Background: Approximately 20% of invasive ductal breast malignancies are human epidermal growth factor receptor 2 (HER2)-positive. These patients receive neoadjuvant systemic therapy (NAT) including HER2-targeting therapies. Up to 65% of patients achieve a pathological complete response (pCR). These patients might not have needed surgery. However, accurate preoperative identification of a pCR remains challenging. A radiologic complete response (rCR) on MRI corresponds to a pCR in only 73% of patients. The current feasibility study investigates if HER2-targeted PET/CT-imaging using Zirconium-89 (89Zr)-radiolabeled trastuzumab can be used for more accurate NAT response evaluation. Methods: HER2-positive breast cancer patients scheduled to undergo NAT and subsequent surgery received a 89Zr-trastuzumab PET/CT both before (PET/CT-1) and after (PET/CT-2) NAT. Qualitative and quantitative response evaluation was performed. Results: Six patients were enrolled. All primary tumors could be identified on PET/CT-1. Four patients had a pCR and two a pathological partial response (pPR) in the primary tumor. Qualitative assessment of PET/CT resulted in an accuracy of 66.7%, compared to 83.3% of the standard-of-care MRI. Quantitative assessment showed a difference between the SUVR on PET/CT-1 and PET/CT-2 (ΔSUVR) in patients with a pPR and pCR of −48% and −90% (p = 0.133), respectively. The difference in tumor-to-blood ratio on PET/CT-1 and PET/CT-2 (ΔTBR) in patients with pPR and pCR was −79% and −94% (p = 0.133), respectively. Three patients had metastatic lymph nodes at diagnosis that were all identified on PET/CT-1. All three patients achieved a nodal pCR. Qualitative assessment of the lymph nodes with PET/CT resulted in an accuracy of 66.7%, compared to 50% of the MRI. Conclusions: NAT response evaluation using 89Zr-trastuzumab PET/CT is feasible. In the current study, qualitative assessment of the PET/CT images is not superior to standard-of-care MRI. Our results suggest that quantitative assessment of 89Zr-trastuzumab PET/CT has potential for a more accurate response evaluation of the primary tumor after NAT in HER2-positive breast cancer.

Adjuvant immunotherapy is currently moving into earlier stages of melanoma. CPG7909, a Toll-like receptor-9 (TLR9) agonist, injected at the primary tumour excision site in patients with clinical stage I/II melanoma, was found to boost loco-regional and systemic anti-melanoma immunity. In an exploratory analysis this was accompanied by a significant decrease in tumour-positive sentinel lymph node (SLN) rates and improved recurrence-free survival (RFS). To confirm these results we performed a randomized, placebo-controlled and double-blind phase II study of TLR9 agonist IMO-2125 in stage II pT3-4/cN0 melanoma. We here report the interim efficacy and safety results. Patients with pT3-4/cN0 melanoma, scheduled to undergo wide local excision and sentinel node biopsy (SNB), were randomized 1:1 to a single intradermal injection of 8mg of IMO-2125 vs. placebo at the primary tumour excision site, 7-10 days prior to surgery. The primary endpoint was SLN tumour status; secondary endpoints were SLN and peripheral blood immune status, RFS and overall survival (OS) after 5 and 10 years. From January 2020 to February 2022, 108 patients received IMO-2125 (n=54) or placebo (n=54). At interim analysis, the primary endpoint had been reached, i.e. IMO-2125 injected patients had a significantly reduced tumour positive SLN rate compared to placebo-treated patients (13% vs. 45%, P<0.0001), exceeding the pre-specified P-value of significance of 0.008. Notably, more patients in the placebo group had ulcerated melanoma, but stratified analysis for ulceration showed a similar trend. IMO-2125 treatment was mainly associated with grade 1 and 2 injection site reactions (54%), flu-like symptoms (46%), chills (41%) and fever (33%), accompanied by grade 3 confusion in one elderly patient. Single-dose IMO-2125 has a reasonable safety profile and significantly reduces tumour positive SLN rates in pT3-T4 melanoma patients. Provided that SLN status accurately reflects future RFS, IMO-2125 has the potential to improve outcomes of early-stage melanoma patients.

