ESTRO 2021 Abstract Book

S273

ESTRO 2021

the ESTRO, AAPM, and ABG have created the working group on Model-Based Dose Calculation Algorithms in Brachytherapy. Its main aim being to facilitate such task to clinical user by creating and analyzing relevant test cases. In this talk, I will describe the new MBDCAs commercially available in clinical practice and their main properties. I will focus on their strong and weak points when dealing with clinical scenarios and I will establish the relevance of the proposed test cases in clinical practice.

SP-0373 Do we have enough data to feed automated dose planning algorithms in brachytherapy? B. Pieters

Abstract not available

SP-0374 Requirements for a clinical treatment planning concept to guide automated brachytherapy planning E. Placidi 1 1 Fondazione Policlinico Universitario A. Gemelli IRCCS, UOC di Radioterapia - Gemelli ART, ROME, Italy Abstract Text The presentation “ Requirements for a clinical treatment planning concept to guide automated brachytherapy planning” will focus on the new aspects of brachytherapy. An overview on the state-of-the-art techniques for modern brachytherapy will be presented by emphasizing the aspects that need further developments. The physicist point of view will also be deepened by discussing how the new algorithm implementations may change the clinical practice. The presentation will also give a closer look to the features of the treatment planning systems that are mostly required in the future. PH-0375 Lymphocyte-sparing in pelvic radiotherapy for prostate cancer: an in-silico planning study M. Baré 1 , S. Poeta 2 , Y. Jourani 3 , F. Otte 4 , P. Lambin 4 , D. Van Gestel 4 , R. Van den Begin 4 1 Institut Jules Bordet - Université Libre de Bruxelles, Radiation Oncology Department , Brussels, Belgium; 2 Institut Jules Bordet - Université Libre de Bruxelles, Medical Physics Department , Brussels, Belgium; 3 Institut Jules Bordet - Université Libre de Bruxelles, Medical Physics Department, Brussels, Belgium; 4 Institut Jules Bordet - Université Libre de Bruxelles, Radiation Oncology Department, Brussels, Belgium Purpose or Objective Irradiation of bone marrow (BM) and large blood vessels, usually not taken into account during RT planning, can cause transient or persistent lymphopenia. Furthermore, lymphopenia correlates strongly with worse oncological outcome. In this planning study, we investigated the feasibility of lymphocytes-rich OAR (LOAR) sparing in pelvic irradiation for localized prostate cancer (PCa). Materials and Methods Twenty consecutive patients previously treated with RT of the prostate (P, n=10, 77Gy/35#) or prostate bed (PB, n=10, 70Gy/35#) between Mar 2019 and Oct 2020 were included. All patients received pelvic nodal irradiation (56Gy/35#). The following LOARs were delineated using semi-automatic segmentation: lumbosacral spine (Ls-BM), ilium (Il-BM), low pelvis (Lp-BM) and the combined whole-pelvis (Wp-BM), using semi-automatic segmentation. Twenty new treatment plans were optimized using Monaco (v5.51, Elekta AB, Stockholm, Sweden) to lower V10Gy, V20Gy and Dmean for Wp-BM, and V40Gy for Ls-BM, Il- BM and Lp-BM, while maintaining strict coverage of the targets and respecting standard OARs dose constraints. Dosimetric parameters were compared between standard-of-care (SOC) and lymphocyte-sparing (LS) plans, using a Wilcoxon signed-rank with a statistical significance level of p<0.05. Additionally, all treatment plans were tested using the institutional standard for patient specific QA. Results The LS plans showed a statistically significant decrease in all LOAR parameters compared to SOC plans (table 2): an absolute decrease of 13.3, 9.7 and 14.4% for the V40Gy of Ls-BM, Il-BM and Lp-BM respectively in P patients, and 14.2, 16.6 and 21.4% in PB patients. For Wp-BM, we observed statistically significant differences for V10Gy, V20Gy and Dmean in favor of LS plans for all patients. These results are also illustrated in figure 1. There was no difference in coverage for PTV70-77 and PTV56 between both treatment plans (table 1). The bladder and femoral heads showed no significant difference either. Regarding the rectum, the V50Gy was better for P but the V70Gy was slightly higher in LS plans for PB, while respecting the constraints. Although the V45Gy of the bowel bag was higher in LS plans, dose constraints remained respected. All plan QAs met our standard for acceptance (>95%, 3%/3mm gamma). Poster highlights: Poster highlights 13: Treatment planning 2