extended Nodal Irradiation (ENI) might be associated with fewer treatment failures than Stereotactic Ablative Radiotherapy (SABR) to the involved node(s) alone. Ultra hypofractionated ENI is yet to be evaluated in this setting, but it may provide a therapeutic advantage if PCa has a low a/b ratio plus patient convenience/resource benefits. This volumetric modulated arc therapy (VMAT) planning study developed a class solution for 5-fraction ENI plus a simultaneous integrated boost (SIB) to involved node(s). Materials and Methods Ten patients with oligorecurrent pelvic nodal disease after radical prostatectomy/post-operative prostate bed radiotherapy were selected. Three plans were produced for each dataset to deliver 25 Gy in 5 fractions ENI plus SIBs of 40, 35 and 30 Gy. The biologically effective dose (BED) formula was used to determine the remaining dose in 5 fractions that could be delivered to re-irradiated segments of OARs based on traditional constraints. Tumour control probability (TCP) and normal tissue complication probability (NTCP) were calculated using the LQ-Poisson Marsden and Lyman-Kutcher-Burman models respectively. Results Six patients had an OAR positioned within planning target volume node (PTVn), which resulted in reduced target coverage to PTVn in six, five and four instances for 40, 35 and 30 Gy SIB plans respectively. In these instances, only 30 Gy SIB plans had a median PTV coverage >90% (inter-quartile range 90-95). No OAR constraint was exceeded for 30 Gy SIB plans,