The SLIM technique: introducing a novel way for the orientation of cavity shaves during breast conserving surgery in breast cancer patients

Introduction

Breast conserving surgery (BCS) has been a significant breakthrough in the management of breast cancer worldwide. In keeping with the current trend of oncologic breast surgery that has geared towards more conservative approaches, BCS in appropriately selected patients in conjunction with post-operative radiotherapy has not only proven to be as effective as mastectomies in terms of survival outcomes but has likewise provided the additional benefits of better cosmesis and improved quality of life (1,2). One of the crucial steps in ensuring the best possible oncological outcome after BCS is achieving negative margins. Failure to do so increases the risk of local recurrence and the incidence of re-excision, reported to be on average 20%, but could even reach up to 60% in other studies (3,4). Re-excision raises the possibility of decreased cosmetic satisfaction, post-operative complications, and delays in adjuvant treatment delivery (3). Apart from the anxiety and emotional distress that patients face, the financial burden of a second surgery is equally unsettling.

With the goal of achieving clear resection margins and avoiding subsequent operations, several techniques has been suggested to reduce margin positivity rates (5). However, some of these techniques, such as intraoperative frozen section of breast margin assessment can be costly and time consuming (6). As a result, not all techniques may be widely accepted. Routine cavity shaving (4), which involves circumferential excision of additional tissue around the cavity, was found to be more effective, as it reduces the rates of positive margins by half and can even detect occult multifocal breast cancer with no significant effect on blood loss, operative time, length of hospital stay, and cosmesis (7-9). Furthermore, it can be easily performed intra-operatively without significant changes to the cost of treatment (10).

There is evidence (11) that cavity margin shaving is routinely practiced at the time of BCS, however, a standardised procedure has yet to be established (12). Cavity shaves have so far been obtained randomly, which can cause confusion and errors in labelling the specimens during the operation. To overcome this, a standard sequence of obtaining cavity shaves using the SLIM (superior, lateral, inferior, medial) technique (13) was first described in 2022. We aimed to assess the outcomes of implementing this standardised technique on re-excision rates. This is the first study to assess the outcomes of SLIM technique, to the best of our knowledge. We present this article in accordance with the STROBE reporting checklist (available at https://gs.amegroups.com/article/view/10.21037/gs-2025-435/rc).

Methods

Sample population

This retrospective cohort study included women aged ≥18 years with biopsy-proven breast malignancies, including ductal carcinoma in situ (DCIS), stage I–III invasive carcinoma, and malignant phyllodes tumours. Participants included those who underwent BCS (i.e., wide excision, wide excision with nipple/areola excision, therapeutic mammoplasty etc.), either as an initial treatment or after neoadjuvant chemotherapy (NACT), from 01 January 2021 to 30 September 2024 at KK Women’s and Children’s Hospital in Singapore. Not included in the study were patients with benign pathologies, metastatic disease, and those treated in other institutions. Those who underwent mastectomy as primary surgery and those with inaccessible patient records were likewise excluded. The intervention group comprised patients who underwent cavity shave excisions, while the control group included patients who did not receive cavity shaves.

SLIM technique

For patients undergoing BCS, intra-operative cavity margin shaves were performed in a standardised fashion; superior, lateral, inferior and medial. They were also sent for histopathological analysis in that order. The acronym SLIM makes the sequence easier to remember, i.e., S (superior), L (lateral), I (inferior), and M (medial) margins, Figure 1. All cavity margin shaves are placed in formalin and sent to the pathology for inking based on local protocols. The anterior and posterior margins are not routinely taken as wide excisions are commonly performed from skin or including skin down to chest wall.

Figure 1 Intra-operative radial cavity margin shaves were performed sequentially in four directions: superior, lateral, inferior, and medial. The final margins were on the outer edge of the shave away from the lumpectomy.

Positive margins

Margin status was classified as positive if there was tumor on the inked side for patients with invasive cancer (14). For patients with DCIS, a 2 mm margin was needed to ensure negative margin (15). All postoperative histology were discussed in a multidisciplinary tumour board; and re-excision was usually performed when there were positive margins.

Data collection

Clinical, radiographic and pathological data of eligible patients were obtained from a prospectively kept database. Re-excision rates, in view of positive resection margins, were also recorded. Participants were divided into two groups: those who had the SLIM technique performed during their BCS, and those who did not. The primary outcomes included re-excision rates and number of incidental breast cancers in the SLIM group.

