Of the 205 tumors, 113 (55%) were classified as LUMA, 34 (17%) as LUMB, 32 (15%) as TN, 8 (4%) as ERBB2, 10 (5%) as LAHH, and 8 (4%) as LBHH. Mean Ki-67 LI was significantly different among LUMA, HER2 and TN molecular classes (p<0.01). Ki-67 LI correlated with the number of mitotic figures (p<0.01) irrespective of the molecular class of the tumor.
Multivariate linear regression analysis of Ki-67 LI found correlations with TN molecular class (p<0.001, mean 61.7±25.0% for TN versus 17.6±14.7% for others), necrosis (p<0.0001 mean 65.7±24.7% with versus 21.0±19.4% without) and extent of lymphoid infiltrate (p<0.0001, means: 14.8±13.8, 23.53±19.8, 49.9±28.2, and 61.8±24.6 for none, mild, moderate, and marked respectively).
Multivariate logistic regression analyses found the following associations: LUMB class with increased lymph node involvement (p=0.0141, 14/27 LUMB versus 45/154 others); ERBB2 class with apocrine differentiation (p=0.0031, 7/8 ERBB2 versus 19/197 others), lymphoid infiltrate (p=0.0118, 8/8 ERBB2 versus 104/197 others) and increased amount of ductal carcinoma in situ (p=0.0129, 5/8 tumors with >25% in situ carcinoma); TN class with both high Ki-67 index (p=0.011, mean 61.7±25.0% for TN versus 17.6±14.7% for others) and with CK5 positivity (p<0.0001).
Multivariate logistic regression for the presence versus absence of lymph node metastasis identified significant correlation only with increasing tumor diameter (p=0.0023, OR=1.99/cm) and LUMB class (p=0.0075, OR=4.69). To test the robustness of these findings, we repeated the analysis as a multivariate linear regression with extent of lymph node involvement scored 0, 1, 2, 3, or 4 for negative, pN1mi, pN1, pN2, or pN3, respectively as the end point. We found the same significant correlations for LUMB (p=0.009) and tumor diameter (p=0.001) with that analysis.
The Ki-67 LI, CK5 immunoreactivity, and comparative morphologic features for various molecular classes are shown in . Other pertinent morphologic details are provided below.
Morpho-immunohistologic findings in breast cancer molecular classes
LUMA Tumors: Of the 113 LUMA tumors, 101 (89%) were ductal, 10 (9%) lobular and 2 (2%) were mixed ductal and lobular types. The ductal tumors were composed of 85 no special type (NST), 3 pure mucinous, 3 pure tubular, 3 solid papillary, 3 mixed NST and mucinous, 1 pure cribriform, 1 NST and cribriform, 1 NST and papillary, 1 tubular and mucinous carcinoma. Among lobular cancers, both classic (6 cases) and pleomorphic (4 cases) tumors were identified in the LUMA category. Although, the average Ki-67 LI was the lowest among all the molecular classes (), few cases showed a high labeling index which correlated with high mitotic activity in these cases ().
A prototype LUMA tumor (A-H&E; B-Anti-ER) with very low Ki-67 labeling index (C, Anti-Ki67).
A higher grade LUMA type of invasive ductal carcinoma (A-H&E; B-Anti-ER) showing an increased Ki-67 labeling index (C, Anti-Ki67) that correlates to the mitotic activity of the tumor.
LUMB Tumors: These tumors were of both ductal and lobular types, generally moderately differentiated, with some degree of PR expression and relatively low Ki-67 LI (). However, 3 of these 34 LUMB tumors showed morphology consistent with the “basal-like” tumors described above and previously in the literature. Two of these tumors were also positive for CK5. These tumors were classified as LUMB because the ER IHC scores for these 3 tumors were 20, 45 and 60, respectively.
These tumors in our series were mainly high-grade with the majority showing at least some degree of apocrine differentiation (). CK5 immunoreactivity was seen in 5 of 8 cases (63%). All of these 5 positive cases showed apocrine differentiation and reactivity with EGFR antibody. Four of these 5 cases were also positive with CK5/6. Some feature classically ascribed to TN-basal like carcinomas such as necrosis and moderate intra-tumoral lymphoid infiltrate [5
] were seen in 37% and 63% cases respectively. The average Ki-67 LI was 27.8%, which was intermediate between labeling index for LUMA and TN tumors.
An ERBB2 tumor showing apocrine differentiation (A-H&E; B-Anti-HER2) and moderate increase in Ki-67 labeling index (C, Anti-Ki67).
Using morphologic criteria, TN tumors could be classified into 3 groups-tumor with sheet-like growth pattern (n=15; 47%, ); ductal NST (n=13; 40%, ); and apocrine carcinomas (n=4; 3%). These included 2 tumors that showed spindle cell metaplastic features. The classic morphologic features of “basal-like” breast carcinoma described in the literature [5
] were predominantly seen in tumors with sheet-like growth pattern (SLGP). However, the CK5 reactivity was seen in 73% (11/15) cases of tumors with SLGP, in 62% (8/13) cases of ductal NST and in 75% (3/4) of apocrine carcinomas. The average Ki-67 LI was highest (71.3%) in tumor with SLGP, slightly lower (58.5%) in ductal NST, and lowest (27%) in apocrine carcinomas.
A TN tumor showing sheet like growth pattern (A, H&E) and a high Ki-67 labeling index (B, Anti-Ki67).
A TN tumor without sheet like growth pattern (A, H&E) and a high Ki-67 labeling index (B, Anti-Ki67).
Other HER2-positive Tumors: Morphologically, both LAHH and LBHH tumors were generally moderately differentiated and predominantly of the ductal, no special type. Average PR expression was higher in LAHH tumors compared to LBHH tumors. LAHH tumors showed an average Ki-67 LI of 16.3% compared to 24.6% LI for LBHH tumors.
Validation of Immunohistologic Criteria: Of the 359 tumors (validation set) treated with NACT, 110 (30.7%) were LUMA, 74 (20.4%) were LUMB, 79 (22%) were TN, 57 (16%) were ERBB2, 15 (4.2%) were LAHH and 24 (6.7%) were LBHH. Complete pathologic response was identified in 33% of ERBB2, 30.3% in TN, 8.3% of LBHH, 1.8% of LUMA, 1.4% of LUMB, and 0% of LAHH tumors (p<0.0001). Average percentage tumor size reduction was also highest in the TN (75%) and ERBB2 (68%) tumors and was statistically significant compared to other classes (p<0.05). The average percentage tumor size reduction in other tumors was as follows: 47% in LBHH, 33% in LAHH, 30% in LUMB, and 23% in LUMA. Further details regarding specific chemotherapy and outcome data is a subject of separate publication (manuscript in preparation).