Abstract Background: Sentinel node biopsy (SNB) is standard in assessing axillary lymph node status in patients with clinically node-negative breast cancer. The 5-year analysis of AMAROS trial showed that if locoregional treatment is advised after a tumor-positive axillary SNB, axillary radiotherapy (ART) is a reasonable alternative for an axillary lymph node dissection (ALND) with less side effects, though follow up was relatively short. Here we present the 10-year follow up data. Methods: From February 2001 to April 2010, patients with primary breast cancer stage cT1-2N0M0 were enrolled in the EORTC phase III non-inferiority AMAROS trial by 34 European sites. Patients were randomized between ALND and ART in case of a tumor-positive SNB. The primary endpoint, axillary recurrence rate (AxR) is now assessed at 10 years in the ITT population using Fine and Gray cumulative incidence method with deaths as competing risks, as well as secondary endpoints: overall survival (OS), distant metastasis free survival (DMFS), second primaries (including cancers other than breast cancers and contralateral DCIS) and locoregional recurrences (LRR). Little extra information beyond 5 years was available concerning Quality of Life and morbidity. Data collection is still ongoing and will be presented later. Results:Of the 4806 patients entered, 1425 patients had a tumor-positive SNB: 744 in the ALND-arm and 681 in the ART-arm, 60% with a macrometastasis. Both treatment-arms achieved a median 10-year follow-up and were comparable regarding age, tumor size, grade, tumor type and adjuvant systemic treatment. In the group who had ALND, the 5-year AxR was 0.41% (95%CI: 0.00;0.88) (4/744) and the 10-year AxR was 0.93% (95%CI:0.18;1.68) (7/744). In the group who had ART, the 5-year AxR was 1.04% (95%CI: 0.27;1.81) (7/681) and the 10-year AxR was 1.82% (95%CI: 0.74;2.94) (11/681) (HR 1.71, 95%CI: 0.67;4.39, p = 0.37). Sensitivity analysis, considering deaths and distant recurrences as competing risks, revealed consistent results. There were no significant differences between treatment arms regarding OS (ALND: 84.6% (95%CI: 81.5;87.1), ART: 81.4% (95%CI: 77.9;84.4), HR 1.17, 95%CI: 0.89;1.52, p= 0.26) and DMFS (ALND: 81.7% (95%CI: 78.5;84.4), ART: 78.2% (95%CI: 74.6;81.3), HR 1.18, 95%CI: 0.92;1.50, p=0.19). Cumulative incidence estimates of 10-year LRR are 3.59% (95%CI: 2.12;5.06) (ALND) versus 4.07% (95%CI: 2.49;5.65) (ART) (p= 0.69). More second primaries were observed after ART: 75/681 (21 contralateral breast) as compared to ALND: 57/744 (11 contralateral breast) (p = 0.035). All results are consistent in the per protocol analysis of patients with a tumor-positive SNB. Conclusion: Axillary recurrence after 10 years in patients with a tumor-positive SNB who were treated with ART is extremely rare and not significantly different from patients who were treated with ALND. OS, DMFS and locoregional control are also comparable. Second primaries including contralateral breast cancers are more frequently encountered after ART, but the difference is still low in absolute numbers. Thus, ART is a safe treatment for breast cancer patients with a tumor-positive SNB. Citation Format: Rutgers EJ, Donker M, Poncet C, Straver ME, Meijnen P, van de Velde CJ, Mansel RE, Blanken C, Orzalesi L, Klinkenbijl JH, van der Mijle HC, Veltkamp SC, van 't Riet M, Albregts M, Marinelli A, Rijna H, Tobon Morales R, Snoj M, Bundred N, Chauvet MP, Merkus JW, Petignat P, Schinagl DA, Coens C, Peric A, Bogaerts J, van Tienhoven G. Radiotherapy or surgery of the axilla after a positive sentinel node in breast cancer patients: 10 year follow up results of the EORTC AMAROS trial (EORTC 10981/22023) [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr GS4-01.

