ESTRO 2021 Abstract Book

S1594

ESTRO 2021

Only 26/60 lesions were initially prescribed a SF of 20.0 Gy by the treating radiation oncologist. While 22/25 NSBs (88%) were treated with a SF, only 4/35 LNs (11%) received a SF treatment. All SF treatments could be delivered as planned. The gross tumor volume (GTV) was significantly larger for the SF lesions, with mean GTVs of 12.5 (0.8–72.6) vs. 9.16 (0.3–56.5) cm³ for the SF and MF lesions, respectively (p=0.01). Plan reporting parameters were clearly superior for the SF treatments (Figure 1), with a mean PTV V100% of 96% (67-99) vs. 64% (0-100), a mean D98% of 98% (76-103) vs. 86% (53-105), a mean D2% of 126% (107-135) vs. 119% (92-157) and a mean D50% of 115% (104-124) vs. 108% (84-139), for the SFs and MFs, respectively. Considering limiting OARs, most restrictions were imposed by maximum dose-constraints of small bowel and sacral plexus (in 35% and 29% of the MF treatments, respectively). Not surprisingly, the minimal distance between PTV and limiting OAR was significantly smaller for the MF treatments: a mean minimal distance of 0.4 cm (0.0-2.7) vs. 3.9 cm (0.0-14.0) for MF and SF, respectively (p<0.001). In an attempt to predict the possibility of delivering SF SABR, constraints for OARs of all 34 MF plans were re-calculated to a single dose. In 17/34 (15 LN and 2 NSB) cases, no dose constraints were violated, and a SF SABR would have been feasible. No correlation existed between GTV and technical feasibility of SF. Distance between PTV and limiting OAR was again larger for the group where SF was technically feasible compared to the others: mean distance of 2.5 cm (0.0-14.0) vs. 0.3 cm (0.0-1.8) for the SF-technically-feasible group and the remaining plans, respectively (p<0.001).

Conclusion The majority of NSB and LN metastases are eligible for SF SABR, but prescribing physicians have difficulties in estimating its potential prior to actual treatment planning. We suggest prescribing a SF for these patients and only converting to a MF schedule in case of violation of constraints. Further research focusses on creating a model to predict the most feasible fractionation schedule. PO-1872 Implementation of semi-automated restricted IMRT and comparison to 3DCRT and VMAT for breast cancer C. Milewski 1 , M. Chéve 1 , N. Fournier-Bidoz 1 , M. Lerousseau 2,3 , E. Deutsch 1,4 , G. Louvel 1 , S. Rivera 1,4 , G. Auzac 1 1 Gustave Roussy Cancer Campus, Radiation Oncology Department, Villejuif, France; 2 Paris Saclay University, Inserm, U1030, Villejuif, France; 3 Paris-Saclay University, CentraleSupélec, Gif-sur-Yvette, France; 4 Paris-Saclay University, Inserm, U1030, Villejuif, France Purpose or Objective To implement a step and shoot semi-automated IMRT technique in order to improve dose distributions without increasing workload and treatment planning duration in case of adjuvant whole breast irradiation. To compare dose distribution of this technique with 3D conformal irradiation (3DCRT) and VMAT. Materials and Methods This monocentric retrospective study included 20 patients receiving adjuvant breast irradiation planned in 3DCRT using RayStation TPS version 9B (RaySearch, Sweden). With the same TPS, restricted IMRT (rIMRT) plans were secondarily created on 2 beams (internal and external tangential manually positioned) using inverse planning restricted optimization parameters (OPs): a maximum of 10 segments in total and a minimal area of 9 cm² by segment. To ensure accuracy of OPs settings and process speed, treatment planning was semi-automated by scripting the creation and optimization steps of rIMRT plans. VMAT plans using 2 partial arcs were also retrospectively calculated for each patient and quantitatively compared to the 3DCRT and rIMRT plans using homogeneity index (HI), target coverage (D98%) and hotspots (D2%) assessment with a paired two samples t-test. P-values < 0.05 were considered as statistically significant. Patient-specific