S6
Track-Topic
ESTRO 2026
Digital Poster 676
breast irradiation (vAPBI) delivered as a single fraction of postoperative multicatheter interstitial high-dose- rate brachytherapy (MIB) in elderly patients with low-
Impact of Applicator–OARs Distance on Dose Constraints in Accelerated Partial Breast Irradiation Using Strut-Adjusted Volume Implant (SAVI). Kauznori Miyaura 1,2 , Ryoichi Notake 3,2 , Hiroyuki Okamoto 4,2 , Ryoichi Yoshimura 5,2 1 Medical Physics Course, Graduate School of Health Sciences,, Showa medical university, Tokyo, Japan. 2 WASAVIs, Working group of APBI with SAVIs in Japan, Tokyo, Japan. 3 Radiology Center,, Tokyo Science University Hospital, Tokyo, Japan. 4 Radiation Safety and Quality Assurance Division, National Cancer Center Hospital, Tokyo, Japan. 5 Department of Radiation Therapeutics and Oncology, Tokyo Science University, Tokyo, Japan Purpose/Objective: Accelerated partial breast irradiation (APBI) using the Strut-Adjusted Volume Implant (SAVI) system allows conformal radiation delivery following breast- conserving surgery. However, the proximity of the applicator to organs at risk (OARs) such as the skin and chest wall can make it challenging to meet dose constraints. This study aimed to quantitatively evaluate the relationship between applicator–OAR distance and dose–volume parameters in APBI with SAVI, based on a multicenter observational dataset from the WASAVIs study in Japan (UMIN ID: 000021237). Material/Methods: Treatment planning data were collected in DICOM format (CT, structure, plan, and dose) from six institutions participating in the WASAVIs study. A total of 31 analyzable cases were included. On the planning CT images, the distances between SAVI and the skin (SAVI–SKIN) and between SAVI and the chest wall (SAVI–CHESTWALL) were measured at the slice where the applicator exhibited maximal expansion. Correlations were then analyzed between these distances and various dose–volume histogram (DVH) parameters, including PTV_EVAL V90%, V150%, V200%, SKIN D1cm3, and CHESTWALL D1cm3. The coefficient of determination (R2) from linear regression analysis was used to evaluate the strength of correlation. Results: The linear regression analysis demonstrated moderate correlations between applicator–OAR distance and OAR doses, with R2 values of approximately 0.5 for SKIN D1 cm3 and CHESTWALL D1 cm3. These findings suggest that shorter distances between the applicator and these OARs were associated with higher doses. In contrast, the correlation coefficients for PTV_EVAL V90%, V150%, and V200% were below 0.3, indicating that the applicator–OAR distance had a limited effect on target coverage parameters.
risk breast cancer. Material/Methods:
Patients aged ≥ 70 years (Balducci score I–II) with low- risk breast cancer (GEC-ESTRO classification) were enrolled in the SiFEBI trial. After lumpectomy, intraoperative catheter implantation was performed and postoperative vAPBI (single fraction, 16 Gy) was delivered. Surveillance occurred twice yearly. The primary endpoint was cumulative incidence of local recurrence rate (ciLR). Secondary endpoints included cumulative incidence of metastasis rate (ciMD), cancer- specific survival (CSS), overall survival (OS), late toxicity, and cosmetic outcome. Endocrine-therapy (ET) use and duration were also analysed; patients were considered adherent if they had taken ET for up to 50
months. Results:
From November 2012 to September 2014, 26 patients were enrolled. Median age was 76.6 years [69.4–88.9]. Median tumour size was 10.4 mm [4–17]. Histology was predominantly invasive ductal carcinoma (76.5%). Most tumours were low histologic grade (65%) and hormone-receptor positive (96%), All patients belonged to the luminal molecular subtype, with a median Ki-67 value of 10% [5–30]. Median surgical margins were 4 mm [2–10]. Median interval from surgery to vAPBI was 7 days [6–15]. After a median follow-up of 137 months [95%CI 134-147], the 10-year ciLR was 95% [95%CI 0.87-1]. Ten-year ciMD, CSS and OS rates were 0%, 100%, 81% [95%CI 67-97] respectively.Ten-year late toxicity occurred in 9 patients (34.6%), with a total of 9 events. Of these, 7 events (77.8%) were grade 1 and 2 (22.2%) were grade 2. Reported late effects included G2 breast pain in 1 pt (cytosteatonecrosis), G1 hypopigmentation in 3 pts, and breast fibrosis in 5 pts (G1: 4 pts; G2: 1 pt). Cosmetic outcomes were reported as excellent in 21 patients (81%) and good in 2 patients (8%).Median duration of ET was 59 months [3–61]. Twelve of 26 pts (46.2%) did not adhere to ET for up to 50 months including 1 pt who experienced local relapse 8 years after vAPBI. Conclusion: With 10 years of follow-up in this elderly, low-risk cohort, single-fraction postoperative MIB vAPBI (16 Gy) yielded excellent oncologic outcomes, notably high local control and cancer-specific survival. Late toxicity and cosmetic outcomes were acceptable. Further studies including larger cohorts with long follow-up would be valuable, particularly to allow meaningful comparison with ultra-hypofractionated external beam radiotherapy regimens. Keywords: Breast cancer, very APBI, single fraction
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