Correlation between ultrasound descriptors and histopathological prognostic markers (Ki-67 score, Nottingham grade, and tumor subtype) in triple-negative breast cancers

INTRODUCTION

Triple negative breast cancer (TNBC) is a subtype of breast cancer that is characterized by the absence of immunohistochemical staining of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor-2 (HER2) receptors. TNBC is known for its aggressive nature, absence of response to hormonal therapy, and poor prognosis. Importantly, when breast cancer occurs in young women, it is more likely to be of the TNBC subtype.^[1] The combination of being an aggressive tumor with a predilection towards younger women makes TNBC a formidable disease to contend with. Furthermore, TNBC can often masquerade as a benign appearing mass, and it is reported that benign appearing TNBCs are more likely to be more aggressive.^[2-4] In this setting, it is important for radiologists to understand the correlation between supposedly “benign” imaging appearances and poor prognostic indicators in TNBCs, so that they are alert even while imaging apparently benign masses. Since the proliferation index “ki67” is a proxy for predicting the biological behavior of a malignancy and its response to adjuvant systemic treatment, our study is aimed at evaluating whether specific ultrasound (US) features correlated with high (>14%) or low (<14%) ki67 score in cases of histopathologically proven TNBC. Additionally, we also evaluated the association between US imaging features and other histopathological markers, including subtypes of TNBC, Nottingham’s Histologic score, and Nottingham’s Histologic grade.

MATERIALS AND METHODS

Ethical Approval was sought and obtained from our Institutional Ethics Committee. The requirement for informed consent was waived as this is a retrospective study. Data confidentiality was maintained using specific numbered identification and password protection.

We retrospectively searched our Institutional Radiology Picture archiving and communication system database for cases that had undergone US-guided core needle biopsy at the Radiology Department at our institution and were proven to have TNBCs from 2017 to 2024. 50 cases were found and included in the study. The US features of the malignant mass (shape, margins, echogenicity, posterior features, orientation, vascularity, presence of echogenic rim, and presence of calcifications) were assessed by two radiologists independently.

Ki67 scores for all patients were documented from the histopathology report and categorized as high or low. At our institution, a Ki-67 score >14% was considered high and a score <14% was considered low.

Other histopathological examination (HPE) data, including subtype of TNBC (basal-like/non-classified), Nottingham’s histological grade, and presence of necrosis on histopathology, were also documented from the histology report for all 50 cases.

Statistical analysis was done using the chi-square test and t-tests to determine whether any specific US features correlated with age at diagnosis or histopathological features (Ki-67 score, Nottingham’s grade, subtype of TNBC, presence of necrosis).

RESULTS

The mean age of patients was 53.4 years (Range: 23-75 years). All patients were female. The mean Nottingham’s Histologic Score was 7.08+1.23 (Range: 4-9). The mean Ki-67 score was 52 (Range: 5-95). All 50 patients showed a mass on US; 22/50 (44%) had suspicious ipsilateral axillary lymph nodes. The US features (shape, margins, orientation, echogenicity, posterior features, and presence of cystic spaces within the mass) and histopathological features (TNBC subtype (BL or NC), ki67 scores, Nottingham’s Grades, and histological type) are summarized in Table 1.

The results of the statistical analysis are presented below:

DISCUSSION

TNBC is considered one of the most aggressive immunohistochemical subtypes of breast cancer, characterized by negativity for ER, PR, and HER2. TNBC is subclassified into six types: basal-like 1 (BL1), basal-like 2 (BL2), mesenchymal (M), mesenchymal stem-like (MSL), immunomodulatory (IM), and luminal androgen receptor (LAR), based on gene expression profiling.^[5] However, in practice, TNBC is often just classified into 2 broad subcategories: basal-like subtype (BL, which is EGFR positive) and non-classifiable subtype (NC, which is EGFR negative).^[6]

Previous studies have shown that the BL subtype has a poorer prognosis, while the NC subtype has a better prognosis.^[7,8] The proliferation index “Ki-67” (a nuclear antigen seen in proliferating cells) is a proxy for predicting the biological behavior of the malignancy and response to adjuvant systemic treatment. In cases of TNBC, high Ki-67 is often associated with more aggressive disease, increased possibility of metastasis, and poor prognosis.^[9] In agreement with this, our study also showed that the BL subtype was found to be significantly associated with a high Ki-67 (p < 0.001).

In our study, 90% of patients had high Ki-67, while only 10% had low Ki-67. This is in agreement with previous studies that also showed a prevalence of higher Ki-67 scores in TNBC patients.^[10]

In our study, 96% of TNBCs were moderately or poorly differentiated (Nottingham Grade 2: n=29/50, 58%; Nottingham Grade 3: n=19/50, 38%). Previous studies have also reported that the majority of TNBCs were categorized as high-grade tumors.^[11,12]

Although some studies reported that Ki-67 was higher in young patients with TNBC, some others reported that Ki-67 did not correlate with age.^[1,9] In our study, there was no correlation between ki67 score and the age of the patient.

