ESTRO 2020 Abstract book

S569 ESTRO 2020

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

1 Fondazione Policlinico Gemelli IRCCS, Radioterapia Oncologica, Roma, Italy ; 2 Fondazione Policlinico Gemelli IRCCS, Chirurgia Senologica, Roma, Italy Purpose or Objective In the last years, thanks to diagnostic advantages, implementation in cancer behaviour knowledge and new target therapies, oliogometastatic disease is an even more common presentation. At the same time, oligoprogression is a reality too, that is raising the issue of maintaining systemic therapies that can be considered still efficacy. In literature, some evidences are reported about the role of radiotherapy in oligoprogression with a known driver of tumor progression, for example, gene addicted lung cancer. On the other side, stereotactic body radiotherapy (SBRT) or Stereotactic Radiosurgery (SRS) are emerging as part of oligometastatic disease in many tumours as prostate cancers. In this preliminary study, we analysed the role of radiotherapy in oligoprogressive breast cancer (OPBC) setting. Material and Methods All the OPBC patients (pts) during systemic therapy that underwent a local not antalgic radiotherapy treatment with VMAT, SBRT or SRT technique were included into the study. Data on age, tumor characteristics at diagnosis, systemic therapy, radiotherapy performed were retrospectively collected. Progression free survival-2 (PFS- 2) was defined as the time (months) from end of radiotherapy to new progression without changing systemic therapies. A Kaplan-Meyer analysis was performed for PFS-2 outcome and a subgroup analysis was conducted on histology, subtype, systemic treatment ongoing and parenchyma irradiated. Results From 2013, we retrospectively found 520 breast cancer metastatic pts who underwent RT. Among these, twenty- five pts were enrolled for this study. Median age was 56 (36-86). Systemic therapies ongoing were: 50% anti-Her2 drugs; 16,6% anti-CDK4/6 agents; 22% cytotoxic agents; 11,4% endocrine therapy. Radiotherapy was administered with VMAT or SBRT/SRS FFF technique on brain (72.2% of pts) with a schedule of 50/30 Gy at 5/3Gy (whole brain with a simultaneous integrated boost on lesions) or 25 Gy/8,5Gy (SBRT of single lesions); lung (16,6% of pts) with a schedule of 50 Gy/10Gy; bone with a schedule of 30 Gy/6Gy. Mean PFS-2 was 10.63 months (1-25) [Figure 1]. At the subgroup analysis histology (p=0.37), subtype (p=0.07), systemic treatment ongoing (p=0.18) and parenchyma treated (p=0.97) did not showed a significant correlation with PFS-2, while a treatment that control deeply a driver of cancer progression showed a positive correlation with it (p=0.02 with an advantage for luminal and HER2 respect triple negative breast cancer) [Figure 2]. Conclusion In this generating hypothesis study, preliminary results show that introducing focal radiotherapy treatments in OPBC treatments, can improve PFS-2, prolonging sistemic therapies, especially in good prognosis subtypes with a driver to target with a drug. Further studies are needed on large series to confirm these preliminary results, also with new target therapies ongoing. PO-0983 Mammary gland and implant changes during daily image guided intensity modulated radiotherapy S. Scharm 1 , S. Kampfer 1 , K.J. Borm 1 , S.E. Combs 1 , M.N. Duma 2 1 Klinikum Rechts der Isar -Technical University Munich, Department of Radiation Oncology, Munich, Germany ; 2 Klinikum Rechts der Isar -Technical University Munich and University hospital of Friedrich-Schiller-University Jena, Department of Radiotherapy and Radiation Oncology, Munich- Jena, Germany

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

Purpose or Objective