Diffusion-weighted MR imaging of locally advanced breast carcinoma: the optimal time window of predicting the early response to neoadjuvant chemotherapy.

BACKGROUND: It is very difficult to predict the early response to NAC only on the basis of change in tumor size. ADC value derived from DWI promises to be a valuable parameter for evaluating the early response to treatment. This study aims to establish the optimal time window of predicting the early response to neoadjuvant chemotherapy (NAC) for different subtypes of locally advanced breast carcinoma using diffusion-weighted imaging (DWI).
METHODS: We conducted an institutional review board-approved prospective clinical study of 142 patients with locally advanced breast carcinoma. All patients underwent conventional MR and DW examinations prior to treatment and after first, second, third, fourth, sixth and eighth cycle of NAC. The response to NAC was classified into a pathologic complete response (pCR) and a non-pCR group. DWI parameters were compared between two groups, and the optimal time window for predicting tumor response was established for each chemotherapy regimen.
RESULTS: For all the genomic subtypes, there were significant differences in baseline ADC value between pCR and non-pCR group (p < 0.05). The time point prior to treatment could be considered as the ideal time point regardless of genomic subtype. In the group that started with taxanes or anthracyclines, for Luminal A or Luminal B subtype, postT1 could be used as the ideal time point during chemotherapy; for Basal-like or HER2-enriched subtype, postT2 as the ideal time point during chemotherapy. In the group that started with taxanes and anthracyclines, for HER2-enriched, Luminal B or Basal-like subtype, postT1 could be used as the ideal time point during chemotherapy; for Luminal A subtype, postT2 as the ideal time point during chemotherapy.
CONCLUSIONS: The time point prior to treatment can be considered as the optimal time point regardless of genomic subtype. For each chemotherapy regimen, the optimal time point during chemotherapy varies across different genomic subtypes.

Background

Neoadjuvant chemotherapy (NAC) has become a standard treatment for locally advanced breast carcinoma. The major clinical benefit of NAC, compared with adjuvant therapy, is the downstaging of large tumor and increased rate of breast-conserving surgery [1, 2]. However, some cases are not sensitive to NAC, and have no significant decrease or even enlarge in tumor size after treatment. Therefore, treatment efficacy should be predicted as early as possible, on which clinicians can tailor the therapeutic strategy and prevent unnecessary treatment, and thus improve the outcome of tumor [3]. At present, the assessment of the size or volume of residual tumor using conventional MRI is an important basis for the prediction of tumor response. But many tumors don’t have a distinct decrease in size until several weeks or months after chemotherapy because of relatively slow tumor shrinkage, therefore, it is very difficult to predict the response to NAC only on the basis of change in tumor size [4].

Diffusion-weighted imaging (DWI) is a functional MRI technique that can reflect the subtle change in extra-cellular water diffusion within the tumor area [5]. Several previous studies [6, 7] have demonstrated that ADC value derived from DWI can be used as a valuable parameter for evaluating the early response to treatment because it is convenient and needn’t require an intravenous injection of an exogenous contrast media [8]. The exploration of the optimal time window of DWI examination is very useful for predicting the response to NAC as early as possible. If ADC value derived from DWI prior to treatment can be used for discriminating pCR and non-pCR group, the baseline time point can be considered as the optimal time point, on which treatment regimen can be adjusted or tailored appropriately before chemotherapy. However, if baseline ADC value does not work, the exploration of the ideal time point during the chemotherapy is also necessary in order to make the best use of DWI in predicting tumor response, especially for the patients who are resistant to NAC. However, there has been no study on the optimal time window for predicting the early response to NAC using DWI.

Methods

In the present study, we aimed to systematically analyze the dynamic change in ADC value before and after chemotherapy initiated in order to establish the optimal time widow for predicting the response to NAC for different subtypes of locally advanced breast carcinoma.

Study design and population

This is a prospective observational clinical single-center study. Our study received approval from institutional ethics committee and written informed consent from all patients. Between January 2013 and April 2016, 155 patients with locally advanced breast carcinoma were recruited to this study.

Inclusion criteria were as follows: ① All patients had histologically proven breast carcinoma. The clinical stage stayed at II or III (requirement for breast conserving surgery). The axillary lymph node metastases were suspected or determined on the basis of imaging studies (ultrasonography and MRI), physical examination or/and fine-needle aspiration biopsy. ② The patients would complete a full course of NAC and subsequently undergo breast-conserving surgery. ③ There was no contraindication to MR examination. ④ The patients were younger than 70 years old.

Exclusion criteria consisted of ① unable to complete the full course of NAC, ② failure to complete all the follow-up MR examinations on schedule, ③ existence of obvious artifacts on DW images.