Statistical analysis

Categorical characteristics between the two groups were compared using the Pearson Chi-squared test, and continuous characteristics were compared using Student’s t-test. Fisher’s exact test was utilized in subgroup analysis when there was smaller sample size. Patients were not randomly assigned to SLIM or non-SLIM groups resulting in selection bias, therefore, to mitigate for this, we have clarified the allocation criteria from 2022 when SLIM was adopted gradually. Since this was a retrospective cohort study, the sample size was determined by, the number of eligible patients who underwent BCS at the given time frame and the availability of complete records. All data were extracted from a prospectively maintained institutional database. Prior to analysis, the dataset was reviewed for completeness. Patients who were lost to follow up before definitive margin status assessment or re-excision were excluded from the analysis. We conducted analyses excluding patients who received NACT to determine if these factors influenced the re-excision rates. Additionally, subgroup analyses were done using Fisher’s exact test for smaller sample sizes to verify the consistency of the results.

Ethical statement

This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. This study obtained approval from the SingHealth Centralised Institutional Review Board (CIRB No. 2019/2419) and informed consent was waived by the Institutional Review Board.

Results

A total of 421 patients were included in the study, with 79.6% (n=335) in the SLIM group and 20.4% (n=86) in the control group. There were no statistically significant differences in terms of mean age [54.2 (13.3) vs. 52.5 (11.6)] years or mean pathologic tumour size [22.5 (12.6) vs. 22.2 (11.5)] mm between the SLIM and control group, respectively. Furthermore, there was no difference in lymph node involvement [46 (13.7%) vs. 19 (22.1%)], or in patients who received NACT [51 (15.2%) vs. 14 (16.3%)] between the SLIM and control group, respectively (Table 1).

Regarding histological subtypes, IDC was the most common subtype in both groups, with 184 out of 335 patients (54.9%) in the SLIM group and 54 out of 86 patients (62.8%) in the control group. DCIS followed as the second most common subtype, with 58 out of 335 patients (17.3%) in the SLIM group and 21 out of 86 patients (24.4%) in the control group. A significant difference was observed only in the distribution of “other” histopathological subtypes between the two groups. This group was made up of mostly malignant phyllodes. In the SLIM group, 45 out of 335 patients (13.4%) had “other” histological subtype, compared to 2 out of 86 patients (2.3%) in the control group, P<0.001.

Additionally, the SLIM technique was associated with the detection of incidental cancer (both DCIS and IDC) in the shave margins in 1.2% (n=5) of patients, highlighting the potential for detecting occult cancer with the SLIM technique.

Subgroup analysis of patients who had re-excisions

Of the patients who had re-excisions, 40 (11.9%) were in the SLIM group, compared to 13 (15.1%) in the control group. There was no statistically significant difference in the mean age [standard deviation (SD)] between the SLIM [54.4 (10.9)] and control [58.1 (11.3)] groups. Similarly, no significant difference was observed in mean (SD) pathologic tumour size [18.4 (8.6) mm in SLIM vs. 15.0 (8.7) mm in the control group], the proportion of patients who received NACT [2 (5%) in SLIM vs. 1 (7.7%) in the control group], or lymph node involvement [2 (5%) in SLIM vs. 1 (7.7%) in the control group].

When considering the histological subtypes for re-excisions, there was no significant difference between the SLIM and control groups for IDC. Specifically, 65% (26/40) of re-excisions in the SLIM group were due to IDC, compared to 61.54% (8/13) in the control group. However, a significant difference was observed for re-excisions due to invasive lobular cancer (ILC), with 5% (2/40) of re-excisions in the SLIM group being due to ILC, compared to 23.08% (3/13) in the control group, P<0.05. No significant difference was observed for re-excisions due to DCIS, with 17.5% (7/40) of re-excisions in the SLIM group and 23.08% (3/13) in the control group (Table 2).

Discussion

In this study, the overall re-excision rate in patients undergoing BCS was low, and below the average reported in the literature (16-19). The characteristics were comparable between the two groups though SLIM was used statistically more frequently in patients with other histological subtypes. In patients with invasive lobular carcinoma, SLIM technique was statistically associated with a lower rate of re-excision. A small proportion of incidental cancers were also detected in patients undergoing margin cavity shaves with the SLIM technique, underscoring the clinical significance of this technique.

The characteristics of the patients, in terms of age, tumour size, lymph node involvement, and patients with NACT, were comparable between the SLIM and control groups This made the groups well-matched and ensured that any differences in outcomes, such as re-excision rates were likely due to the SLIM technique itself, rather than inherent differences between the groups. The SLIM technique group had a significantly higher proportion of patients with “other” histological subtypes (11.94%) compared to the control group (2.33%). A possible explanation for this was that SLIM was used more often with other types of breast pathologies, such as malignant phyllodes etc. where there was no consensus on optimum surgical margins, in order to achieve margin clearance in this sub-group. There was a slightly lower rate of re-excisions when cavity margin shaves were taken using the SLIM technique, however this was not significant.

The SLIM technique was associated with a lower proportion of re-excisions for ILC, suggesting that SLIM may reduce the need for re-excision in cases of invasive lobular carcinoma. This was an important finding since in ILC, which is the second most common subtype of breast cancer, the issue of positive margins is a particularly prevalent problem, given that it can be more challenging to detect and achieve clear margins. Up to 60% of women with ILC who undergo BCS will have positive margins (20-22), and re-excision was required to clear positive margins (23). Therefore, the SLIM technique may offer improved margin management in these cases.

