ESTRO 2020 Abstract book

S222 ESTRO 2020

trial has Although the Z0011 trial has been critized 8 , most authors agree that there is probably a subset of patients with a positive SN, in whom axillary treatment can be omitted. How to identify these patients is however still not yet clear. Haffty et al 9 suggested to replace an ALND by less or more extensive regional RT, dependent on the estimated risk group. Similarly, current Dutch guidelines recommend to apply axillary treatment in patients with a positive SN according to three risk groups: low, intermediate and high risk, with no axillary treatment in the low risk group, treatment of axilla level 1&2 in the intermediate risk group either by ART or ALND, and to add RT of axilla level 3 and 4 in combination with the CW-RT to treatment of axilla level 1&2 (again: either RT or ALND) in the high risk group. Especially in this intermediate risk group, there is a subset of patients in whom CW-RT is not clearly indicated after an ALND, i.e. the subgroup of patients that was eligible for the SUPREMO trial, where the additional value of CW-RT is being investigated. In this specific risk group, replacement of an ALND by ART, regional RT without CW- RT can be considered. A similar situation may occur in patients with cT1-2N0, ypT1- 2N1mi(SN) luminal A breast cancer without risk factors. In conclusion: Although CW-RT is indicated in the majority of patients with LABC, there may be a small selected group of patients where regional RT may be considered without CW-RT. 1. http://www.nccn.org/professionals/physician_gls/pdf/b reast.pdf 2. Simos D et al. Curr Opin Support Palliat Care 2014;8:33– 8. 3. Recht A et al. JCO 1999: 17(6):1689-700. 4. Mamounas EP et al. JCO 2012: 30(32):3960-6. 5. Giuliano et al. JAMA 2017: 318(10): 918–26. 6. Galimberti et al. Lancet Oncol 2018; 19: 1385–93. 7. Donker et al. Lancet Oncol 2014: 15(12): 1303–10. 8. Kühn & Poortmans, Breast Care 2011;6:154–57. 9. Haffty BG et al. JCO 2011: 29(34):4479-81. SP-0383 Protons, photons or MR linacs for breast cancer? B. Vrou Offersen 1 1 Aarhus University Hospital, Experimental Clinical Oncology, Aarhus N, Denmark Abstract text Radiation therapy (RT) of locally advanced breast cancer (LABC) patients reduces risk of local, regional and distant failures and improves survival. The planning of RT in these high-risk patients is challenging and involves advanced techniques including detailed target volume delineation and respiratory gated strategies to ensure optimal dose to targets and as low dose as possible to organs at risk (OAR). For decades, photon-based RT has been worldwide standard of RT of LABC patients. Therefore, gains and risks from RT of LABC reported in meta-analyses are based on photons. Photon RT is relatively easy to plan and robust to deliver in most patients, and anti-cancer effects and acute and late side-effects have been reported in numerous randomized trials. There is no doubt that photon RT is a good solution for most LABC patients. However, there are some patients, where individual factors hamper optimal photon RT planning and delivery. These factors involve for example anatomical characteristics, poor cooperation with respiratory gated technique combined with the location of tumor bed, all leading to a poor dose distribution to relevant targets and/or high dose to OAR. Such patients constitute around

Symposium: Controversies in locally advanced breast cancer

SP-0382 Regional nodal irradiation without chestwall radiotherapy for breast cancer L. Boersma 1 1 Maastricht University Medical Centre+, Radiation Oncology MAASTRO- GROW School for Oncology and Developmental Biology-, Maastricht, The Netherlands Abstract text In the first part of this presentation, the definition of Locally Advanced Breast Cancer (LABC) will be discussed. A short review will be given on the distribution of locoregional recurrences after a modified radical mastectomy (MRM) in the absence of radiation treatment (RT), to identify regions with the highest risk on a recurrence. From these data the choice for target volumes of post mastectomy radiotherapy (PMRT) will become clear. In the second part, replacing an axillary lymph node dissection (ALND) by axillary RT (ART) is discussed, which may result in some specific situations where regional RT is applied without chestwall (CW)-RT. Definition of LABC and distribution of locoregional recurrences: The term LABC usually describes a breast cancer that has progressed locally but has not yet spread outside the breast and local lymph nodes. The exact definition varies from Stage III breast cancer 1 to patients with ≥cT3, and/or ≥cN2 2 disease, to even patients with only cT1-2N+ patients. In most guidelines, locoregional treatment of LABC consists of breast surgery, usually MRM including an ALND, and at least CW-RT, but mostly locoregional RT. The CW is always included in the target volume, since several studies have shown that after MRM the risk on developing CW-recurrences is the highest, compared to nodal recurrences: in an analysis of 1099 patients treated with an MRM and systemic treatment in four ECOG trials where adjuvant RT was not allowed, Recht et al 3 found a 10 year CW-recurrence rate of 12%, with 8% in the supraclavicular nodes, 4% in the axillary nodes, and only 0.2% in the internal mammary chain. Manounas et al 4 found similar results in an analysis of the NSABP 18&27 trials: in 513 cT3 patients treated with systemic treatment and an MRM without PMRT, they found 50 (10%) CW- recurrences and only 14 (3%) regional recurrences at 10 years after diagnosis. For the 609 patients with cT1-2 tumours a similar pattern was seen, although less pronounced: 38 (6%) CW-recurrences vs 24 (4%) regional recurrences. Although these studies thus also included patients with stage II disease, they strongly support the fact that if RT is indicated, it should at least consist of CW- RT. Replacing ALND by ART: However, it can be wondered whether these conclusions also hold true in the absence of an ALND. Since the publication of the ACOSOG Z0011 trial 5 , the IBSCG 23-01 6 trial and the AMAROS 7 trial, tables have turned with respect to axillary treatment: the AMAROS trial showed that an ALND can safely be replaced by ART in case a positive sentinel node (SN) and pT1-2 disease, with significant less lymphedema after ART than after an ALND 7 ; the Z0011 and IBSCG 23-01 trials suggest that axillary treatment is not required at all in case of ≤2 macrometastases (Z0011) or micrometastases (IBSCG 23- 01) in the SN, in patients with pT1-2 disease. It should be noted however that the majority of these patients was treated with breast conserving therapy, including breast RT and adjuvant systemic treatment. Although the Z0011