Neoadjuvant chemotherapy and classification for response to treatment

All the patients were treated with 4 to 8 cycles according to their regimen protocols and physical situation. A flow chart of the study design depicting number of patient and time-points they were measured/examined was seen on the Fig. 1. There were two types of NAC regimens administered in 3-week long cycle as follows: (1) Taxane-based with anthracyclines delivered in four, six or eight cycles. (2) Anthracycline/taxane-based consisting of four+four cycles, where anthracycline treatment and cyclophosphamide were followed by taxanes (CA-T), or vice versa (T-CA) [9, 10]. In some patients with Her2/Neu-positive lesions, trastuzumab was used in combination with NAC regimen (but not concurrently). According to the chemotherapy regimens, the patients were classified into three groups including the group started with taxanes, started with anthracyclines and started with taxanes and anthracyclines..

Fig. 1

A flow chart of the study design depicting number of patient and time-points measured/examined

The tumor size of the lesion before or after treatment was measured and compared by two radiologists in consensus. Tumor size was defined as the largest diameter of the lesion measured with electronic calipers on the largest cross section of the tumor. After surgery, an experienced pathologist blindly assessed all specimen slices. According to Miller and Payne grading system [11], the pathologic response to NAC was classified into five grades as follows: grade 1, there were some changes of individual malignant tumor cells, but no reduction in overall cellularity; grade 2, there was a minor loss of invasive tumor cells(< 30%), but overall cellularity was still high; grade 3: there was a considerable reduction in tumor cells(30%~ 90%); grade 4: there was a marked disappearance of invasive tumor cells(> 90%) such that only small clusters or widely dispersed cells could be detected; grade 5: there was no invasive tumor cell identifiable in the sections from the site of the previous tumor, only a little ductal carcinoma in situ or tumor stroma remained. All the patients were divided into pCR and non-pCR group. Grade 1 to 4 was regarded as non-pCR group, and grade 5 as pCR group.

MR technique and image analysis

MR examination was performed on a 3.0 T scanner (Discovery MR 750, GE Healthcare, USA) with an 8-channel phased-array breast coil. The time points of MR examinations were seen on the Fig. 1. The time points included before chemotherapy (baseline), 21 days (postT1, after 1 cycle), 42 days (postT2, after 2 cycles), 63 days (postT3, after 3 cycles), 84 days (postT4, after 4 cycles), 126 days (postT5, after 6 cycles) and 168 days (postT6, after 8 cycles) after chemotherapy initiated.

Conventional sequences included axial fat-suppression T2WI, axial fat-suppression T1WI and axial multi-phase dynamic contrast-enhanced (DCE) sequence. The parameters for axial fat-suppression T2WI were as follows: TR/TE: 4787/85.0 ms; FOV: 30x30cm; matrix size: 256 × 256; section thickness: 5 mm; inter-slice gap: 1 mm; number of excitation (NEX): 2. The parameters for axial fat-suppression T1WI were as follows: TR/TE: 640/1.7 ms; FOV: 32x32cm; matrix size:256 × 256; section thickness: 5 mm; inter-slice gap: 1 mm; NEX: 1. For DCE-MRI sequence, transverse 3D Vibrant-Flex was scanned before and repeated 8 times after intravenous administration of 0.1 mmol/kg Gd-DTPA (Magnevist; Bayer, Berlin, Germany) at 2 mL/s (followed by a flush of 20-mL saline solution) via a power injector with a 15-s timing delay. Axial diffusion-weighted MR images were acquired using a single shot echo-planar imaging (SS-EPI) sequence. The parameters were as follows: four b values: 0, 300, 600 and 1000s/mm2; TR/ TE: 2400/62 ms; field of view (FOV): 300x250mm; matrix size: 128 × 160; section thickness: 4 mm; inter-slice gap: 1 mm; receiver bandwidth: 250 kHz; parallel imaging (ASSET) factor: 2; scanning duration: 1 min and 58 s.

The original data were transferred to GE AW 4.6 post-processing work station. ADC images were produced automatically with software (MADC Function tools, GE Healthcare, USA). The whole volume of interest (VOI) was isolated using manual segmentation by two radiologists with more than 7 years’ experience on breast MRI diagnosis. T2WI and DCE-MR images were used as the references to determine the extents of lesion on the corresponding ADC maps. The radiologists manually contoured the edge of target lesions slice by slice with the help of DCE-MR images using the segment tool. The VOI encompassed the profiles of mass as much as possible and avoided recognizable necrotic, hemorrhagic and cystic areas. Two radiologists were blinded to each other’s results to allow measurement of inter-observer variability. Eventually, the measurements of all parameters for each VOI were recorded.