BACKGROUND:Marking the axilla with radioactive iodine seed and sentinel lymph node (SLN) biopsy have been proposed for axillary staging after neoadjuvant systemic therapy in clinically node-positive breast cancer. This study evaluated the identification rate and detection of residual disease with combined excision of pretreatment-positive marked lymph nodes (MLNs) together with SLNs. METHODS:This was a multicentre retrospective analysis of patients with clinically node-positive breast cancer undergoing neoadjuvant systemic therapy and the combination procedure (with or without axillary lymph node dissection). The identification rate and detection of axillary residual disease were calculated for the combination procedure, and for MLNs and SLNs separately. RESULTS:At least one MLN and/or SLN(s) were identified by the combination procedure in 138 of 139 patients (identification rate 99·3 per cent). The identification rate was 92·8 per cent for MLNs alone and 87·8 per cent for SLNs alone. In 88 of 139 patients (63·3 per cent) residual axillary disease was detected by the combination procedure. Residual disease was shown only in the MLN in 20 of 88 patients (23 per cent) and only in the SLN in ten of 88 (11 per cent), whereas both the MLN and SLN contained residual disease in the remainder (58 of 88, 66 per cent). CONCLUSION:Excision of the pretreatment-positive MLN together with SLNs after neoadjuvant systemic therapy in patients with clinically node-positive disease resulted in a higher identification rate and improved detection of residual axillary disease.

We investigated the acute toxicity of accelerated partial breast irradiation using external beam (EB-APBI) or intraoperative radiotherapy (IORT) techniques in elderly breast cancer patients.

Axillary lymph node dissection (ALND) is frequently performed for node-positive (cN+) breast cancer patients. Combining positron emission tomography/computed tomography (PET/CT) before-NST and the MARI (marking axillary lymph nodes with radioactive iodine seeds) procedure after neoadjuvant systemic therapy (NST) has the potential for avoiding unnecessary ALNDs. This report presents the results from implementation of this strategy.

Abstract PURPOSE We investigated the acute toxicity of Accelerated Partial Breast Irradiation using external beam (EB-APBI) and intraoperative radiotherapy (IORT) technique. MATERIALS AND METHODS Women ≥60 years with unifocal breast tumors of ≤30 mm undergoing breast conserving therapy were eligible for this prospective multi-centre cohort study. After informed consent, patients were treated with IORT in one institute, EB-APBI was applied in other institutes. IORT was applied with electrons directly on the tumor bed following lumpectomy (21 Gy at 90% isodose). EB-APBI was delivered using 3D-conformal radiotherapy or IMRT in 10 daily fractions of 3.85 Gy. Acute toxicity was scored using the CTCAE v.3, initially by the treating physician and retrospectively by blinded researchers. The highest reported toxicity until 3 months after treatment was used. Patient reported symptoms were analysed using Visual Analogue Scales (VAS) for pain and fatigue (scale 0-10), and single items from the EORTC QLQ-C30 and Breast Cancer module questionnaires at different time points up till 3 months. P<0.01 was deemed significant. RESULTS Between January 2011 and August 2016, 622 patients were eligible, eventually 268 patients underwent IORT and 207 underwent EB-APBI. Of those, 267 (IORT) and 206 (EB-APBI) were available for toxicity analysis. Patient characteristics are shown in table 1. Although more patients experienced >grade 2 acute toxicity in the IORT group (10.4% IORT and 4.9% EB-APBI; p=0.027), grade 3 toxicity was low and did not significantly differ between groups (3.3% IORT and 1.5% EB-APBI). No grade 4 toxicity occurred. After IORT more wound infections requiring oral antibiotics (5.2% versus 1.5%) and more seroma requiring aspiration (2.2% versus 0.5%) were observed. Results for EORTC symptoms and VAS scores are displayed in table 2. Patients treated with EB-APBI experienced significantly less