Differentiation Between Benign and Metastatic Breast Lymph Nodes Using Apparent Diffusion Coefficients

Abstract

The aim of this study was to determine the range of apparent diffusion coefficient (ADC) values for benign axillary lymph nodes in contrast to malignant axillary lymph nodes, and to define the optimal ADC thresholds for three different ADC parameters (minimum, maximum, and mean ADC) in differentiating between benign and malignant lymph nodes. This retrospective study included consecutive patients who underwent breast MRI from January 2017-December 2020. Two-year follow-up breast imaging or histopathology served as the reference standard for axillary lymph node status. Area under the receiver operating characteristic curve (AUC) values for minimum, maximum, and mean ADC (min ADC, max ADC, and mean ADC) for benign vs malignant axillary lymph nodes were determined using the Wilcoxon rank sum test, and optimal ADC thresholds were determined using Youden's Index. The final study sample consisted of 217 patients (100% female, median age of 52 years (range, 22-81), 110 with benign axillary lymph nodes and 107 with malignant axillary lymph nodes. For benign axillary lymph nodes, ADC values (×10^-3 mm²/s) ranged from 0.522-2.712 for mean ADC, 0.774-3.382 for max ADC, and 0.071-2.409 for min ADC; for malignant axillary lymph nodes, ADC values (×10^-3 mm²/s) ranged from 0.796-1.080 for mean ADC, 1.168-1.592 for max ADC, and 0.351-0.688 for min ADC for malignant axillary lymph nodes. While there was a statistically difference in all ADC parameters (p<0.001) between benign and malignant axillary lymph nodes, boxplots illustrate overlaps in ADC values, with the least overlap occurring with mean ADC, suggesting that this is the most useful ADC parameter for differentiating between benign and malignant axillary lymph nodes. The mean ADC threshold that resulted in the highest diagnostic accuracy for differentiating between benign and malignant lymph nodes was 1.004×10^-3 mm²/s, yielding an accuracy of 75%, sensitivity of 71%, specificity of 79%, positive predictive value of 77%, and negative predictive value of 74%. This mean ADC threshold is lower than the European Society of Breast Imaging (EUSOBI) mean ADC threshold of 1.300×10^-3 mm²/s, therefore suggesting that the EUSOBI threshold which was recently recommended for breast tumors should not be extrapolated to evaluate the axillary lymph nodes.

Keywords: MRI; apparent diffusion coefficient; breast cancer; diffusion-weighted imaging; lymph nodes; prognostic factors.

Figure 1

68-year-old woman presented for high-risk screening breast MRI exam. She had a family history of cancer, and was BRCA1 and ATM positive. Breast MRI shows a benign appearing left axillary level 1 lymph node: (A) ADC, (B) T1-weighted non-fat saturated, and (C) T2-weighted fat saturated axial sequences.

Figure 2

49-year-old woman underwent routine screening breast MRI exam. She received a dose of the COVID vaccine in the right arm a few months prior to her breast MRI. Enlarged right axillary lymph nodes were identified on breast MRI. (A) T2-weighted fat saturated image, (B) T1-weighted fat saturated post-contrast image, and (C) ADC. She subsequently underwent diagnostic ultrasound and ultrasound-guided fine needle aspiration (D, E), which yielded benign results.

Figure 3

64-year-old woman with right breast 12:00 axis triple negative invasive ductal carcinoma and right axillary lymphadenopathy as seen on (A) ADC, (B) T1-weighted fat saturated post-contrast image. (C) Targeted ultrasound shows borderline cortical thickening of the right axillary lymph node. (D) Ultrasound-guided fine needle aspiration confirmed metastatic adenopathy. Note the difference in ADC values between primary breast and right axillary adenopathy, e.g., mean ADC 0.739 versus 1.111.

Figure 4

Boxplots for (A) mean ADC, (B) maximum ADC, and (C) minimum ADC for the differentiation between benign and malignant axillary lymph nodes.

Figure 5

Receiver operating characteristic curve for (A) mean ADC, (B) maximum ADC, and (C) minimum ADC for the differentiation between benign and malignant axillary lymph nodes.