Neoadjuvant chemotherapy (NAC) in breast cancer provides the advantages of de-escalating surgical management, decreasing the risk of micrometastases, and allowing the assessment of in situ response.1 The United States National Cancer Database was queried in 2018 and found an overall complete pathologic response rate of 19%, ranging from 0.3% in luminal A cancers to 38.9% in human epidermal growth factor receptor 2-positive (HER2+) cancers.2 For tumors with high response rates, NAC also informs use of adjuvant therapies such as capecitabine and ado-trastuzumab emtansine (T-DM1) to maximize prognosis. T-DM1 is a conjugate of trastuzumab and the microtubule inhibitor emtansine. The combination allows intracellular delivery of emtansidine to HER2 positive cells.3,4

Guidelines for the assessment of pathologic response are sparse and non-specific. American Society of Clinical Oncology (ASCO) guidelines5 recommend frequent clinical examinations in NAC patients with supplemental imaging as required using the modality that best visualized the tumor at original diagnosis. Modality and timing in relation to chemotherapy completion are not specified. As such, interpretation of these guidelines and resultant clinical care is variable, both within and between institutions.

Magnetic resonance imaging (MRI) has been shown to be the best predictor of pathologic complete response (pCR).6,7 However, this predictive ability is imperfect; a study in 2019 by Sener et al. demonstrated that 27% of patients with radiologic complete response on MRI harbored residual disease (invasive cancer and/or ductal carcinoma in situ [DCIS]).8 Comparatively, mammography has shown less success in predicting the extent of residual disease, often due to the presence of residual microcalcifications.9,10,11,12

Calcification burden can be dynamic or stable as NAC progresses, and its response does not reliably correlate with disease response.10 Two retrospective reviews have demonstrated that the extent of mammographic microcalcifications correlates only weakly to moderately with maximum dimension of residual invasive disease.11,13 They are often believed to represented DCIS, which may be eradicated by NAC but the associated calcifications usually persist.14

Microcalcifications, defined as calcifications individually measuring <1 mm, are found in 30–50% of malignant breast lesions.13 Some will advise complete excision given the risk that they represent the full extent of tumor burden.15 Other biological explanations suggest that residual calcifications after NAC can represent necrotic non-viable tumor, DCIS, hematoma, or fat necrosis.11,16 As such, using extent of residual calcifications post chemotherapy to guide surgical excision may be misleading and incongruent with oncologic goals and cosmesis.

Moreover, given that hormone-sensitive patients have higher proportions of residual microcalcifications than hormone-negative patients,11 and that disease response to chemotherapy varies by receptor subtype, there may be a group of patients whereby presence of residual calcifications could be incompletely excised, allowing for breast-conserving surgery without oncologic compromise.13

Recognizing the current ambiguous relationship between imaging parameters, receptor subtypes, and optimal surgical management, the aim of this study was to explore the relationship between post-NAC microcalcifications, the corresponding MRI findings, and the presence of residual disease on surgical pathology. In contrast to prior studies, comparisons will be stratified by receptor subtypes to better delineate the relationship of calcification extent with oncologic safety.

Methods

Patient Population

This retrospective study included patients who received NAC between 1 January 2014 and 31 December 2019 at Sunnybrook Health Science Center, Toronto, who have documented calcifications on pre- and post-NAC mammogram associated with a biopsy-proven invasive carcinoma (ductal, lobular, mixed-type, or non-specified). Patients were identified from a database of patients who have received NAC that has a standing Research Ethics Board (REB) approval for data use with the authors. Patients with no available surgical pathology or inflammatory breast cancer were excluded.

Objectives

The primary objective of this study was to correlate the maximum dimension of post-NAC mammographic calcifications with the maximum dimension of residual invasive disease on surgical pathology, stratified by type of calcification (pleomorphic or other, including linear, amorphous, fine, dystrophic, non-described), receptor subtype, and type of chemotherapy. Receptor subtypes were categorized as (1) estrogen receptor (ER)- or progesterone receptor (PR)-positive and HER2-negative (hormone receptor-positive [HR+]/HER2−); (2) ER- and PR-negative and HER2+ (HR−/HER2+); (3) triple-negative breast cancer (TNBC); or (4) triple-positive breast cancer (TPBC).

