ESTRO 2020 Abstract Book

S523 ESTRO 2020

Results Table 1 shows the global results. The use of bra resulted in a significant dose reduction in lung (Dmean and V16Gy reduced by 7% and 11%, respectively), in heart (V10Gy reduced by 9%) and in liver (60% reduction in Dmean). Regardingbreast volume and ptosis grade, the highest benefits were seen in large breasts> 2000 ml and in GIII ptosissubgroup. Reproducibility was excellent, with minorpositioning shifts during treatment. Image 1. No acute skin toxicities > G2 and no differences were observed compared to patients treated without bra.

Purpose or Objective To quantify the dosimetric impact of the variations in target volume delineation of axillary lymph nodes (L2,L3,L4) in breast cancer locoregional treatment. Material and Methods 18 radiotherapy (RT) centres were asked to elaborate a locoregional treatment plan on their own planning target volume (PTV), namely Single Centre PTV (SCPTV), created from the clinical target volumes (CTV) they had delineated in a previous investigation, that is Single Center CTV (SCCTV). The SCCTV comprised the axillary nodes of three patients with different anatomy (P1,P2,P3 with increasing complexity). The corresponding gold standard CTV (GS- CTV) from an expert consensus outline validated with STAPLE algorithm. . For each patient, the GS PTV (GSPTV) was created by applying margins to the GS-CTV identical to those of each partecipating centre. Each centre planned the treatment on their own SC-PTV, then the SC-PTV was replaced with the GS-PTV (with margins given by the single centre). Datasets were imported into MIM software version 6.1.7 (MIM Software, Cleveland, OH), where DVH were extracted and differences were analyzed in terms of dose delivered to 99%, 98%, 95%, 2% and 1% of the volume (respectively D99, D98, D95, D2, D1), the mean dose delivered (Dmean) and the volume receiving 95% of the dose (V95). Statistical analysis included mean value, standard deviation (SD), 95% confidence intervals (95% CI) and p-value. Results 17/18 centres used exclusively Intensity Modulated RT (IMRT). The CTV to PTV margins ranged from 0 to 10 mm, with median 5 mm. Twelve centres used isotropic expansion. Results are shown in table 1. D99, D2, Dmean and V95% were chosen to analyze the results. No difference between SC-PTV and GS-PTV was observed in the maximum dose (D1%), which was under 110% in all the cases. Mean D98≥95% for SCPTV versus GSPTV was 92% (SD 5.80) and 57% (SD 18.55) for P1, 92% (SD 6.18) and 70% (SD 13.24) for P2, 91% (SD 7.59) and 57% (SD 2.71) for P3. SCPTV and GSPTV differences were significantly larger for P3 taking into consideration all the dosimetric parameters: regarding Dmean, the value was 4.35% (95%CI 2.48-6.22) compared to 3% seen for P1 and P2. Level 4 showed lower coverage for P2 and P3 in the GS-CTV. In the range of CTV to PTV expansion of IMRT, the coverage of GS-CTV by 95% isodose did not show a linear correlation with the margins width of GS-PTV. Conclusion All 54 but one plans were done with IMRT. Axillary nodes CTV delineation variation led to significant reductions in dose delivered to GSPTV. GS-PTV planning objectives were not achieved (only 1 plan out of 54). L4 more penalized in GS-PTV. IMRT relied on high-precision contouring and margins width did not compensate for contouring issues. The results highlighted the need for delineation training and standardization. Table 1

Conclusion Our results show that the use of Chabner XRT® Radiation Bra during WBI for patients with large or pendulous breastsimprovedall the main dosimetric factors evaluated. The treatment reproducibility was excellent and no acute toxicities > G2 were observed. PO-0981 Variability of axillary nodal CTV contouring in breast cancer (PART II – dosimetric impact) M.C. Leonardi 1 , F. Cattani 1 , M. Pepa 1 , R. Luraschi 2 , S. Vigorito 2 , S.G. Gugliandolo 1 , D. Ciardo 1 , R. Orecchia 3 , B.A. Jereczek-Fossa 1 , A. Breast Cancer Study Group 4 1 IEO- European Institute of Oncology IRCCS, Division of Radiotherapy, Milan, Italy ; 2 IEO- European Institute of Oncology IRCCS, Department of Medical Physics, Milan, Italy ; 3 IEO- European Institute of Oncology IRCCS, Scientific Directorate, Milan, Italy ; 4 Associazione Italiana di Radioterapia Oncologica, Airo, Milan, Italy

PO-0982 Preliminary analysis of radiotherapy role on PFS-2 in oligoprogressive breast cancer F. Marazzi 1 , V. Masiello 1 , T. Zinicola 1 , S. Luzi 1 , L. Boldrini 1 , G. Franceschini 2 , F. Moschella 2 , D. Smaniotto 1 , S. Reina 1 , S. Chiesa 1 , A. Petrone 1 , B. Corvari 1 , M. Balducci 1 , R. Masetti 2 , M.A. Gambacorta 1 , V. Valentini 1