ESTRO 2020 Abstract Book

S631 ESTRO 2020

Purpose or Objective Low-dose rate brachytherapy (BT) utilizing permanent 125 iodine radioactive seeds is a highly effective treatment option for low and intermediate risk prostate cancer patients. In prostate brachytherapy, optimal implantation is not always achieved, due to edema or seed loss. One way to improve seed placement is the use of stranded seeds called intra-operatively built custom linked seeds (IBCLS) in opposition to loose seeds (LS) but there is no data comparing acute and late toxicities between these 2 techniques. The aim of this study was to compare the acute and late toxicities of both procedures in a matched paired population. Material and Methods Patients were considered for BT following the European guidelines. Patients with previous external radiotherapy to the pelvis or those who received androgen deprivation before the procedure were not considered. Between 2003 and 2018, 105 Patients in the “loose seeds” cohort were individually matched to 105 patients in the IBCLS cohort following the: age, prostate volume, preoperative IPSS score, clinical stage and Gleason score. Toxicities were assessed using the International Index of Erectile Function five items (IIEF5) and the International Prostate Symptom Score (IPSS) forms. Results Patients characteristics were comparable. There was no differences in the D90Gy (180.01Gy vs 179.78Gy, p=0.733), but the percentage of the prostate volume that receives 100% (V100%) and 150% (V150%) of the prescribed dose were higher in IBCLS (p<0.001). The organ at risks dose were lower for both rectum and urethra in IBCLS group (p<0.001). There was no difference in the IPSS at 1, 3 and 6 months between two groups, but at one year, the IPSS was lower in the IBCLS group (10.04 vs 7.77) (p=0.028). The sexual and gastrointestinal toxicity were similar in both groups. There are no grade 2 toxicities in the IBCLS group. Conclusion This study shows that prostatic brachytherapy by intra operatively built custom linked seeds is a safe technique, with a lower impact on IPSS score at 12 months compared to brachytherapy with loose seeds. PO-1199 Rectal toxicity prostate cancer treated with Brachytherapy: a radiomics-machine learning based NTCP G. Dissaux 1,2 , M. Ibrahim 2 , F. Lucia 1,2 , V. Bourbonne 1,2 , N. Boussion 1,2 , O. Pradier 1,2 , D. Visvikis 2 , A. Valeri 2,3 , J. Bert 2 , M. Hatt 2 , U. Schick 1 1 University Hospital, Radiation Oncology, Brest, France ; 2 Univ Brest, LaTIM INSERM UMR 1101, Brest, France ; 3 University Hospital, Urology, Brest, France Purpose or Objective Dose-volume histograms (DVH) do not account for the spatial relationships between voxel doses. We aimed to compare the prediction of rectal toxicities following prostate cancer (PC) Low Dose Rate Brachytherapy (LDR- BT) using the conventional approach based on DVH parameters with the use of dosimetric radiomic features using 4 ML machine learning algorithms and 3 deep learning (ML, DL) techniques as well majority voting techniques. Imbalanced class problem was also taken into account in our workflow. Material and Methods The records of 188 consecutives patients with more than 2 years follow up (including clinical data, DVH and rectal toxicities event) who underwent LRT-BT for localized PC were collected prospectively. 21 patients had ≥ grade 2 late rectal toxicity in our whole dataset. In our workflow we have used 4 ML algorithms such as Random Forest, Xtreme Gradient Boosting, LightGBM, CatBoost and 3 DL classifiers: Deep Neural Network, Deep Autoencoder+RF, Deep Variational Autoencoder+RF, as well as majority

each time point from baseline: soon after the end of RT, at one month after RT and at 12 ad 24 months. Results Overall, patients’ tolerance was assessed as satisfactory across all the considered time points, with no residual toxicity exceeding G2 at 6 months after the end of treatment, except for one patient who developed G3 GI symptoms. The most relevant deterioration in IPSS from baseline was reported after 1 month from the end of treatment, although a sizable recovery towards baseline value was assessed at 12 months. Boxplots reporting IPSS modification are shown in Figure 1. The QLQ-PR25 Urinary Symptoms score was also analyzed to evaluate the urinary function. A deterioration of GU symptoms from baseline was observed already after the end of RT and maintained at one month after, with a recovery towards baseline value at 12 months. Interestingly patients with a bladder volume under the median showed a worsening of symptoms after RT, whereas patients with a volume equal or greater than the median reported a significant decreasing trend of deterioration of symptoms with a median complete recovery at 12 months (Figure 2). The analysis of QLQ-C30 and IIEF-5 showed a non-statistically significant change of QoL from baseline. Only one patient out of the 65 patients died and not for disease-relating cause, leading to an OS of 98%. Biochemical progression-free survival (b-PFS) was of 97% at 2-years. Since 2014, only 2 patients experienced biochemical and clinical relapse.

Conclusion The median follow-up is still too short to make our data about the oncological outcomes consistent, but the toxicity results and the relative assessed QoL appear to be encouraging. The increasing dose to the DIL does not compromise the RT toxicity and at the same time opens the question about the possibility of an even more escalate treatment. PO-1198 Comparison between built custom linked seeds and loose seeds in prostate brachytherapy L. Ollivier 1 , F. Lucia 1 , C. Lucas 2 , V. Bourbonne 1 , J. Marolleau 2 , N. Boussion 1 , G. Goasduff 1 , G. Fournier 2 , G. Dissaux 1 , O. Pradier 1 , A. Valeri 2 , U. Schick 1 1 Radiation Oncology department- University Hospital- Brest- France-, Radiation Oncology department- University Hospital- Brest- France-, Brest, France ; 2 Urology Department - CHU Brest - Brest - France, Urology Department - CHU Brest - Brest - France, Brest, France