Pathologic analysis

After the surgery, sections were cut and stained with hematoxylin and eosin (HE) according to standard histologic protocols. Positivity for the HER-2 protein was evaluated according to the criteria of the Hercep Test. HER-2 membrane staining intensity and pattern were evaluated using the 0 to 3+ scale, and 3+ (uniformly intense membrane staining in at least 30% of tumor cells) was regarded as positive [12]. The percentage of nuclei with immunoreactivity to estrogen receptor(ER), progesterone receptor (PR) and Ki-67 was classified as continuous data from 0 to 100%. ER-positive and PR-positive cases showed staining in at least 10% of the tumor cell nuclei. Ki-67 was defined as low if ≤20% Ki-67 was detected and as high if > 20% Ki-67 was detected [13]. Lesions were classified into four subtypes according to immuno-cytochemical characteristics: Luminal A (ER+ and/or PR+, plus HER2-, and low-expression of Ki-67), Luminal B (ER+ and/or PR+, plus HER2+, and high-expression of Ki-67), HER2-enriched (ER- and PR-, plus HER2+) and Basal-like(ER- /HER2-) [12, 13]. Percentage of stroma was determined according to the criteria established by Mesker et al. [14]. Visual fields were scored only where both stroma and tumor were present and where tumor cells were seen on all the slides of the microscopic image field. Percentage of stroma was classified into stroma-rich (≤50% tumor percentage) and stroma-poor group (> 50% tumor percentage). Three stromal components, including collagen, fibroblasts and lymphocytes, were evaluated. The presence of a central fibrotic focus was defined as a characteristic tumor stroma with scarlike features or a radiating fibrosclerotic core surrounded by invasive carcinoma cells [15]. MVD was determined from the CD34 immunohistochemical-staining slides. A single countable vessel was defined as any positively stained endothelial cell or cell cluster separate from adjacent microvessels or tumor cells. The vessels containing erythrocytes in the lumen were excluded. Five high power fields were counted, and the average was determined [16].

Statistical analysis

Statistical analyses were performed using statistical software (SPSS, version 22.0; IBM Corp., Armonk, NY, USA). Inter-observer agreement on measurement of ADC value was analyzed using Bland-Altman method [17], and the mean difference, standard deviation (SD), 95% limits of agreement and intraclass correlation coefficient (ICC) were calculated. The distribution of DWI parameter was determined using Kolmogorov–Smirnov test. In this study, ADC values and △ADC for the subgroups didn’t fit a normal distribution. Consequently, the multiple comparisons of parameters between pCR and non-pCR group or among different time points were performed using Mann-Whiney U test or Friedman test. A level of p value < 0.05 was regarded as statistically significant.

The correlations between DWI parameters and histological response to neoadjuvant chemotherapy were analyzed using Spearman correlation test. The potency of DWI parameters for discriminating pCR and non-pCR was assessed using a receiver operating characteristic (ROC) analysis. The resulting threshold value was used to calculate the sensitivity and specificity.

Results

Patient characteristics

Of 155 patients, 13 patients were excluded because of no completion of the full course of chemotherapy (n = 2), failure to undergo follow-up MR examinations on schedule (n = 3), image distortions (n = 2), no surgery after NAC (n = 2), surgery before completion of chemotherapy (n = 2), distant metastasis (n = 1), and lack of proper pathological result (n = 1). Eventually, 142 patients were included into this study. According to pathological findings, 40 cases were regarded as pCR, and 102 cases as non-pCR.

The demographic, clinical and pathological characteristics for pCR and non-pCR group were summarized in Table 1. The table showed that there were no differences in mean age, menopausal status, histologic type, cycles of NAC and surgery method between pCR and non-pCR group(p < 0.05). There was a higher percentage of tumors staged at III in non-pCR than in pCR group (p = 0.03); there was a difference in constituent ratio of genomic subtype between pCR and non-pCR group (p = 0.04). Luminal A subtype had the lowest pCR rate (20%, 5/25), and Basal-like subtype had the highest pCR rate (32.5%, 13/40).