fatigue direct postoperatively (EORTC p=0.003, VAS p<0.001). After 3 months, this difference dissolved, and only pain according to the VAS scale was significantly worse in the EB-APBI group (p=0.003). Table 1. Patient characteristics IORT, n=267EB-APBI, n=206p-valueAge(median; range) 68; 59-9067; 59-860.683pT stagepTis7%13%0.020 pT184%74% pT29%13% pN stagepN093%97%0.092 pN17%3% ER statusPositive93%93%0.122Systemic therapyNo59%62%0.509 Table 2. EORTC and VAS scores BaselinePostoperativeAfter EB-APBI3 months after treatment IORTEB-APBIIORTEB-APBIEB-APBIIORTEB-APBIPain771920111013Fatigue1192916241723Breast pain4425211068Swollen breast122516554Oversensitive breast33362715811Skin side effects1147835VAS pain(median) 3.03.02.02.03.0VAS fatigue(median) 4.03.04.03.04.0EORTC: Scores of “quite a bit” and “very much” were scored positive. Percentage of patients that scored positive is shown. CONCLUSION Even though more clinically relevant toxicity is seen in IORT treatment direct postoperatively, acute toxicity in both IORT and EB-APBI treatment is acceptable. IORT patients seem to recover more rapidly regarding fatigue than EB-APBI patients. This may be explained by the fact that IORT treatment is completed in one day. Citation Format: Jacobs DHM, Marijnen CAM, Speijer G, Straver M, Marinelli A, Merkus J, Roelofzzen EMA, Zwanenburg LAG, Fisscher U, Petoukhova AL, Mast ME, Koper P. Toxicity analysis of elderly breast cancer patients using different accelerated partial breast irradiation techniques [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P2-11-10.

BACKGROUND:In multifocal breast cancer, guidelines recommend basing adjuvant systemic treatment decisions on characteristics of the largest lesion, disregarding multifocality as an independent prognosticator. We assessed the association between multifocal disease and both the 70-gene signature (70-GS), and distant metastasis-free survival (DMFS) in clinical low-risk breast cancer patients enrolled in the European Organisation for Research and Treatment of Cancer 10041/BIG 03-04 Microarray In Node-negative and 1 to 3 positive lymph node Disease may Avoid ChemoTherapy (MINDACT) trial. PATIENTS AND METHODS:The analysed population consisted of enrolled patients in the MINDACT trial with clinical low-risk disease, defined by a modified Adjuvant! Online cut-off for the 10-year risk of recurrent disease or death. Eligibility criteria of MINDACT dictate that patients with multifocal disease could be included if the different lesions had similar pathological characteristics. The presence of multifocal disease was deducted from the case report form (CRF)-question for sum of diameter for all invasive tumour foci. Clinicopathological characteristics and gene expression of patients with unifocal and multifocal (largest lesion) disease were compared. Subsequently, the association between multifocal disease and the 70-GS was evaluated as well as the association between multifocality and 5-year DMFS. RESULTS:The study included 3090 clinical low-risk patients with unifocal and 238 patients with multifocal disease. Apart from a higher prevalence of lobular tumours (21.8% versus 10.8%, by local pathology), we did not observe differences in baseline characteristics between multifocal and unifocal tumours. Patients with multifocal tumours were more likely to be at high genomic risk as compared to patients with unifocal tumours (22.7% versus 17.3%, odds ratio [OR] 1.45, 95% confidence interval [CI] 1.02-2.07, P = 0.038). We did not find a significant association between tumour focality and DMFS (97.1% for unifocal versus 96.9% for multifocal, hazard ratio [HR] = 1.55, 95% CI 0.68-3.46, P = 0.172), nor a signal for a potential interaction between the prognostic effect of the 70-GS and focality of the tumour regarding DMFS. CONCLUSION:In the group of clinical low-risk MINDACT patients, multifocal tumours were more likely to have a high-risk 70-GS profile compared to unifocal tumours. We did not observe a significant interaction between multifocality and the 70-GS with respect to survival without distant metastasis in these patients.