The secondary objectives were to assess correlation of the maximum dimension on final pathology with both the maximum dimension of (1) post-NAC mammographic mass (if present), and (2) post-NAC MRI, mass enhancement (ME), and non-ME (NME). Maximum dimension on final surgical pathology included the dimensions of invasive disease, DCIS, and the tumor bed.

Data Collection

Demographics

Patient age at diagnosis, menopausal status (pre/peri or post), and self-reported population group were collected from the electronic patient record. T and N stage (per the 8th edition of the American Joint Committee on Cancer [AJCC] TNM staging system) and histologic grade were also collected.

Imaging

Pre- and post-NAC mammographic characteristics were recorded, including morphology (as above), maximum dimension of calcifications, and maximum dimension of associated focal asymmetry (if present). If a focal asymmetry was present on the mammogram but no dimension was recorded, then the maximum dimension was taken from mass on the corresponding same-day ultrasound. The maximum dimension of ME and NME on MRI were also recorded for both pre- and post-NAC imaging. Tomosynthesis was considered as equivalent to mammography for this study.

Pathology

Receptor subtype and presence of DCIS were recorded from pre-NAC core needle biopsy (CNB). ER and PR were considered positive if ≥1% of cells were stained by immunohistochemistry (IHC). HER2+ status was deemed positive with 2+ or 3+ grading on IHC (with fluorescence in situ hybridization [FISH] confirmation for 2+). Results were categorized as follows: (1) ER+ or PR+, HER2− (HR+/HER2−); (2) ER− and PR−, HER2+ (HR−/HER2+); (3) TNBC; (4) TPBC: ER+ or PR+, HER2+.

From the final surgical pathology, margin status, residual disease cellularity (%), and maximum dimension of the tumor bed of residual invasive disease and DCIS were recorded. A positive margin was defined as ink present on the tumor; the dimension of residual invasive disease was defined as the maximum dimension of the largest focus of disease; and the maximum dimension of the tumor bed was defined as the dimension of residual disease and any intervening fibrosis. All measurements were reported in millimeters. Additionally, location of the calcifications was recorded as being associated with (1) benign tissue; (2) invasive disease; (3) DCIS; or (4) any combination of the aforementioned categories. For tumors where residual disease was present, cellularity was recorded.

Neoadjuvant Chemotherapy (NAC)

The NAC regimen was recorded as the original regimen the patient started on, in the following categories: (1) adriamycin, cyclophosphamide and paclitaxel (AC-T); (2) 5-fluorouracil, epirubicin, cyclophosphamide and docetaxel (FEC-D); and (3) other (e.g., AC with weekly T, Taxotere and carboplatin). The administration of trastuzumab for HER2+ patients and treatment completion were recorded as binary (yes/no).

Data Analysis

Demographic variables were reported as proportions. Mammographic and MRI dimensions were all recorded in millimeters and correlated with final pathology using absolute percentage differences ([maximum dimension on final pathology − post-NAC imaging maximum dimension] / post-NAC imaging maximum dimension). A negative percentage difference indicated that imaging overestimated pathology. If data points were unavailable, the patient was excluded from the group/subgroup analysis. A Pearson correlation (R2 correlation) was calculated for each comparison. Correlation was considered strong if it was >70%, moderate if it was 25–70%, and weak if it was <25%.17

Results

Demographics

A total of 343 patients were identified in the NAC database, of whom 186 met our inclusion criteria. Reasons for exclusion were as follows: 147 patients for lack of pre-NAC calcifications, 2 patients for inflammatory breast cancer, and 8 patients for lack of appropriate imaging reports.

Patient and tumor characteristics are reported in Table 1. The HR+/HER2− group represented 38% of patients. HR−/HER2+ patients represented 12%, TNBC patients represented 18.3%, and TPBC patients represented 32%. Three patients had bilateral breast cancers (each breast was considered separately), one of whom was a male patient. Most cancers were of the ductal subtype, and there were 14 (8%) patients who were lobular, mixed ductal/lobular, or non-specified.

