Introduction

The heterogeneity in breast cancer is related to aggressiveness and poor prognosis (1). Tumor aggressiveness can also be assessed based on gene-expression profiling, which establishes a classification of breast tumors in four different molecular subtypes: luminal A; luminal B; HER2-enriched; and triple negative/basal-like (2-4). More aggressive tumor subtypes, like Her2-enriched and triple negative, have a propensity for metastatic disease and thus require effective treatment (5-7). Diffusion-weighted imaging (DWI) measures the random motion of water molecules with diffusivity providing a surrogate marker for tissue cellularity, microstructure and tumor heterogeneity (8, 9). The diffusivity can be quantified by calculation of the apparent diffusion coefficient (ADC) and previous studies show that ADC values are associated with tumor aggressiveness and prognosis (10-13). Tumor heterogeneity on DWI can be qualitatively evaluated with visual assessment, yet it is subjective and may be prone to inter- and intra-observer variability. Histogram analysis, a method with which one can quantitatively evaluate the distribution of different ADC values within a lesion, may be used to characterize tumor heterogeneity and thereby identify more aggressive molecular breast cancer subtypes. In this context, the aim of this study was to investigate if quantitative objective histogram analysis or qualitatively visual assessment of tumor heterogeneity on DWI could differentiate molecular subtypes of invasive breast cancers.

Methods

In this Health Insurance Portability and Accountability Act compliant and Institutional Review Board approved study, we retrospectively selected consecutive patients from January 2011 to January 2013 with invasive ductal carcinoma of the breast who underwent MRI with DWI with ADC mapping at our institution. The exclusion criteria were 1) lesion with less than 1 cm, 2) previous treatment for breast cancer, 3) pathology report unavailable, and 4) poor DWI image quality. A total of 91 patients were included in the study. Two radiologists specializing in breast imaging independently evaluated the MRI studies. In patients with more than 1 lesion, only the largest was evaluated. A region of interest was drawn on ADC maps in consensus on the slice with the largest diameter covering the whole lesion, taking care to avoid biopsy markers. Histogram analyses was then performed and mean, standard deviation, median, 25% quartile, 75% quartile, 10% quartile, 90% quartile, kurtosis and skewness of ADC values were calculated. For the qualitative analysis, breast tumors were first visualized on contrast-enhanced T1-weighted images and then on DWI and classified according to the degree of heterogeneity. Tumor heterogeneity on DWI was graded on a scale from one to four, being 1=homogeneous, 2=mildly heterogeneous, 3=moderately heterogeneous, and 4=highly heterogeneous. Molecular breast cancer subtypes were derived via IHC surrogates. Tumors were classified as luminal A if either ER or PR was positive and HER2 was negative, Luminal B if either ER or PR was positive and HER2 positive, HER2-enriched if ER and PR were negative and HER2 positive and basal-like if ER, PR and HER2 were negative. Mann-Whitney statistical tests were used to compare quantitative and qualitative results among different molecular subtypes.

Results

The ADC values from HER2-enriched tumors were greater than other subtypes, and were statistically significant for ADC 75% and ADC 90% quartiles when compared to luminal tumors (p=0.0180 and 0.0197, respectively) and when compared to triple negative/basal-like (p=0.0429 for both). The histogram analysis quantitative markers of heterogeneity, which included standard deviation, kurtosis and skewness, were not statistically significantly different among molecular tumor subtypes (Figure 1). Qualitative visual assessment of tumor heterogeneity on DWI independently performed by two radiologists demonstrated a greater heterogeneity of more aggressive molecular subtypes, i.e. HER2-enriched and triple negative/basal-like tumors compared to luminal cancers. However, there was also no statistically significant difference among molecular subtypes with respect to heterogeneity classification (Figures 2 and 3).

Discussion

Previous studies have investigated histogram analysis of breast lesions for differentiation of benign from malignant lesions according to heterogeneity (14). Whereas there are differences in heterogeneity between benign and malignant lesions, gradations of tumor heterogeneity within molecular subtypes on DWI have not yet been investigated. Our results are in agreement with a previous study by Park et al. (15) which did not show a significant difference on kurtosis and skewness between less aggressive ductal carcinoma in situ and invasive carcinoma. However, with the advent of high-throughput methods to extract multiple imaging parameters, a more sophisticated assessment of tumor heterogeneity through radiomics analysis of other functional MRI parameters might be possible and further studies are warranted.

Conclusion

In conclusion, neither quantitative objective histogram analysis nor qualitatively visual assessment of tumor heterogeneity on DWI can differentiate molecular subtypes of invasive breast cancers.

Acknowledgements

This study received funding from the NIH/NCI Cancer Center Support Grant (P30CA008748), DOD BCRP W81XWH-09-1-0042 grant, the Breast Cancer Research Foundation and the Susan G. Komen Breast Cancer Foundation.

References

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