Intraoperative margin assessment is crucial for reducing the likelihood of positive margins, yet there is no universally agreed-upon method for its implementation. Research suggested that using frozen section and cytology during surgery could provide greater accuracy than other techniques, but these methods are both time-consuming and resource-demanding (24,25), with a high percentage of “indeterminate” margin rate requiring further excision and subsequently found to be negative on paraffin (25). Emerging technologies, such as radiofrequency measurement, electrical impedance of tissues, and rapid evaporative ionization mass spectrometry, have demonstrated potential (11,26-29). However, these technologies still need further investigation, and their integration into standard clinical practice has been slow. As a result, optimising existing surgical techniques and maximizing the use of current resources is key to improving outcomes.

A recent survey among the breast surgeons, radiologists and pathologists (30) revealed a general dissatisfaction with the status quo in the operating theatre-radiology-pathology chain. Notable discrepancies were observed in the responses from pathologists, radiologists, and surgeons regarding specimen orientation and marking methods. Even within the same institution, there seemed to be a lack of consensus on the adequacy and interpretation of these methods. One study showed that margin face discordance using intra-operative suture orientation between surgeons and pathologist could be as high as 33% (31). The complexity of the procedure explained why nearly half of all re-excisions resulted in negative surgical margins in the final report. Furthermore, surgical margins can be affected by several factors, including how the specimen was transported and stored before pathological analysis, the techniques used for orientation, and the role of mammography with specimen compression. When breast tissue was compressed during specimen handling, its dimensions were altered and reduced by approximately 50% in the direction of compression and expanded about two times in the perpendicular direction (30). To thoroughly examine all the edges of a typical spherical lumpectomy specimen, it was estimated that around 3,000 tissue sections would be needed (32).

This discrepancy highlighted the possibility of differing interpretations and approaches to surgical specimen handling, underscoring the importance of standardised protocols and improved communication among multidisciplinary teams to achieve more consistent and precise handling and evaluation of specimens. As such, adopting a standardised method like SLIM to determine the sequence of removal of the shave margins could help minimize these inconsistencies and reduce confusion in operating theatre on the nature of the specimen especially when several shave margins were obtained simultaneously. Use of SLIM technique could also reduce re-excision in certain histological subtype as shown in our study. In our institution orientation is indicated using sutures: a long suture laterally, a short suture superiorly, and a loop suture anteriorly. We are exploring the feasibility of surgeon-performed inking to improve orientation accuracy in future practice.

Strengths of this study include it being the first to evaluate outcomes associated with the SLIM technique for sequential labelling of circumferential cavity margin shaves. Importantly, the technique offers a dual advantage: the routine use of cavity shaves and the application of standardised, systematic orientation and labelling, which may reduce errors, inconsistencies, and communication breakdown between surgeons and pathologists. The study also examined the use of the SLIM technique in a subgroup of patients with non-carcinomatous histology, thereby extending its potential applicability beyond conventional breast carcinomas.

The limitations include the relatively small sample size and the retrospective study design, which may introduce inherent biases, including selection bias in the use of the SLIM technique. Nevertheless, this is mitigated by the largely comparable baseline characteristics between the SLIM and control groups.

Conclusions

The SLIM technique was particularly beneficial for invasive lobular carcinoma, reducing the need for re-excision in this challenging subtype. The SLIM technique also helped detect incidental cancers in the margin cavity shaves. Given the variability in current practices for margin evaluation, standardising margin assessment during BCS using the SLIM technique can minimise inconsistencies.

Acknowledgments

This work was presented at St Gallen International Breast Cancer Conference, Vienna, 2025 and Global Breast Cancer Conference, Korea, 2025.

Footnote

Provenance and Peer Review: This article was commissioned by the Guest Editors (Visnu Lohsiriwat and Chayanee Sae-Lim) for the series “Transformative Approaches in Breast Surgery: Cutting-Edge Innovation, and Practice” published in Gland Surgery. The article has undergone external peer review.

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://gs.amegroups.com/article/view/10.21037/gs-2025-435/rc

Data Sharing Statement: Available at https://gs.amegroups.com/article/view/10.21037/gs-2025-435/dss

Peer Review File: Available at https://gs.amegroups.com/article/view/10.21037/gs-2025-435/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://gs.amegroups.com/article/view/10.21037/gs-2025-435/coif). The series “Transformative Approaches in Breast Surgery: Cutting-Edge Innovation, and Practice” was commissioned by the editorial office without any funding or sponsorship. The authors have no other conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. This study obtained approval from the SingHealth Centralised Institutional Review Board (CIRB No. 2019/2419) and informed consent was waived by the Institutional Review Board.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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