Table 1 Patient and tumor characteristics
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Correlation of Post-NAC Calcifications with Maximum Dimension of Invasive Disease

The maximum dimension of calcifications on post-NAC mammography had a weak correlation with the maximum dimension of invasive disease on pathology, as demonstrated in Fig. 1. By weighted percentage difference, the dimension of calcifications overestimated the dimension of invasive disease by 57%.

Fig. 1
figure 1

Correlation of maximum diameter of mammographic calcifications post-NAC with the maximum diameter of invasive disease on pathology. NAC neoadjuvant chemotherapy

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When analyzed by receptor subtype, TNBCs (n = 9) were the only group that demonstrated a strong correlation (83%) between calcifications and final dimension of invasive disease. Microscopically, most calcifications in this subgroup (n = 6) were associated with a combination of benign tissue, invasive disease, or DCIS on final pathology. In this group, calcifications overestimated the final dimension of invasive disease by 41%. Overestimation of disease on final pathology was more pronounced in HR+/HER2− patients (70%) and HR−/HER2+ patients (86%) [Table 2].

Table 2 Summary of correlation analyses by subgroup for mammographic calcifications with invasive disease, tumor bed, and in-situ disease
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When divided by morphology of calcifications, the correlation was weak with all three pathology parameters. When analyzed by subgroup for chemotherapy type, there were too few patients per group to perform an analysis (n ≤ 5).

Correlation of Post-NAC Calcifications with Maximum Dimension of Tumor Bed and Ductal Carcinoma in Situ

The dimension of calcifications on post-NAC mammography correlated moderately (50%) with the dimension of the tumor bed (Fig. 2). On average, the calcifications overestimated the tumor bed dimension by 15%. When divided by subgroup, calcifications in the TPBC (n = 10) and TNBC (n = 7) subgroups demonstrated strong correlation (>70%) with tumor bed dimension (Table 3).

Fig. 2
figure 2

Correlation of maximum diameter of mammographic calcifications post-NAC with the maximum diameter of the tumor bed on pathology. NAC neoadjuvant chemotherapy

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Table 3 Distribution of calcifications on final pathology, divided by receptor subtypes
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The dimension of mammographic calcifications correlated poorly with DCIS (3%) and overestimated the dimension of DCIS by 46%. When divided by subgroup, the TNBC (n = 6) subgroup demonstrated strong correlation (>70%). In this subgroup, calcifications overestimated the DCIS dimension by 18% (Table 3).

Correlation of Mammographic Focal Asymmetry with Final Pathology

The post-NAC mammographic focal asymmetry, if present (n = 51), demonstrated weak correlation with the maximum dimension of invasive disease (R2 = 6%), the tumor bed (R2 = 18%), and DCIS (R2 = 4%). The mammographic asymmetry underestimated residual pathologic disease by 28%, 56%, and 40%, respectively. When subdivided by HR subgroup, correlations between mammographic asymmetry and final pathology (invasive disease, DCIS, and tumor bed) were weak to moderate.

Correlation of Magnetic Resonance Imaging with Final Pathology

Mass Enhancement (ME)

MRI ME correlated weakly with the maximum dimension of invasive disease (Fig. 3), the tumor bed, and DCIS. Respective correlations were 13%, 3%, and 18% (Appendix 2). When divided by tumor receptor subgroups, TNBCs demonstrated the highest correlation with invasive disease (R2 = 38%). The HR−/HER2+ group had the lowest correlation with invasive disease but the highest correlation with DCIS (R2 = 45%). The HR+/HER2− and TPBC groups had weak correlations with both invasive disease and DCIS.