Abstract PURPOSE: gene-expression profiles, such as the 70-gene signature (70-GS), are increasingly used as adjunct to conventional clinicopathological prognostic factors to guide adjuvant chemotherapy (CT) decisions. The Dutch guideline suggests use of validated gene-expression profiles in estrogen-receptor (ER) positive (+) early stage breast cancer patients without overt lymph node metastases. We aimed to assess the impact of the 70-GS on CT decisions in ER+ early stage breast cancer patients. PATIENTS AND METHODS: In this prospective observational multicenter study physicians were asked for their opinion whether to administer or omit adjuvant CT before deployment and after obtaining the test result of the 70-GS in this guideline delineated group of patients. RESULTS: Between January 1 2013 And December 31 2015 660 patients, treated in 31 hospitals, were enrolled. Based on the clinicopathological postoperative findings physicians would administer CT in 41%, withhold CT in 16% of patients and refrained from formulating an advice in the remaining 43% of patients letting their recommendation depend on the result of the 70-GS. Estimated 5-year survival benefit of CT administration was 0.8%, 0.6% and 0.7% respectively (p 0,585). The 70-GS result hardly varied in relation to the initial advice of the physician: 56% and 59% had a low-risk profile in patients in whom CT was recommended or discommended respectively (r =0.021, Table 1). In 51% of patients in whom a pre-test recommendation was formulated incorporation of the 70-GS test result changed the initial advice. Adherence to the test result was high for the three groups (range 94-97%). Table 1. Concordance between pre-test CT recommendation of the oncologist and the 70-gene signature (GS) test result. 70-GS test resultPre-test CT recommendationno. patientsLow RiskHigh RiskNo CT10763 (59%)44 (41%)CT270152 (56%)118 (44%)Depends on 70-GS result283174 (61%)109 (39%)Note. Agreement between pre-test oncologist CT recommendation and the 70-GS test result: Pearsons r = -0,031 95%CI (-0,11 – 0,045). Table 2. CT recommendation before vs. after obtaining the 70-GS test result and the actual administration of CT. Post-test CT recommendationAdherence to test result*Actual administered CTAdherence to test result*Pre-test CT recommendationno. patientsNo CTCT%No CTCT%No CT10769 (65%)38 (35%)94%73 (68%)34 (32%)91%CT207156 (58%)114 (42%)97%162 (60%)108 (40%)90%Depends on 70-GS result283173 (61%)110 (39%)95%185 (65%)98 (35%)91%*Percentage of patients in whom the post-test recommendation/actual administered CT was in line with the 70-GS test result (i.e. no CT in case of a low-risk profile and CT in case of a high-risk profile. Note. Change in CT recommendation in patients with a CT or no CT pre-test recommendation McNemar's chi-square test p < 0.001 and p < 0.001 for actual administered CT CONCLUSION: guideline-directed use of the 70-GS in Dutch ER+ early breast cancer patients influenced CT treatment decision in the majority of patients. The physician's tentative CT advice was not associated with the 70-GS test-result. Citation Format: Kuijer A, Straver M, Elias S, Smorenburg C, Wesseling J, Linn S, Rutgers E, Siesling S, van Dalen T. Impact of 70-gene signature use on adjuvant chemotherapy decisions in early breast cancer patients: Results of the prospective symphony triple A study [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P6-09-03.

