ESTRO 38 Abstract book

S833 ESTRO 38

Our patients have tolerated treatment better than expected with no sub-acute or late toxicity. Biochemical and PSMA responses are promising. We continue to accrue patients at dose level 2 (38Gy in 6#). EP-1545 Prostate cancer EBRT: adaptive strategy and use of robust optimization for geometrical uncertainties E. Ferrara 1 , D. Beldì 1 , J. Yin 2 , G. Loi 3 , M. Krengli 1,4 1 University Hospital "Maggiore della Carità", Radiotherapy, Novara, Italy ; 2 Sichuan Cancer Center, Radiotherapy, Chengdu, China ; 3 University Hospital "Maggiore della Carità", Medical Physics, Novara, Italy ; 4 University of "Piemonte Orientale", Translational Medicine, Novara, Italy Purpose or Objective Intensity modulated radiotherapy is sensitive to errors, mainly due to steep beam dose gradients and organ motion. Conventional margins are often insufficient to ensure robustness of treatment plans. The aim of this work is to develop and validate a new adaptive treatment method for prostate cancer (PCa) radiotherapy (RT), using an off-line strategy to design a patient-specific internal target volume (ITV) based on the study of organ motion obtained by serial Cone-Beam CT (CBCT) images and management of the geometrical uncertainties with min- max robust optimization. Material and Methods 20 patients (pts) with intermediate-high PCa treated with radical RT were enrolled in this study. The prescribed dose was 78 Gy to prostate (CTV78) and 60 Gy to prostate plus seminal vesicles (CTV60) delivering 2 Gy/fraction (fx). The CBCTs were acquired during the first 5 fxs, then once a week for the remaining treatment. The CBCTs were imported in the Raystation treatment planning system (TPS) and co-registered with the planning CT using the on- line match rigid transformations provided by the OBI system on the treatment unit (Varian Trilogy TX). Then the deformable image registration (DIR) algorithm ANACONDA was applied to propagate the CTV78 and CTV60 volumes from the reference planning CT to the first 5 CBCTs. The reliability of the DIR mapped ROIs was assessed by radiation oncologists and the contours were used to generate the ITV. The original plan was re-optimized using a min-max robust algorithm based on the worst scenario optimization assuming an isotropic 5 mm maximum setup error. Then CTV coverage and OARs sparing achieved with the robust plan (RP) were analyzed and compared with the original standard plan (SP) calculating the dose distributions on the residual CBCTs. For each isocenter perturbation of 1 mm, the dose was recalculated and the following parameters were acquired: D99, D98 and D95 to evaluate target volume coverage and mean dose and D2 to evaluate OARs doses. Results Our adaptive strategy and RP showed to achieve optimal coverage of CTV also in the worst scenario (geometric error up to 5 mm) with D99>95% of prescribed dose and significant less dose to rectum and bladder. The analysis on all the residual CBCT acquired during the treatment showed that CTV coverage of RP was statistically better than SP in terms of D99 and D98 (p=0.008 and p=0.02, Wilcoxon test). Statistically (T-student test) significant mean dose reduction and D2 reduction was noted for rectum (p<0.05) and for bladder (p<0.009). Moreover, RP appeared to be less sensitive to bladder and rectal filling. Conclusion Robust optimization is a feasible and safe approach in prostate treatment. It may be successfully used to adapt the treatment with better target coverage and OARs sparing than standard PTV based planning during the treatment course.

Conclusion Delivery of linac-based SBRT boost is feasible and well tolerated with low rates of early toxicity and promising PSA responses. A second transient peak in moderate irritative GU toxicity was observed at 18 months. No urethral strictures have been reported to date. Longer term follow-up is required to document late toxicity and tumour control with this approach. A randomized trial comparing this approach with SBRT monotherapy is under development. EP-1544 Focal Linac-based SBRT Re-treatment for local recurrence of Ca P following previous definitive RT G. Hruby 1 , A. Kneebone 1 , T. Eade 1 , A. Le 1 , J. Booth 1 , J. Hunter 1 , C. Kwong 1 , C. Brown 1 1 Royal North Shore Hospital, Radiation Oncology, Sydney, Australia Purpose or Objective to determine the safety and tolerability of focal linac based SBRT re-treatment with real time imaging for local only recurrence of prostate cancer after definitive RT Material and Methods A prospective, ethics approved feasibility and toxicity trial with nested sequential dose escalation component. In order to be eligible, men had to have biopsy proven recurrence limited to both PSMA-PET and mp-MRI region of suspicion and disease confined to less than half a lobe. We mandated a disease free interval of at least 4 years from initial RT and a PSA < 15 prior to enrolment. Results Between March 2016 and October 2018, 20 men were treated with 36 or 38Gy in 6 2nd to 3rd daily fractions. Two men had received LDR monotherapy and 2 HDR boost brachytherapy, the rest (16) EBRT. Median time from initial RT was 8 years (range 4.5-12). Original tumour details are presented in Table 1. At focal SBRT, median age and PSA were 73 and 5.6 respectively. All 20 completed SBRT; 5, 3 and 2 men with grade 1 fatigue, GU and GI toxicity, respectively. All toxicity was self- limiting and to date there has been no late toxicity. 19 of 20 patients are biochemically controlled with PSA response. Nine men have undergone per-protocol repeat PSMA-PET at 12 months, only one (also with BF = PSA nadir +2) has failed within the prostate. Eight of 9 have demonstrated metabolic PSMA response to focal SBRT (see Figure 1). Table 1: Tumour Features at original RT T stageNPSANGSNT190-10963T2910-204713T3220 or more783 91 Figure 1 16/11/2015 2 months before SBRT – local recurrence R lobe 3/6/2017: 12 months after SBRT re-treat

Conclusion