ESTRO 2024 - Abstract Book

S3667

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2024

in institute B [3] and returned to institute A for planning were considered: the first (labelled “sensitive rectum region”, SRR) was in the rectum and associated with rectal bleeding, while the second (labelled “sensitive bladder region”, SBR) was the union of two bladder subregions associated with late dysuria (DYS_LAT) and retention (RET_LAT) respectively (Fig.1).

A KB (RapidPlan Varian, Inc.) model was used to obtain clinically suitable plans using VMAT with four complete arcs (KB-plan). Then, keeping the same beam ballistic, we further optimized plans using MCO (Trade-Off Exploration, Varian Eclipse v. 16.1) to reduce the dose to the considered SRSs. We established a set of clinical goals: for low (pelvic nodes), intermediate (seminal vesicles) and high dose (prostate) PTV, we set to 95% the minimum value for V95%. We also chose objectives for MCO. Compared to the KB plan: a) dose homogeneity for PTVs should be unchanged; b) DVHs of the whole rectum/bladder and other OARs cannot be worsened; c) gEUD of SRSs should decrease as much as possible while respecting the previous criteria. When choosing gEUD as the optimization objective, the volume-effect parameter 'a' was selected as 2.0 for the SBR and 4.8 for the SRR , based on previously found results [4,5] (of note ‘a’ was determined in [5] for the Trigone, a region that broadly corresponds to the identified SBR). Finally, we generated a database of optimal plans and navigated to find the best-compromised plan, giving the same importance to SRR and SBR (KB+gEUD plan). We randomly selected a subset of forty-five patients to analyze the optimization performance and to serve as a collection of suitable plans to train a new KB Rapidplan model. We will use the fifteen patients left for an external validation. For the training subset, we computed and compared (Wilcoxon non-parametric test) the dose volume histograms (DVHs) resulting from KB vs KB+gEUD plans for SRSs and organs at risk (OARs), corrected for the 2Gy-equivalent dose (EQD2). For the structures under consideration, we extracted selected dose-volume descriptors, specifically: mean dose, gEUD, the EQD2 to 1% of the volume (EQD2_1%, taken as the maximum dose), V20Gy, V40Gy, V60Gy, V70Gy. We also evaluated differences in Monitor Units (MU) between KB and gEUD plans, as indication of the plan complexity.

Results:

The gEUD differences ranged from -11.7 to 0.0 Gy for the SBR (median -5.7 Gy) and from -8.0 to -3.6 Gy (median - 1.9 Gy) for the SRR. EQD2_1% differences spanned from -13.5 to 1 Gy (median -1.5 Gy) for the SBR and from -7.6 to 0.8 Gy for the SRR (median -0.8 Gy). Figure 2 presents mean DVHs for OARs and SRSs in both KB and KB+gEUD plans, accompanied by dose difference plots.