Table 1

The demographic and pathological characteristics for non-pCR and pCR group

Variables	pCR (n = 40)	non-pCR (n = 102)	P-value
Mean age (yrs)	47.3 ± 11.0	43.3 ± 10.0	0.10
Menopausal status			0.09
Premenopausal	23(57.5%)	58(56.9%)
Postmenopausal	17(42.5%)	44(43.1%)
Histologic type			0.06
IDC	29(72.5%)	80(78.4%)
ILC	11(27.5%)	22(21.6%)
Clinical stage			0.03
IIa	8(20.0%)	9(8.8%)
IIb	11(27.5%)	10(9.8%)
IIIa	12(30.0%)	30(29.4%)
IIIb	4(10.0%)	27(26.5%)
IIIc	5(12.5%)	26(25.5%)
Axillary lymph node metastases			0.07
yes	17(42.5%)	32(31.4%)
no	23(57.5%)	70(68.6%)
Cycles of NAC			0.06
4 cycles	8(20.0%)	16(15.7%)
6 cycles	23(57.5%)	59(57.8%)
8 cycles	9(22.5%)	27(26.5%)
Surgery			0.07
Breast-conserving surgery	15(37.5%)	36(35.3%)
Modified radical mastectomy	25(62.5%)	66(64.7%)
Genomic subtype			0.04
Luminal A	5(12.5%)	20(29.4%)
Luminal B	14(35.0%)	30(33.3%)
Basal-like	13(32.5%)	27(15.7%)
HER2-enriched	8(20.0%)	25(23.5%)

Note: pCR: pathologic complete response; IDC: invasive ductal carcinoma; ILC: invasive lobular carcinoma; NAC: neoadjuvant chemotherapy

Inter-observer agreement on ADC measurement

The mean value, standard deviation (SD), mean difference, 95% limits of agreement for measurements and intra-class correlation (ICC) were summarized in Table 2. The statistical analyses showed that a good agreement between two observers was obtained in terms of ADC measurement at each time point.

Table 2

Inter-observer agreements on ADC measurement

ADC value (10−3 mm2/s)	ADC value (10−3 mm2/s)	Mean Difference	95% limits of agreement	ICC†
Base-line
Observer 1	0.9021 ± 0.42	0.0151	−1.512, 1.474	0.9925–0.9974
Observer 2	0.9172 ± 0.37
PostT1
Observer 1	1.0784 ± 0.52	0.0113	−1.1203, 1.308	0.9782–0.9824
Observer 2	1.0671 ± 0.53
PostT2
Observer 1	1.2607 ± 0.67	0.0157	−1.1214, 1.312	0.9897–0.9568
Observer 2	1.2764 ± 0.70
PostT3
Observer 1	1.3645 ± 0.71	0.0104	−1.1124,1.212	0.9901–0.9969
Observer 2	1.3745 ± 0.72
PostT4
Observer 1	1.4753 ± 0.74	0.0036	−1.1313, 1.239	0.9912–0.9981
Observer 2	1.4789 ± 0.79
PostT5
Observer 1	1.5286 ± 0.88	0.0087	−1.1412,1.3129	0.9899–0.9965
Observer 2	1.5373 ± 0.94
PostT6
Observer 1	1.5541 ± 0.89	0.0112	−1.1423,1.3278	0.9798–0.9908
Observer 2	1.5653 ± 0.90

Note: Data are mean ± standard deviations; †: ICC = intra-class correlation coefficient

Baseline measurement

For all the genomic subtype, there were significant differences in ADC value between pCR and non-pCR group(p < 0.05). For Luminal A, Luminal B, Basal-like and HER2-enriched subtype, the areas under the curves (AUCs) of ROC for baseline ADC value in discriminating pCR and non-pCR were 0.556, 0.538, 0.534 and 0.601 respectively.

Measurement during chemotherapy for the group started with taxanes

The differences in ADC values between pCR and non-pCR group were found only at the minority of time points (p < 0.05), while the differences in △ADC between two groups were found at the majority of time points (p < 0.05). For Luminal A or Luminal B subtype, the difference in △ADC between pCR and non-pCR group achieved significance as early as postT1, and the correlation between △ADC and treatment efficacy achieved the highest level at postT1(Spearman coefficient:0.679, 0.618). For Basal-like or HER2-enriched subtype, the difference in △ADC between pCR and non-pCR group achieved significance as early as postT2, and the correlation between △ADC and treatment efficacy achieved the highest level at postT2 (Spearman coefficient:0.647, 0.629)(Table 3).