Although TNBC is often associated with a younger age of diagnosis, the mean age in our study population was 53.4 years, comparable to that in Muller’s study, where the mean age of patients with TNBC was 58 years.^[2] A few studies, however, have reported a lower mean age of presentation of TNBC.^[13,14] Over and above the mean age of incidence of TNBC, the important concept to be understood is that when young women (<40 years) develop breast cancer, they are more likely to have the triple negative subtype than other subtypes.^[1]

Radiologically, the primary concern is that TNBC has imaging characteristics that can mimic benign masses.^[14,15] Müller et al reported that 14.5% of TNBC cases in their study population were originally reported benign on ultrasound.^[2]

In our study, 22% of patients had oval/gently lobulated masses with circumscribed margins. This is consistent with the rates (12-27%) reported in the literature.^[16,17] It is important to note that most TNBCs present with irregular margins, but amongst the tumors that have circumscribed margins, TNBC is the most frequent subtype.^[2,18]

Posterior enhancement has also been reported to be frequently associated with TNBC.^{[2,4,14,17-19]} In our study, 58% of masses showed posterior enhancement. Posterior acoustic enhancement is caused by the high cellularity in these high-grade tumors.^[20]

Previous studies have shown that both circumscribed margins and posterior enhancement are associated with higher Ki-67 scores and higher tumor grade in TNBCs.^[3,4,20] In our study too, both circumscribed masses and posterior enhancement were found to be significantly associated with higher mean Nottingham score (p = 0.046 and p = 0.041, respectively), and posterior enhancement was also significantly associated with higher Nottingham grade (p = 0.032). Circumscribed masses also showed higher mean Ki-67 than non-circumscribed masses and more frequent necrosis on pathology, although these two associations were not statistically significant.

In particular, a combination of oval/round shape, circumscribed margins, and posterior acoustic enhancement was reported to be highly specific for TNBC (98.4%).^[4,15,19,20] 10/11 of our circumscribed masses also showed posterior enhancement and parallel orientation. This makes for an alarming combination as these masses can easily be mistaken for benign entities like fibroadenomas and phyllodes tumors [Figure 1a and b].

Close

Figure 1: (a) US image of a 23-year-old lady who presented with new onset left breast pain revealed an oval circumscribed hypoechoic mass in parallel orientation with mild internal vascularity and posterior acoustic enhancement, measuring 2 cm. Histopathological examination: Invasive mammary carcinoma, no special type, with 25% areas of necrosis, Nottingham’s histologic score 9/9, and Nottingham’s histologic grade 3. Immunohistochemical studies revealed a triple-negative basal-like molecular subtype, with a Ki-67 score of 70%. (b): US image of a 32-year-old lady who presented with a right breast lump revealed an oval circumscribed hypoechoic mass in parallel orientation with cystic changes within, mild internal vascularity, and posterior acoustic enhancement. Histopathological examination: Invasive mammary carcinoma, no special type, Nottingham’s histologic Score 7/9, and Nottingham’s histologic grade 2. Immunohistochemical studies revealed a triple-negative basal-like molecular subtype, with a Ki-67 score of 70%. US: Ultrasound.

Export to PPT

Another point of concern is that circumscribed masses in our study were also significantly associated with younger age compared to non-circumscribed masses (p = 0.006), which were common in older patients [Figures 2a and b]. 36.4% (4/11) of patients with circumscribed margins were between the ages of 23 and 35. This is particularly concerning because, at a younger age, a new onset circumscribed mass is more likely to be mistaken for a benign entity than if it appeared in a patient of older age.

Close

Figure 2: (a) US image of a 74-year-old lady who presented with a right breast lump revealed an irregular mass with indistinct and angular margins, mixed posterior features, and mild internal vascularity. Histopathological examination: Invasive mammary carcinoma, no special type, Nottingham’s histologic score 6/9, and Nottingham’s histologic grade 2. Immunohistochemical studies revealed triple-negative basal-like molecular subtypes, with a Ki-67 score of 20%. (b) US image of a 65-year-old lady who presented with a left breast lump revealed an irregular mass with indistinct and angular margins, surrounded by an echogenic rind, showing posterior shadowing and mild internal vascularity. Histopathological examination: Invasive mammary carcinoma, no special type, Nottingham’s histologic score 8/9, and Nottingham’s histologic grade 3. Immunohistochemical studies revealed a triple-negative basal-like molecular subtype, with a Ki-67 score of 40%. US: Ultrasound.

Export to PPT

CONCLUSION

Although the combination of circumscribed margins, parallel orientation, and posterior acoustic enhancement is classically seen in fibroadenomas, radiologists must be aware that there are subsets of aggressive malignancies that present with a similar imaging appearance, even in a young population. Such entities help us understand why even convincingly benign appearing masses in the US need to be categorized as BI-RADS 3 on the index visit and advised follow-up imaging as per guidelines. Knowledge about such entities and their adverse outcomes inspires appreciation for the guidelines that are designed to help clinicians and radiologists navigate these unique clinical dilemmas and walk the fine line between remaining cautious and avoiding overcalls.