Fig. 3
figure 3

Correlation of maximum diameter of MRI mass enhancement post-NAC with the maximum diameter of invasive disease on pathology. MRI magnetic resonance imaging, NAC neoadjuvant chemotherapy

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Non-ME

MRI NME performed similarly with weak correlations to invasive disease, the tumor bed, and DCIS. Respective R2 values were 10%, 0.0%, and 18% (Appendix 3). MRI NME overestimated the final dimension of invasive disease by 44%, but underestimated the tumor bed by 16%. When divided by tumor receptor subgroups, correlation with invasive disease was highest in TNBC (R2 = 28%) and lowest in the HR+/HER2− group (R2 = 2%). Correlation with DCIS was weak for all subtypes.

Relationship of Calcifications to Residual Disease on Final Pathology

Calcifications were associated with benign tissue alone in 25% of patients. This percentage was highest in HR−/HER2+ patients, at 48%. In patients with residual disease identified on surgical pathology (n = 97), average residual cellularity on surgical pathology was highest in TNBCs, at 28% (range 5–90%). HR+/HER2− and HR−/HER2+ groups had moderate cellularity at 18% (1–70%) and 19% (1–80%), respectively, while the TPBC group had lowest average cellularity at 11% (0.2–75%).

Discussion

We present a retrospective cohort study describing the correlation of mammographic and MRI features with residual pathologic disease in breast cancer patients undergoing NAC, subdivided by receptor subtype. Overall, there was only weak to moderate correlation of the maximum dimension of post-NAC calcification to the maximum dimension of invasive disease, DCIS, and residual tumor bed. These results persisted across all receptor subtypes, although the results were comparatively more robust in TNBCs.

Our demographics align with studies reporting predominantly ductal carcinomas15 and increased association of calcifications with HR+/HER2− tumors compared with other receptor subtypes.9,11 This association has been challenged in other retrospective studies, with some suggestion that HER2 positivity may be increasingly associated with calcifications.11,15

Despite poor overall correlation to invasive disease, DCIS, and tumor bed on surgical pathology, mammographic calcifications consistently overestimated the maximum dimension of invasive and in situ disease. This trend persisted across all receptor subtypes. Similarly, Yim et al. demonstrated overestimation of invasive and in situ disease in 74% of patients (n = 207) undergoing NAC, and suggested greater overestimation in patients with calcifications found outside the focal asymmetry.1 The propensity of mammographic calcifications to overestimate pathologic findings suggests to surgeons that consistently aiming for oncologic resection of all known calcifications will permit excision of residual tumor with a generous margin. Conversely, this also suggests that excision of all visualized calcifications may be excessive, unnecessarily detrimental to cosmesis, and could potentially be avoided.

In the case of a focal asymmetry on mammography, the asymmetry alone (without considering the dimension of the calcifications) should not be used for surgical planning given the high risk of underestimating residual disease. Some studies have suggested that a decrease in the mammographic focal asymmetry of >25% of the original dimension correlates with pCR.9 Our study did not evaluate for this finding.

The correlation of MRI NME and ME to all three pathological measurements was weak to moderate. In the literature, MRI has been shown to be a stronger correlate to pathology compared with mammography.7,18 In contrast to our direct linear correlation of MRI to pathology, the small retrospective studies by Rosen et al. and Yeh et al. examined MRI’s predictive value for pCR and demonstrated MRI to have superior predictive value compared with mammography (correlation of 0.75 and agreement rate of 71%, respectively), but often has low negative predictive values (NPVs).19,20 NPVs indicate the probability of a true negative result. Having a low NPV indicates a high likelihood of false negative imaging, potentially compromising oncologic safety. For MRIs in allcomers, Vriens et al. reported an NPV of 26% for HR+ tumors and 58% for HR− tumors.21

By subtype, in our study, TNBCs had the strongest performance. There was a strong correlation of mammographic calcifications to invasive and in situ disease. MRI ME and NME correlated moderately with invasive disease. The higher accuracy of MRI in TNBCs has been demonstrated in other retrospective reviews,22,23 including the iSPY study.24 In the latter, TNBCs had the lowest number of discrepancies between MRI (using combinations of ME and NME) dimension and final pathology, but 39% of their patients still had discrepancies of ≥2 cm (two-thirds of which were overestimations).24 These studies have included all NAC patients, not just those with microcalcifications. Additionally, not all studies are consistent with this; Vriens et al. and Scheel et al. showed similar correlations across all receptor subtypes.21,23 In TNBCs where correlation is suggested to be strongest, NPV or MRI has been reported in the range of 58–100%.25

