Phyllodes tumors (PTs) are rare (0.3-1% of all primary breast tumors) fibroepithelial breast neoplasms that carry a risk of recurrence and metastasis (1,2). PTs are classified into benign, borderline and malignant-grade disease according to histopathological criteria of stromal cellularity and atypia, mitotic count, stromal overgrowth and the nature of the tumor border (3,4). Histopathological grading is important for prognostication and management: Recurrence rates increase with higher tumor grades [10-17, 14-25 and 23-30% for benign, borderline and malignant PTs, respectively (3,4)]; benign tumors only very rarely metastasize (3); and malignant tumors may benefit from adjuvant therapy such as postoperative radiotherapy, although the optimal management remains uncertain (5). Metastasis of PTs heralds a usually dismal prognosis and death from the disease (6), but in almost all cases, metastasis only occurs when the primary tumor is graded as malignant (3,6).

Although complete excision is the gold standard treatment for PT and complete excision with an adequate margin reduces the subsequent recurrence risk, it is still being debated what constitutes an adequate margin to reduce the risk of recurrence (5). Other clinicopathological features, such as cellular atypia, mitotic rate and stromal overgrowth, have similarly been examined as predictive factors for recurrence, with conflicting results (3,5). Other protein biomarkers such as matrix metalloproteinase (MMP)-14, Six-1, PAX3, FoxC2, TWIST, C-X-C chemokine receptor 4, vascular endothelial growth factor, stromal Yes-associated protein, cellular E-cadherin and CD10 have been associated with recurrent PT, while others have not [e.g., epidermal growth factor receptor (EGFR), human EGFR 2, membranous E-cadherin; MMP-1, -2, -7, -9, -11 and -13; and tissue inhibitor of metalloproteinase (TIMP)-1, -2 and -3] (7). There are still no definitive markers of recurrence or other outcomes such as survival to guide management, despite the persistent risk of recurrence, even in patients with benign disease. Although benign disease does not usually result in clinically serious sequelae, it would still be helpful to identify all individuals at risk of recurrence, even those with benign disease, to tailor surveillance strategies (8).

Ki67 is a nuclear antigen that is expressed in all phases of the cell cycle (G1, S, G2), and it is a reliable and widely used immunohistochemical biomarker of proliferation in routine histopathological practice, including in breast cancer (9). Although practice varies, Ki67 is routinely used as a prognostic biomarker in invasive ductal carcinoma, and the Italian Association of Medical Oncology, European Group on Tumor Markers, European Society for Medical Oncology and the National Institute for Health and Care Excellence either recommend or advise considering Ki67 measurement for prognostication in patients with breast cancer (10). Furthermore, the European Society for Medical Oncology supports the use of Ki-67 expression as a predictive biomarker of response to neoadjuvant chemotherapy (10). Similarly, Ki67 is prognostic and used routinely in patients with other tumors, such as in pancreatic neuroendocrine neoplasia, where it predicts both recurrence and metastasis (11-13). While the mitotic index only reflects cells in the M-phase, Ki67 detects other cells at different stages of proliferation (or arrest) and therefore provides different information about proliferation in the histopathological snapshot of tumor biology captured in a tumor section (9). In PT, stromal expression of Ki67 has been reported to be 5-25% in benign tumors and 15-100% in malignant tumors, and this association with the grade has been reported in several studies (14). However, whether Ki67 has prognostic significance in PT remains uncertain.

Therefore, in the present study, a systematic review and meta-analysis were performed to examine whether Ki67 is associated with adverse clinical outcomes, particularly recurrence, in patients with PT.

This meta-analysis is reported according to the 2020 Preferred Reporting Items for Systematic reviews and Meta-Analyses statement (15).

