ESTRO 38 Abstract book

S310 ESTRO 38

1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands ; 2 UMC Utrecht, Pathology, Utrecht, The Netherlands ; 3 St Antonius Hospital, Pathology, Nieuwegein, The Netherlands ; 4 Diakonessenhuis, Pathology, Utrecht, The Netherlands ; 5 UMC Utrecht, Surgery, Utrecht, The Netherlands ; 6 St Antonius Hospital, Surgery, Nieuwegein, The Netherlands ; 7 Rivierenland Hospital, Surgery, Tiel, The Netherlands; 8 Diakonessenhuis, Surgery, Utrecht, The Netherlands; 9 UMC Utrecht, Medical Oncology, Utrecht, The Netherlands ; 10 UMC Utrecht, Radiology, Utrecht, The Netherlands; 11 UMC Utrecht, Nuclear Medicine, Utrecht, The Netherlands ; 12 The Royal Marsden NHS Foundation Trust, Radiotherapy, Sutton, United Kingdom; 13 UMC Utrecht, Imaging, Utrecht, The Netherlands Purpose or Objective This single arm interventional multi-center cohort study was designed to evaluate the pathologic and radiologic response after MR-guided single dose ablative preoperative partial breast irradiation (PBI) in early-stage breast cancer patients with low risk on local recurrence. Material and Methods In the ABLATIVE study (ClinicalTrials.gov: NCT02316561), we included women with unifocal, estrogen-receptor positive and HER2-negative tumors with a maximum diameter of 20mm (age 50-70 years), or 30mm (age ≥70 years) as assessed on MRI. Furthermore, patients were required to have a tumornegative sentinel lymph node biopsy, no history of breast cancer and no indication for chemotherapy. Patients were treated with MR-guided single dose ablative preoperative PBI: 20Gy to the gross tumor volume (GTV) and 15Gy to the clinical target volume (CTV=GTV+2cm margin). GTV and CTV were expanded by 3mm to create the planning target volume. Breast-conserving surgery (BCS) was originally performed 6 months after PBI. After the first 15 patients this interval was prolonged to 8 months to evaluate whether this would increase the rate of pathologic complete response (pCR). Primary outcome was the rate of pCR, which was defined as no residual tumor cells. Secondary outcomes were radiologic response and treatment-induced toxicity. Radiologic response was evaluated by dedicated breast radiologists on tumor morphology, diffusion restriction and contrast enhancement kinetics using DCE- and DW-MRI every 2 months until BCS was performed. Toxicity was assessed according to CTCAE v4.03. Results From May 2015 until November 2017, 32 patients were recruited. Median follow-up was 18 months (range 9-36). The median tumor size was 13mm (range 5-20) and median age 64 years (range 51-78). pCR was reported in 5 and 7 patients (33% and 41%), and near pCR in 5 and 6 patients (33% and 35%), after 6 and 8 months respectively (table 1). Radiologic response assessment showed an increase in complete response during follow-up. Radiologic complete response was only observed in patients with pCR or near pCR, however, 11 of the 23 patients with pCR or near pCR did not show a radiologic complete response (figure 1). Five patients received preoperative endocrine treatment which was initiated after PBI; 2 of these patients had pCR. Only mild treatment-induced toxicity (i.e. grade 1-2) was observed and all grade 2 toxicity was transient. Observed toxicity was grade 1 fibrosis (91%), grade 1 breast pain (59%) and transient grade 2 breast pain (3%).

Conclusion MR-guided single dose ablative preoperative PBI resulted in pCR in one third of the patients. Radiologic complete response on MRI was not recognized in all patients with pCR. Further assessment of MRI and other response monitoring techniques should be considered in single-dose ablative preoperative PBI. OC-0592 5 year results of the Preoperative Accelerated Partial Breast Irradiation (PAPBI) trial S. Bosma 1 , F. Leij van der 1 , S. Vreeswijk 1 , M. Vijver van der 2 , S. Rivera 3 , T. Foukakis 4 , D. Bongard van den 5 , E. Rutgers 6 , A. Scholten 1 , H. Bartelink 1 , P. Elkhuizen 1 1 The Netherlands Cancer Institute, Radiation oncology, Amsterdam, The Netherlands; 2 Amsterdam UMC, Pathology, Amsterdam, The Netherlands; 3 Institute Gustave Roussy, Radiation Oncology, Paris, France; 4 Karolinska institutet, Radiation Oncology, Stockholm, Sweden ; 5 UMCU, Radiation Oncology, Utrecht, The Netherlands ; 6 The Netherlands Cancer Institute, Surgery, Amsterdam, The Netherlands Purpose or Objective Accelerated Partial Breast Irradiation (APBI) is a treatment option for low-risk early stage breast cancer. Preoperative ABPI (PAPBI) provides several advantages compared to conventional postoperative radiotherapy (RT), including improved target localization, accurate tumor delineation, the possibility for response evaluation and excision of the high dose irradiated volume. Here, we present the 5-year results of the PAPBI trial providing novel data from a large set of preoperatively irradiated patients. Material and Methods In the multicentre PAPBI trial, 138 patients were included between 2010-2017. Women aged ≥ 60 years with an invasive, unifocal ≤ 3 cm on MRI, (non-lobular) adenocarcinoma of the breast and a negative SN received PAPBI (40 Gy in 10 fractions in 2 weeks or 30 Gy in 5 fractions in 1 week). 6 weeks after RT a wide local excision was performed. The primary endpoints were breast fibrosis and cosmetic outcome. Local recurrences (LR) should not exceed 4% at 5 years. Toxicity was scored 3- monthly in the first year, then every 6 months for the first 5 years and yearly thereafter. Cosmetic outcome was evaluated by the treating physician and the (Dutch) patients before start of treatment, 6 months after treatment and then yearly. Results 133 patients were analyzed with a median follow-up of 4.6 yrs (0.9-8.3 yrs). Baseline characteristics are displayed in table 1. 78 (59%) patients were treated with 10x4 Gy and 55 (41%) with 5x6 Gy. A postoperative complication occurred in 14% of patients, of which 4 patients required re-surgery. 11% had a postoperative infection requiring treatment. Acute skin toxicity was absent in 65% of patients and 34% of patients had grade 1 skin toxicity . At 1 year of follow-up, induration/fibrosis at the tumor area was absent in 30(26%) and mild in 69(60%) of 115 patients. At 3 (n=94) and 5 years (n=55) of follow-up, respectively