In this study, early clinical outcomes of APBI are reported, and VMAT and 3D-CRT are compared. A total of 82 patients were selected for APBI, with treatment planning performed using 2-arc VMAT for all the patients. No grade 2 or higher adverse events were observed, and all the patients completed treatment.

Accurate identification of the tumor bed is critical for planning APBI treatment. In our practice, surgical clips are routinely placed during breast-conserving surgery, which enables the CTV to be defined via these clips in combination with preoperative imaging. In approximately 10% of the cases in this study, however, clips were not placed, which was primarily because of metal allergies. In such cases, the CTV was delineated on the basis of postoperative changes in the breast that were observed on treatment planning CT, which was supplemented by preoperative MRI or ultrasound when available. This method, however, introduces uncertainty, thus necessitating the consideration of additional margins. For patients without clips, the CTV was defined using a margin of approximately 2–3 cm from the edge of the tumor bed, with alignment confirmed against postoperative changes in the breast.

Additionally, uncertainty in defining tumor margins may arise for two main reasons. First, recent breast-conserving surgeries have increasingly incorporated oncoplastic techniques, which aim to improve cosmetic outcomes while maintaining oncologic safety [9,10,11,12]. At our institution, however, clips are routinely placed in the tumor bed following breast-conserving therapy, thereby reducing this uncertainty. The second source of uncertainty arises when the tumor bed is closed with sutures for cosmetic reasons, which may lead to displacement of the clips. Difficulties in accurately localizing the tumor bed after partial mastectomy with oncoplastic techniques have been well documented [13,14,15,16,17]. Secondary displacement may result in misalignment between the original tumor site and the final clip location [13, 15, 16]. In various studies, the reliability of clip positions for defining CTVs after oncoplastic reconstruction has been questioned, and compensating for this uncertainty by enlarging the boost volume or increasing the dose has been cautioned against [13]. Conversely, in a phase II trial of APBI performed concurrently with breast reconstruction, similar concerns were reported, but a strategy of contouring a more generous CTV was adopted. In this trial, no association between oncoplastic reconstruction and local control was demonstrated [17]. Taken together, these findings suggest that APBI and oncoplastic reconstruction are generally compatible. However, considerable caution is needed in CTV delineation. When the tumor bed cannot be reliably identified, we consider WBI to be the most appropriate treatment approach.

In this study, treatment plans were generated via both 3D-CRT and VMAT for all patients who underwent APBI, with irradiation ultimately delivered via VMAT. On the basis of the dosimetric parameters, VMAT consistently satisfies target dose constraints for organs at risk outside the breast, whereas 3D-CRT does not. Comparisons of the dose distributions between 9-field 3D-CRT and 2-arc VMAT are shown in Fig. 4. PTV coverage, particularly in deeper regions away from the chest wall, improved with 2-arc VMAT compared with 9-field 3D-CRT. In addition, compared with 9-field 3D-CRT, 2-arc VMAT reduced the dose to the heart and lungs. Taken together, these findings suggest that 2-arc VMAT is preferable to 9-field 3D-CRT for treatment planning in APBI. The Florence trial [6], which was a phase III study in which IMRT was administered, served as the basis for determining our institutional dose prescription and fractionation schedule. In alignment with this evidence, the National Comprehensive Cancer Network (NCCN) guidelines [8] emphasize that the irradiation technique and fractionation schedule must be carefully considered when delivering APBI and recommend IMRT/VMAT for 30 Gy in 5 fractions every other day. This recommendation is also derived from the RAPID trial [5], in which APBI with 38.5 Gy in 10 twice-daily fractions was used. Although the RAPID trial demonstrated comparable local control between the APBI and WBRT groups, the APBI group experienced higher rates of late toxicity and poorer cosmetic outcomes. These adverse effects were attributed to the fractionation regimen and the use of 3D-CRT for APBI.

No grade ≥ 2 acute adverse events were observed following APBI, which supports the safety of this treatment approach. The incidence of grade 1 dermatitis in this study was higher than that in previous reports [4,5,6], which is likely attributable to variability in grading criteria, whereby even minor changes were categorized as grade 1. Importantly, cosmetic outcomes are subject to considerable subjective variation. To address this, in future studies at our institution, validated patient-reported outcome measures such as the BREAST-Q will be incorporated to improve the objectivity of cosmetic assessments.

APBI has become the standard treatment worldwide for radiation therapy following breast-conserving surgery, and we consider it to be highly beneficial for Japanese women as well. Compared with conventional or hypofractionated WBI, APBI requires only five treatment sessions, thus offering a short-course regimen that reduces the burden of hospital visits and lowers healthcare costs. For short-term postoperative radiotherapy, WBI at 26 Gy in 5 fractions was employed in the UPBEAT trial [18]. While the total number of treatment sessions is the same, in patients with early-stage breast cancer who are fit for APBI, WBI is unnecessary. The limited target volume of APBI is advantageous for reducing adverse events. Moreover, the shorter treatment duration enables radiotherapy resources to be redirected to other patients in facilities with limited capacity.

At our institution, APBI was administered to 75% of fit patients, which reflects a high level of patient preference and demand. The reduced number of treatment visits compared with conventional fractionated or hypofractionated WBI likely contributed to this preference. Among eligible patients who chose WBI, the most frequently cited reasons were the weak level of recommendation for APBI in the Japanese Breast Cancer Society clinical practice guidelines for breast cancer, concerns about oncologic outcomes, and the toxicity associated with APBI. Internationally, however, both the ASTRO and NCCN guidelines [7, 8] support the use of APBI on the basis of its safety and efficacy. In a Japanese study of APBI using 3D-CRT at 38.5 Gy in 10 fractions, a potentially increased risk of recurrence in patients with axillary lymph node metastasis was reported; however, the 10-year local recurrence rate was only 4% [19]. When calculated using the linear–quadratic model, assuming an α/β ratio of 4 Gy for the tumor and 3 Gy for normal tissues, 38.5 Gy in 10 fractions and 30 Gy in 5 fractions yielded comparable biologically effective doses (BEDs). Therefore, although long-term follow-up is needed to confirm these findings, similar local control and favorable cosmetic outcomes are expected. APBI that is delivered at 30 Gy in 5 fractions every other day could soon become the standard treatment in Japan.

This study has several limitations. This was a retrospective study with relatively few patients and a short follow-up period, and adverse events might have been underreported. Extended follow-up beyond the present analysis is needed to validate long-term outcomes; however, on the basis of similar reports, this treatment appears to be effective for both local control and the management of adverse events.

In conclusion, APBI with VMAT was delivered to all the patients in this real-world setting, and no severe acute toxicity was observed. Long-term follow-up is warranted to assess oncological outcomes and late toxicity.