ESTRO 2020 Abstract Book

S206 ESTRO 2020

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 5-7% of LABC patients, and these patients may be better treated with protons. Protons are characterized by virtually no entry and exit doses, thus sharp dose gradients can be achieved. A major challenge using proton RT is the need for robustness, thus minor anatomical differences happening to the patient during the RT course may seriously influence the delivery and safety of the proton therapy. There are only few published results from small underpowered and usually retrospective studies investigating protons in LABC patients. In LABC patients, proton therapy is currently being tested in randomized trials, since the therapy is still considered experimental. The primary endpoint in those trials is late effects, primarily major cardiac disease and risk of second cancer, but it is also expected that proton therapy has potential to improve breast cancer distant failures due to better target coverage especially of the internal mammary nodes. The integration of MRI with a linear accelerator (MR-linac) offers great potential for high-precision delivery of RT. High-precision RT is needed in LABC patients who e.g . are candidate for nodal boost. Nodal boost is often difficult to deliver because of the proximity to the brachial plexus, but with higher precision in daily RT delivery it may be possible to use narrower margins and potentially a higher dose. At present there is no data supporting use of MR- linacs for breast cancer RT, however, it is likely that studies will be initiated to demonstrate the potential of MR-linacs for nodal boost and perhaps also in patients treated with pre-operative RT. Indeed, further studies are highly needed for both proton RT and MR-linacs. SP-0384 Re-irradiation versus mastectomy for recurrent breast cancer D. Gabrys 1 1 Maria Sklodowska-Curie Memorial Cancer Center and Institute of Oncology, Radiotherapy Department, Gliwice, Poland

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 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.