As PT is a rare tumor, there are no large, prospective studies on PTs available. However, previous retrospective studies have reported that various clinicopathological factors, including adequacy of resection, histopathological features and tumor protein expression, are associated with clinical outcomes in patients with PT (7,8). Although PTs that clearly lie at the extreme ends of the diagnostic spectrum, such as those that are difficult to differentiate from fibroadenomas or those with overt features of malignancy, allow straightforward clinicopathological correlation and management planning, the disease still poses pathological and clinical challenges in practice. The absence of clear prognostic factors means that there are still no evidence-based guidelines for the follow-up of patients with PT, and accordingly, certain patients with benign lesions are subjected to unnecessary routine follow-up imaging, which may not be required (8). Any indicator of an adverse prognostic course, particularly in patients with clinically benign lesions, would be useful to tailor follow-up and spare the costs and resources associated with surveillance imaging or, conversely, to divert resources to those at greatest risk of future recurrence.

This situation inspired the present systematic review and meta-analysis of the prognostic significance of the Ki67 proliferation marker in patients with PT. Although only five studies that specifically examined the prognostic value of Ki67 in patients with PT were identified, the present analysis indicates that patients with PT with high expression of Ki67, here defined as >10 or >11.2% expression in tumor stroma cells, have a significantly increased risk of an adverse outcome, particularly recurrence. Although the small sizes of the included studies and a lack of relevant data precluded formal subgroup analysis of the prognostic significance of Ki67 in patients with benign and borderline disease alone, the available data appear to suggest that the association between high Ki67 and recurrence is not limited to malignant disease. Indeed, all seven recurrences reported by Chan et al (20) occurred after the resection of benign primary lesions, four of which had Ki67 indices >10%. In the other included reports, even though some initially benign primary lesions went on to recur, their Ki67 status was unclear. The present analysis suggests that a Ki67 index >10% may be an indicator of the need for extra vigilance during follow-up, regardless of other clinical or histopathological features. Even though locally recurrent PT often presents clinically and not through imaging (8,25), establishing Ki67 as an adverse prognostic indicator could be helpful for identifying the subset of patients who may benefit from focused radiological follow-up.

There have been previous attempts to devise predictive models of the clinical behavior of PT. For instance, Tan et al (6) developed a nomogram that included atypia, mitoses, stromal overgrowth and surgical margin status that predicted recurrence-free survival up to 10 years, which was subsequently validated in several independent cohorts from around the world (26-28). Of note, stromal mitotic activity was usually associated with recurrence even when the other included parameters were not (26,27). Given the present findings and that proliferation as measured by mitosis counts are likely to be critical factors related to recurrence in PT, there is a need to re-evaluate the relationship between Ki67 indices and clinical outcomes to establish whether this simple and cost-effective ancillary diagnostic test should be integrated into clinical nomograms, particularly with regard to predicting responses in benign and borderline tumors.

This meta-analysis has several limitations. As noted above, only five studies were available for analysis, all of which were retrospective, and thus, selection bias cannot be excluded. The sample size in the meta-analysis was relatively small, limiting the level of evidence. Due to the limited number of available studies, it was required to combine results for similar but slightly different Ki67 thresholds (10 and 11.2%), which may have introduced a certain imprecision into the results. As the different Ki67 cut-off points were not considered, the optimal threshold for prognostication remains uncertain. Although the results of the meta-analysis remained the same when the study reporting OS outcomes was excluded, it was necessary to combine outcomes to fully evaluate the small number of available studies. Likewise, it was not possible to separate local from distant recurrences to evaluate the impact of the Ki67 index on the risk of distant recurrence/metastasis. Finally, Ki67 measurements were not standardized across laboratories, which may have influenced the results.

In conclusion, the present study was the first meta-analysis of the predictive value of Ki67 in PT of the breast and the results clearly show that a relatively high Ki67 index (>10%) is associated with recurrence. Given that Ki67 is widely used in clinical practice, it seems timely to re-evaluate the prognostic value of the marker in large retrospective cohorts with long follow-up to firmly establish whether it could contribute to identifying patients at risk of recurrence, particularly those with histologically benign disease. Doing so could impact clinical practice by refining follow-up recommendations based on quality evidence and sparing low-risk individuals from unnecessary imaging.