Abstract PURPOSE: A decade ago intrinsic biological breast cancer subtypes have been identified which have proven to be of clinical importance in terms of outcome and response to systemic treatment. The aim of the current study is to assess concordance between breast cancer subtypes determined by local immunohistochemistry (IHC) assessment of estrogen receptor (ER), progesterone receptor (PR) and Her2-receptor status and microarray based molecular subtyping in a subset of ER+ early stage breast cancer patients. PATIENTS AND METHODS: In this prospective observational multicenter study information on local pathology assessment and BluePrint/TargetPrint results were obtained in ER+ Dutch early stage breast cancer patients in whom a 70-gene profile (MammaPrint) was used as they were enrolled in clinical trial based on the existence of controversy regarding the additional value of adjuvant CT. Local IHC assessment of ER, PR and Her2 status were compared with microarray based assessment (TargetPrint/BluePrint) of these characteristics. Reclassification of ER and PR overexpression was assessed by a McNemars test and by Spearman correlation. Furthermore, concordance between the clinical subtypes based on local pathology (Luminal-type: ER+/PR+/Her2-; Her2-type: Her2+ disease) and molecular subtyping was assessed. RESULTS: Between January 2013 And December 2015 660 patients, treated in 31 hospitals, were enrolled. In 564 (85%) BluePrint and/or TargetPrint was performed in addition to the 70-GS. The majority of patients had ER+/Her2- disease and TargetPrint reclassified 1% (n = 7) of patients as ER-negative (r = 0,250, p <0,001). TargetPrint reclassified 7% (n = 40) and 2% (n = 11) of patients for PR and Her2 status respectively (table 1, r = 0,580, p <0,001 for PR Table 1. Concordance between immunohistochemistry and TargetPrint. TargetPrint result (ER, PR and Her2 resp.) ImmunohistochemistryPositiveNegativeOverall discordance (%)p-value*Estrogenreceptor status Positive557 (99%)6 (1%) Negativen.a.n.a.1%n.a.Progesterone receptor status Positive474 (96%)18 (4%) Negative22 (31%)49 (69%)7%0,636Her2 receptor status Positive3 (30%)7 (70%) Negative4 (3%)546 (97%)2%0,549Equivocal0 (0%)3 (1%) * P-value represents results of the McNemar test.). Based on IHC 545 (98%) patients were regarded as luminal-type and the remaining 2% as Her2-type. BluePrint reclassified 2% of the clinical luminal-type patients: 4 (1%) patients were reclassified as basal-type and 3 (0%) patients as Her2-type. Of the clinical Her2-type patients 80% (n=8) was reclassified by BluePrint as molecular luminal-type. Table 2. Concordance between clinical subtyping and molecular subtyping according to BluePrint. BluePrint resultClinical SubtypeNo. ptsLuminalBasalHer2Luminal545539 (99%)4 (1%)3 (0%)Her2108 (80%)02 (20%)Note. Overall discordance 3%. Conclusion: In the current study we observe a high concordance between microarray-based assessment of ER, PR and Her2 and local pathology in Dutch ER+ early stage breast cancer patients. In the small subset of ER+ patients who are considered candidates for 70 GS use and who have HER2+ tumors by IHC molecular typing of HER2 status is of additional value. Citation Format: Kuijer A, Straver M, Elias S, Smorenburg C, Wesseling J, Linn S, Rutgers E, Siesling S, van Dalen T. Concordance of local immunohistochemistry with TargetPrint microarray based assessment of ER, PR and Her2 and BluePrint molecular subtyping in the Symphony Triple A study [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P1-03-04.

BACKGROUND:The treatment of axillary lymph node metastases after neoadjuvant systemic therapy (NST) remains debatable and axillary lymph node dissection (ALND) is still the standard of care. Marking axillary lymph nodes with radioactive iodine seeds (MARI procedure) is accurate in restaging the axilla after NST (false-negative rate 7 per cent). Here, the potential of tailored axillary treatment, determined by combining the results of PET-CT before NST with those of the MARI procedure after NST, was analysed.METHODS:A cohort of axillary node-positive patients was used to construct a hypothetical treatment algorithm based on a combination of PET-CT and the MARI procedure. In the algorithm, the number of fluorodeoxyglucose (FDG)-avid axillary lymph nodes (1-3 versus 4 or more) before NST and the tumour status of the MARI node (positive versus negative) after NST were used to tailor axillary treatment. All patients in the cohort underwent ALND, allowing estimation of potential overtreatment and undertreatment.RESULTS:A total of 93 patients were included in the study. Between one and three FDG-avid axillary lymph nodes were observed in 59 patients, and four or more in 34 patients. The MARI node was tumour-negative in 32 patients and showed residual disease in 61. Treatment according to the constructed algorithm would have resulted in 74 per cent of patients avoiding an ALND, with potential undertreatment in three patients (3 per cent) and overtreatment in 16 (17 per cent).CONCLUSION:Tailored axillary treatment after NST in node-positive patients, by combining PET-CT before NST and the MARI procedure after NST, has the potential for ALND to be avoided in 74 per cent of patients.