Although TNBC pathology in this study and many others has the strongest correlation to radiographic findings, the variability in the existing literature suggests that predictable relationships of imaging and pathology cannot yet be gleaned. Large cohorts of subtype-specific data with consistent data collection are desperately needed to further elucidate these trends and clarify these data, in order for clinicians to better use radiologic data to predict pathologic response and plan optimal operations.

In this study, the dimension of the tumor bed was overestimated by the dimension of calcifications, but underestimated by MRI ME and NME dimensions. Promising results of early feasibility trials for non-operative management would support that the residual tumor bed does not need to be fully excised but should have adequate biopsy samples in order to safely select patients. In patients harboring calcifications, MRI risks underestimating the area required for adequate samples. This should be taken into consideration when selecting patients, and the calcification dimension should be used to supplement the MRI.

On final pathology, 25% of patients harbored calcifications associated with benign changes only. In the literature, similar estimates are variable but have been reported to be as high as 62%9,14,15 or as low as 4.1%.11 HER2 positivity was particularly associated with benignity of calcifications, both in our study and in a previous review by Feliciano et al.

The residual cellularity evaluated in our study was highest in TNBCs, which suggests that TNBCs are biologically complex. Although they are more likely to have congruent imaging and pathology findings, and with a tendency towards good response to NAC compared with other subtypes, the higher average cellularity in those who do not have pCR may support more aggressive surgical management and the need for adjuvant capecitabine.

The findings of this study should be interpreted with caution given its retrospective, single-center design and small sample size, especially notable on the subgroup analyses. The heterogeneity in available imaging for each patient contributed to the small sample size and adds limitation to our analysis. In addition, on imaging evaluation, not all dimensions were considered, nor were all calcifications, only the largest dimension and calcifications in association with the mass biopsied as invasive disease. Similarly, pathology evaluation is challenging post-NAC; it is difficult to account for non-concentric tumor shrinkage, to measure residual tumor bed while accounting for fibrosis, and to ascertain whether calcifications are associated with benign ducts or ducts that have become benign as a result of NAC. Therefore, the correlation of pathology with calcification volume, pattern, and multifocality cannot be ascertained from this analysis. Finally, the importance of lymph node evaluation in managing NAC patients cannot be understated but was not evaluated in this paper.

Upcoming research is examining the value of prediction models, which have had good early outcomes, especially considering the challenges of integrating data with overall poor correlation. Kim et al. demonstrated good correlation of their prediction model using hormone negativity, Ki-67, MRI tumor size, MRI tumor enhancement, and MRI lesion to background enhancement ratio26. Other upcoming factors that may influence imaging use and guidelines include (1) the positive effect of Ki-67 expression on MRI accuracy;27 (2) the increased accuracy of PET/CT in post-NAC patient evaluation;6 and (3) the emergence of abbreviated protocols for MRI with currently understudied accuracy.28

Conclusion

These observations suggest that mammography and MRI are individually poor predictors of pathologic response. Current post-NAC imaging modalities appear to overestimate the extent of residual disease. Surgical excision of all residual microcalcifications may not only maximize oncologic safety but may also represent overtreatment. Ongoing feasibility trials on surgical omission in exceptional responders rely on post-NAC imaging to determine trial candidacy. Our data suggest that potentially eligible patients are likely being excluded, as post-NAC imaging overestimates the extent of residual disease in the majority of individuals receiving NAC.

Further studies are needed to determine the most accurate imaging correlate of residual disease after NAC to plan better breast-conserving surgery, identify candidates for surgical omission, and establish optimal post-NAC imaging guidelines. Current imaging modalities appear most accurate for TNBCs and may be more reliable at identifying exceptional responders for surgical omission in this subtype.