ESTRO 2021 Abstract Book

S1107

ESTRO 2021

hypofractionated RT with focal dose escalation influences tumour control probability (TCP) and normal tissue complication probability (NTCP). Materials and Methods 10 patients with primary PCa, who underwent 68Ga PSMA-PET/CT and mpMRI followed by radical prostatectomy were enrolled. Intraprostatic tumour volumes were contoured manually based on both imaging techniques (GTV-MRI and -PET) using validated contouring techniques and GTV-Union was created by merging both. Segmentation of prostate and organs at risk was based on ESTRO-ACROP guidelines. Three intensity modulated RT (IMRT) plans were generated with 60 Gy to the whole prostate and a simultaneous integrated boost up to 70 Gy to GTV-Union in 20 fractions. The dose concept was adapted from and constraints were in accordance with the DELINEATE trial. Plan 1 did not consider dose constraints for urethra, plan 2 considered dose constraints for urethra and plan 3 considered dose constraints for planning organ at risk volume for urethra (PRV=urethra + 2mm expansion). Distance of GTV-Histo and GTV-Union to urethra was determined. Calculation of TCP-Histo was based on PCa distribution in co-registered histology (GTV-Histo) applying a linear- quadratic Poisson model. Calculation of NTCP urethra was calculated applying a Lyman-Kutcher-Burman model. Additional complication free tumour control probability (P+) was calculated. Furthermore, the intrafractional movement was considered. Results Median overlap of GTV-Union and PRV-Urethra was 1.6% (IQR 0-7%). Median minimum distance of GTV-Histo to urethra was 3.6 mm (IQR 2 – 7 mm) and of GTV-Union to urethra was 1.8 mm (IQR 0.0 – 5.0 mm). The specified prescription doses and dose constraints were reached in all three plans. Urethra-sparing in plans 2 and 3 reached significantly lower NTCP-Urethra (p=0.002) without significantly affecting TCP-GTV-Histo (p>0.28), NTCP-Bladder (p>0.85) or NTCP-Rectum (p=0.85), resulting in a significant improvement of P+ from 88,8% to 91,9% (p=0.006). Intrafractional movement simulation yielded even higher P+ values for Plans 2 and 3 compared to Plan 1. Conclusion Urethral sparing in mpMRI- and PSMA-PET/CT-defined focal dose escalated MHRT is feasible and improves the therapeutic ratio. Therefore, urethral sparing should be implemented in focal RT dose escalation concepts. PO-1349 Separation and rectal dosimetry with a pre-rectal spacer inserted during prostate HDR brachytherapy R. Davda 1 , D. Sandys 2 , W. Kinnaird 3 , M. Boutros 4 , D. Heffernan Ho 5 , D. Pendse 5 , A. Mitra 1 , C. Allen 5 , H. Payne 1 1 University College London Hospital, Oncology, London, United Kingdom; 2 University College London Hospital, Radiotherapy Physics, London, United Kingdom; 3 University College London Hospital, Radiotherapy, London, United Kingdom; 4 University College London Hospital, Radiotherapy physics, London, United Kingdom; 5 University College London Hospital, Radiology, London, United Kingdom Purpose or Objective In men treated with IMRT, pre-rectal spacers have been shown to reduce the high dose (V70) delivered to the rectum, and late patient reported toxicity. The ICEMAN trial is investigating insertion of a pre-rectal spacer in 30 men receiving hormone therapy, high dose rate (HDR) brachytherapy boost and IMRT to the pelvis. We report findings on prostate – rectal separation and rectal dosimetry using a novel technique to insert a spacer (SpaceOAR) during brachytherapy. Materials and Methods Participants underwent a multi-parametric MRI < 2 weeks prior to brachytherapy and spacer insertion, and 5 days afterwards. Brachytherapy procedures were via a transperineal approach under transrectal ultrasound guidance. Immediately prior to brachytherapy catheter implantation, a 15 cm Kellett needle was inserted in the midline between Denonvillier’s fascia and the anterior rectal wall to the prostate midgland and the 5Fr Kellett sheath left in-situ. After HDR implant catheter placement, hydrodissection of the pre-rectal space was achieved by injecting saline via this sheath with subsequent injection of 10 cc hydrogel spacer prior to planning. CT planning was used to define CTV, organs at risk and spacer. Brachytherapy plans were produced using graphical (GRO) and inverse planning by simulated annealing (IPSA). HDR prescription was 15 Gy to the CTV with mandated dose constraints to the rectum, bladder and urethra. Separation between posterior prostate and anterior rectal wall was measured by a Uro-radiologist on MRI prior to and after the procedure on T2 axial 3 mm slices using sagittal planes for reference at the apex, mid-gland, base and seminal vesicles. HDR rectal dosimetry with spacer inserted and for 30 matched patients without a spacer is reported. Results The technique allowed clear visualisation of the prostate during HDR catheter insertion. Complete 10cc spacer injection was successful in 29 / 30 patients and incomplete (<10 cc) in 1 case due to concerns of sheath placement. All men received 15 Gy HDR boost as planned. No complications related to the procedure have been observed. Prostate-rectal separation increased after spacer insertion at all levels of the prostate (Table 1).