In England (NICE 2002) and the United States (NCCN 2018), learned societies recommended considering breast reconstruction after a mastectomy for all patients once the treatment plan for breast cancer is shared. In France, INCa has recommended using this approach since 2015 which was validated by a law voted in 2019. Before these recommendations, the proportion of performed breast reconstructions in France was low (27.4 % in 2012 according to Régis et al.) [1].

Numbers could be higher if immediate breast reconstruction (IBR) was automatically considered during initial cancer management. The proportion of IBR increases in industrialised countries, driven by the objective to improve patients’ quality of life [2,3]: self-image, social and sexual wellbeing and post-operative psychological recuperation. IBR mastectomy (breast implants, musculocutaneous flaps) includes preservation of the skin envelope and, possibly, of the areola mammae to decrease surgical impact and improve cosmetic outcomes [4]. Several studies have established the oncological safety of IBR when managing breast cancers with a good prognostic (in situ, early stages I and II cancers) [4,[5], [6], [7]]). They do not show a decrease in overall survival, recurrence free survival, nor do they show an increase in recurrence rate (local, locoregional, distant) for these patients [8], [9], [10].

Until now, IBR was not recommended when adjuvant radiotherapy indication was available. Indeed, the increased risk of experiencing post-operative complications after IBR could delay adjuvant therapies [10] initiation. Post-operative complications risk is higher with some specific risk factors (e.g. age, active smoking or high BMI are highlighted in the literature [11,12]. However, several pieces of work have suggested using musculocutaneous flaps or breast implants before radiotherapy in the management of advanced breast cancers (stage IIB, III, IV) with no increase of the oncological risk [11], [12], [13], [14].

Recommendations made by INCa in 2022 enable us to offer IBR to patients with a mastectomy indication according to their risk factors of post-operative complications, including to patients who could receive an adjuvant therapy [15]. The indication must consider the risk factors of post-operative complications associated with the risk of increased timeframe before initiation of adjuvant therapy.

We have conducted a safety study on our practices by comparing the rates of early post-operative complications between patients who received a simple mastectomy (SM) and patients who received an IBR mastectomy. Secondly, we have evaluated the impact of the preoperative risk factors on the rate of post-operative complications. With the change of guidelines and the expansion of IBR's indications, we also wanted to estimate the proportion of patients among SM patients who could have benefitted from an IBR indication without threatening the oncological management. This study has been approved by local ethic committee (CNIL, Hospices Civils de Lyon), under number 21_5201.1

A retrospective, descriptive and unicentric study was conducted at Hôpital de la Croix-Rousse (Lyon, France). Patients who received a mastectomy for their breast cancer between 2016 and 2020 were eligible. Having automatically received a bilateral mastectomy for a bilateral breast cancer also allowed inclusion. Male patients, prophylactic mastectomies, history of breast surgery, mastectomies for non-breast cancer or recurrence of breast cancer after autologous reconstruction were excluded.

When the patients received a mastectomy without immediate reconstruction, they were included in the simple mastectomy (SM) group. When the breast reconstruction (musculocutaneous flaps, breast implants - BI - or expanders) was conducted during the same operative time as the mastectomy, patients were included in the mastectomy and immediate breast reconstruction (IBR) group.

Patients characteristics matching usual risk factors of post-operative complications were as follows: >70 years old, active smoker, treated diabetes, treated high blood pressure, BMI >25, immunosuppressant therapy, bleeding risk therapy, history of homolateral chest radiotherapy [16].

Mastectomy indication was studied, alongside oncological characteristics of the tumour (pathology, lymph node status, emboli, grade, hormone receptor and HER2 status) and the therapy (chemotherapy, radiotherapy, hormone-therapy). The reconstruction's technique in IBR group was also studied.

The use of chemotherapy was categorised according to the setting: neoadjuvant, adjuvant, before the mastectomy. Chemotherapy before mastectomy is used when cancers are managed by initial tumorectomy with unhealthy margins (R1), for which it was decided to use chemotherapy before revision mastectomy.

Finally, early complications within 3 months post surgery after inclusion were categorised according to Clavien-Dindo system from grade II (Fig. B.1). Data about grade I complications was not exhaustive and not relevant enough compared to the risk of delay in adjuvant therapies initiation. Several complication grades could be gathered for one specific patient. Tracking of oncological and surgical data of patients operated on at Hôpital de la Croix-Rousse was updated until June 2021 at Hospices Civils de Lyon (HCL), at Centre Léon Bérard (CLB) or by community healthcare professionals: adjuvant therapies, delayed breast reconstruction (DR), diagnosed breast cancer recurrence within a year of mastectomy

The descriptive analysis was focusing on the number of patients within each group, and the proportion of overall breast reconstruction in our population. Population's overall and oncological characteristics were gathered and compared between the two groups. Qualitative data was detailed with percentages and compared with chi-squared and Fisher tests. Quantitative data was detailed by a median and interquartile, then compared through Wilcoxon signed-rank tests. Significance threshold was established at 0.05 for all statistical analyses, conducted through software R.

In the main analysis, complication rates were compared between SM and IBR according to each grade (Fig. B.1). In a secondary univariate analysis, the proportion of post-operative complications was studied for each risk factor. The same statistical tests were used. Finally, among patients who had simple mastectomies before 2022, the number of patients eligible to IBR using new INCa's guidelines was calculated. Patients with one or less risk factor of post-operative complications (cf “study population” paragraph) were included, according to the current practices at Hôpital de la Croix-Rousse and to the safest oncological management. Among those patients, having inflammatory cancer (T4D) was the exclusion criteria.