Most IORT studies focus on oncological outcomes, particularly local recurrence rates, while results for long-term toxicities are less common. Although the cosmetic outcome after breast cancer treatment is a key factor for quality of life [26, 27] and an important parameter in shared decision-making, data on cosmetic outcomes remain limited for breast cancer patients with a low-energy X‑ray IORT boost. Specifically, objective assessment of cosmetic results is challenging. To date, there is currently no standardized objective tool available to evaluate the cosmetic outcomes after breast-conserving treatment. Traditional approaches to assess the cosmetic outcomes of breast-conserving surgery often rely on subjective evaluations by patients themselves or examination of photos by one or more experts, as do recently published and ongoing trials [22, 28,29,30].

Various methods have been employed to assess cosmetic outcomes. Harris et al. (1979) introduced a subjective scoring system categorizing outcomes as excellent, good, fair, or poor [24]. First attempts at objective evaluation after breast surgery were made shortly after, with the Breast Retraction Assessment (BRA) by Pezner et al. [31]. Cardoso et al. furthered this approach by creating a software (BCCT.core) utilizing digital images to evaluate cosmetic outcomes of breast-conserving treatment. BCCT.core combines objective measures of asymmetry, skin color, and scarring to generate an overall cosmetic score based on Harris et al.’s classification system [25]. The validated software therefore enables reproducible, objective evaluation of esthetic outcomes in breast cancer conservative treatment using digital photos [32]. We chose an objective tool of cosmetic evaluation in order to minimize any kind of bias that may come with subjective evaluation. Although one may argue that a patient’s subjective assessment is the most important, an objective analysis of this often-overlooked secondary outcome is crucial for establishing a standardized basis of comparison. A recent study by Corica et al. compared four different cosmetic rating scales, subjective as well as objective, and showed little agreement between them. Especially the patient’s evaluation did not always correlate with the others. They therefore recommend using a combination of subjective and objective evaluations in future analyses [33].

Our evaluation with BCCT.core shows IORT boost to be a procedure with satisfying long-term cosmetic outcomes. The pooled analysis consistently showed a majority of good to excellent cosmetic outcomes at all timepoints, indicating that the use of an IORT boost does not negatively impact cosmetic outcomes. While the separate analysis of each cohort revealed differences in cosmetic outcomes, which are discussed in detail later, the cosmetic results remained consistently stable over time for each group. Smaller subjective assessments [22, 28,29,30] as well as recent studies that analyzed cosmetic outcomes with BCCT.core after IORT as a single high dose in the TARGIT‑A trial [33, 34] provide additional evidence for a good cosmetic outcome. Intraoperative radiation therapy as a single-shot radiotherapy without WBRT has been shown to result in better cosmetic outcomes than standard WBRT [35]. Consequently, several studies have also demonstrated superior cosmetic results with IORT alone compared to IORT followed by WBRT [30, 34]. One contributing factor is the discoloration from hyperpigmentation and telangiectasia, which is linked to the higher doses and larger treatment volumes used in WBRT [12, 34]. Wang et al. compared two recent boost techniques: simultaneous integrated tumor bed boost (SIB) and sequential boost during hypofractionated whole-breast irradiation after breast-conserving surgery: SIB outcomes were significantly better compared to sequential boost, with low deterioration rates after radiation for both techniques (9.8% vs. 7.6%; p = 0.22) [36]. Deterioration rates were 17% in the TARGIT-BQR cohort and 12% in the ROKSM cohort (6 months vs. 5 years).

Specifically, Goerdt et al. investigated long-term cosmetic outcomes after IORT boost followed by WBRT using the same cohort from the University Hospital Mannheim as in the TARGIT-BQR study. However, in contrast to this study, they conducted a cosmetic analysis by evaluating photos taken from three different positions, assessed by three different experts and not by using the BCCT.core software. The study also had a shorter follow-up period of only 4 years, with slightly different follow-up timepoints. Similar to the findings in this study, the majority of results for each timepoint were rated as good or excellent [22]. Our study can confirm these findings through a different, objective assessment method, also extending the long-term results by several years and providing a comparison with an additional cohort.

Further evidence on alternative IORT boost techniques also supports favorable cosmetic outcomes. Studies investigating intraoperative electron radiotherapy (IOERT) followed by whole-breast irradiation consistently reported good to excellent results, in some cases superior to those achieved with external-beam boosts, with low rates of high-grade toxicity [37,38,39]. These findings support our results with low-energy X‑ray IORT and underscore the potential of IORT boosts as an esthetics-preserving treatment option.

While both cohorts underwent the same radiotherapeutic regimen (BCS with IORT boost followed by WBRT), we observed significant differences in cosmetic outcomes between them. A comparison of both studies suggests that despite following the same therapeutic steps, cosmetic outcomes are likely influenced by numerous additional factors. In this comparison, the ROKSM study consistently reported poorer cosmetic outcomes. Several factors may account for this disparity. The ROKSM cohort demonstrated significantly more continuous and thorough photo documentation for each patient compared to the subgroup of the TARGIT-BQR study. Since the TARGIT-BQR study is a so-called no-budget registry study (meaning it operates without funding, with all services provided purely out of academic interest), it is possible that the less-structured invitation and photography process led to fewer documented cosmetic outcomes overall. However, numerous factors influence cosmetic outcomes, including radiotherapy, patient factors, tumor characteristics, and surgical technique. Both studies offered oncoplastic procedures for breast reconstruction, yet it is unclear from the registry data whether these were utilized. The ROKSM study involved two different surgical sites with two distinct surgical teams, offering a potential explanation. Additionally, the literature suggests that while oncoplastic surgeries may improve cosmetic outcome [40, 41], there is an imprecise definition of the term, as these techniques are individualized based on breast characteristics [12, 42]. While patient age was evenly distributed in both studies, the tumor size was significantly different between them, and more re-excisions occurred in the ROKSM study. Larger tumor size is a well-established risk factor for higher re-excision rates, as achieving clear margins becomes technically more challenging and microscopic extensions may be more common. This potentially indicates larger resected volumes, which could lead to poorer cosmetic outcomes. Surprisingly, the TARGIT-BQR study achieved better cosmetic outcomes, despite employing significantly larger applicators and longer irradiation times—parameters usually linked to poorer results due to greater tissue impact. This unexpected finding underscores the considerable influence of other factors. Our results show that surgical technique and initial tumor size might be crucial determinants of cosmetic outcome, with larger tumors and more frequent re-operations in the ROKSM study likely causing more pronounced tissue defects that counteracted the potential negative effects of the radiotherapy parameters in the TARGIT-BQR group.

Our findings indicate that while radiotherapy and its techniques might play an important role in shaping cosmetic outcomes, they are possibly not the sole determinants. Surgical technique, photo documentation quality, and patient-specific characteristics might also be considerable factors. This underscores the need for future prospective studies to systematically evaluate these variables.