ESTRO 2021 Abstract Book

S1563

ESTRO 2021

PO-1834 Dosimetric impact of introducing revised rectal constraints in 20 fraction prostate radiotherapy I. Gleeson 1 , H. Chantler 2 , K. Hutchinson 2 1 CRUK RadNet Cambridge, Medical Physics, Cambridge, United Kingdom; 2 Cambridge University Hospital NHS Trust, Medical Physics, Cambridge, United Kingdom Purpose or Objective Prostate radiotherapy 60 Gy in 20 fractions is still currently widely used in the UK. The CHHiP trial demonstrated its non inferiority versus 74 Gy in 37 fractions. This trial largely used a forward planned Intensity Modulated Radiotherapy (IMRT) technique with specific dose constraints for organs at risk (OARs) and targets. In January 2020, revised rectal dose constraints were published based on CHHiP data which were introduced into our department late October 2020. Prior to this, our OAR constraints were based on PIVOTALboost and CHHiP. The purpose of this study is to quantify the dosimetric impact of using these revised rectal dose constraints in our clinic. Materials and Methods The clinical planning database was searched for patients who had 6 MV Volumetric Modulated Arc Therapy (VMAT) 60 Gy in 20 fractions (prescribed to median of PTV60) before the introduction of revised rectal constraints and after. Revisions were also made to some existing constraints at this time (rectum and PTV coverage). Population Dose Volume Histograms (DVHs) were created for 50 patients planned with the old constraints and 37 with the new constraints. PTV60 was prostate + 0.5 cm and PTV48 was seminal vesicles + 1 cm. Groups were compared for statistically significant differences using two sample

t-tests. Results The results in Table and Figure show improvements in target coverage and lower rectum V 60Gy

. Additionally the case with

the highest rectal V 60Gy to 0% whilst maintaining similar target coverage and rectal DVH overall. The high dose was lowered only and the normal tissue complication probability (NTCP) for late rectal effects (TD50 Gy =76.9, m=0.13, n=0.09) reduced from 2% to 1% illustrating the potential clinical effects. (2.9% from before group) was re-planned to reduce the V 60Gy

Conclusion Revisions to our constraints were successfully implemented showing statistically significant improvements in target coverage and rectal V 60 Gy . No other changes were significantly different due to plans already achieving the OAR objectives apart from the rectum V 60 Gy (92% of plans V 60Gy < 0.1% when using this objective vs 24% previously without using it). This lower high dose may help reduce rectal toxicity as shown by NTCP reduction. This work also allows for analysis of potential areas where further tightening of OAR constraints may be possible. PO-1835 Can we deliver hypofractionated RT for PCa with a single arc? G. Guibert 1 , B. Yanes 2 , T. Risse 3 , P. Weber 4 , B. De Bari 2 1 Réseau Hospitalier Neuchâtelois RHNE , Radio-Oncologie, La Chaux-de-Fonds, Switzerland; 2 Réseau Hospitalier Neuchâtelois RHNE, Radio-Oncologie , La Chaux-de-Fonds, Switzerland; 3 Réseau Hospitalier Neuchâtelois RHNE, Radio- Oncologie, La Chaux-de-Fonds, Switzerland; 4 Réseau Hospitalier Neuchâtelois RHNE, Radio-Oncologie , La Chaux-de-Fonds , Switzerland

Purpose or Objective Hypofractionated RT is a standard of care in prostate cancer (PCa), as reduces the overall treatment time, but it could