Abstract Background The increasing use of primary systemic treatment (PST) for patients with breast cancer enables more breast conserving surgery. In addition, PST converts node-positive into node-negative disease in 20-40% of patients. However, the current guidelines still recommend axillary lymph node dissection (ALND) for clinical node-positive disease (cN+), even if it became node-negative after PST, since false-negative rates of sentinel lymph node biopsy after PST range from 5-30%. Recently, an alternative technique has been introduced to stage the axilla after PST: the MARI-procedure (sensitivity 97%; FNR 7%), in which a tumour-positive lymph node is marked with a radioactive iodine seed before the start of PST and selectively removed after PST. In the present study, we propose a new strategy for treatment of the axilla in cN+ patients by combining results of the pre-PST PET/CT with the post-PST MARI-procedure. Material and methods All patients who received a MARI-procedure from July 2014 until May 2016 were included. Before the start of PST a PET/CT was performed for axillary staging and the detection of distant metastasis. A radioactive iodine seed was placed in a proven tumour-positive axillary lymph node (MARI-node), after which PST was given according to Dutch national guidelines. At our institute, we have implemented a protocol in which results of the pre-PST PET/CT and the post-PST MARI-procedure determine the type of axillary treatment. Patients with 1-3 positive axillary lymph nodes (ALNs) on PET/CT and a tumour-negative MARI-node receive no further axillary treatment. Patients with ≤3 positive ALNs on PET/CT and a tumour-positive MARI-node receive axillary radiotherapy, as well as patients with >3 positive ALNs on PET/CT and a tumour-negative MARI-node. An ALND is only performed in patients with >3 positive ALNs on PET/CT and a tumour-positive MARI-node. Results In total 168 patients received a PET/CT and a MARI procedure, of whom 43% were hormone receptor positive, 28% triple negative and 29% Her2-positive. One hundred and eight patients (64%) showed ≤ 3 and 60 patients (36%) >3 suspected ALNs on PET/CT before the start of PST. The axillary pathologic complete response was 39%. In 134 patients (80%) an ALND was omitted; of these patients 94 (56%) were treated with axillary radiotherapy and 40 patients (24%) received no further axillary treatment. In 34 patients (20%) an ALND was performed (Table 1). The median number of positive additional nodes at ALND was 5 (range 0-16). During a median follow-up of 6 months there were no local recurrences. Axillary treatmentSuspective ALNs on PET/CTOutcome MARIAxillary Treatment NoneRadiotherapyALND + Radiotherapy≤3Negative40-- Positive-68->3Negative-26- Positive--34Total409434ALN: axillary lymph nodes; ALND: axillary lymph node dissection; MARI: Marking the Axilla with Radioactive Iodine Seeds Conclusion Combining pre-PST axillary staging with PET/CT and post-PST staging with use of the MARI-procedure results in a reduction of 80% of axillary lymph node dissections in breast cancer patients with clinical node-positive disease. Citation Format: van der Noordaa MEM, Straver M, van Duijnhoven FH, Groen E, Stokkel M, Vrancken Peeters M-JTFD. Selective elimination of axillary surgery after primary systemic treatment in clinically node-positive breast cancer patients by combining PET/CT and the MARI procedure (marking the axilla with radioactive iodine seeds) [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P2-01-07.