Table 3

The correlations between ADC/△ADC value and final tumor response to NAC started with taxanes

Items	Luminal A	Luminal B	HER2-enriched	Basal-like
The time point when there was a significant correlation between ADC and tumor response	baseline(−0.324)	baseline(r = − 0.346)	baseline(− 0.324)	baseline(− 0.378)
	postT1(0.348)	postT1(r = 0.396)	postT2(0.431)	postT2(0.397)
	postT4(0.357)	postT3(r = 0.323)	postT5(0.368)	postT4(0.334)
	postT6(0.334)	postT6(r = 0.335)	postT6(0.412)	postT6(0.345)
The time point when there was a significant correlation between △ADC and tumor response	postT1(0.679)	postT1(0.618)	postT2(0.629)	posT2(0.647)
	postT2(0.548)	postT2(0.478)	postT3(0.545)	postT3(0.521)
	postT3(0.538)	postT4(0.556)	postT4(0.526)	postT4(0.506)
	postT4(0.593)	postT5(0.538)	postT5(0.534)	postT5(0.547)
	postT5(0.556)	postT6(0.512)	postT6(0.498)	postT6(0.456)

Note: The data in the parentheses are presented as Spearman coefficient

Measurement during chemotherapy for the group started with anthracyclines

The differences in ADC values between pCR and non-pCR group were found only at the minority of time points (p < 0.05), while the differences in △ADC between two groups were found at the majority of time points (p < 0.05). For Luminal A or Luminal B subtype, the difference in △ADC between pCR and non-pCR group achieved significance as early as postT1, and the correlation between △ADC and treatment efficacy achieved the highest level at postT1(Spearman coefficient:0.647, 0.578). For Basal-like or HER2-enriched subtype, the difference in △ADC between pCR and non-pCR group achieved significance as early as postT2, and the correlation between △ADC and treatment efficacy achieved the highest level at postT2 (Spearman coefficient: 0.637, 0.646)(Table 4).

Table 4

The correlations between ADC/△ADC value and final tumor response to NAC started with anthracyclines

Items	Luminal A	Luminal B	HER2-enriched	Basal-like
The time point when there was a significant correlation between ADC and tumor response	baseline(−0.326)	baseline(r = −0.348)	baseline(− 0.332)	baseline(− 0.313)
	postT1(0.357)	postT1(r = 0.391)	postT2(0.423)	postT2(0.358)
	postT4(0.335)	postT3(r = 0.347)	postT5(0.368)	postT4(0.347)
	postT6(0.329)	postT6(r = 0.348)	postT6(0.389)	postT6(0.349)
The time point when there was a significant correlation between △ADC and tumor response	postT1(0.647)	postT1(0.578)	postT2(0.646)	posT2(0.637)
	postT2(0.526)	postT2(0.487)	postT3(0.543)	postT3(0.549)
	postT3(0.538)	postT4(0.532)	postT4(0.527)	postT4(0.536)
	postT4(0.587)	postT5(0.522)	postT5(0.524)	postT5(0.546)
	postT5(0.554)	postT6(0.487)	postT6(0.498)	postT6(0.495)

Note: The data in the parentheses are presented as Spearman coefficient

Measurement during chemotherapy for the group started with taxanes and anthracyclines

The differences in ADC values between pCR and non-pCR group were found only at the minority of time points (p < 0.05), while the differences in △ADC between two groups were found at the majority of time points (p < 0.05). For Luminal B, HER2-enriched or Basal-like subtype, the difference in △ADC between pCR and non-pCR group achieved significance as early as postT1, and the correlation between △ADC and treatment efficacy achieved the highest level at postT1(Spearman coefficient:0.667, 0.628, 0.609). For Luminal A subtype, the difference in △ADC between pCR and non-pCR group achieved significance as early as postT2, and the correlation between △ADC and treatment efficacy achieved the highest level at postT2 (Spearman coefficient: 0.656)(Table 5).

Table 5

The correlations between ADC/△ADC value and final tumor response to NAC started with taxanes and anthracyclines

Items	Luminal A	Luminal B	HER2-enriched	Basal-like
The time point when there was a significant correlation between ADC and tumor response	baseline(−0.326)	baseline(r = −0.367)	baseline(−0.368)	baseline(− 0.349)
	postT1(0.358)	postT1(r = 0.387)	postT2(0.425)	postT2(0.393)
	postT4(0.351)	postT3(r = 0.345)	postT5(0.398)	postT4(0.326)
	postT6(0.329)	postT6(r = 0.331)	postT6(0.367)	postT6(0.319)
The time point when there was a significant correlation between △ADC and tumor response	postT2(0.656)	postT1(0.667)	postT1(0.628)	posT1(0.609)
	postT3(0.556)	postT2(0.469)	postT3(0.541)	postT3(0.546)
	postT4(0.539)	postT4(0.534)	postT4(0.529)	postT4(0.529)
	postT5(0.587)	postT5(0.529)	postT5(0.518)	postT5(0.538)
	postT6(0.531)	postT6(0.486)	postT6(0.492)	postT6(0.476)

Note: The data in the parentheses are presented as Spearman coefficient

The optimal time window of predicting response to chemotherapy

According to the results mentioned above, baseline time point could be considered as the optimal time point regardless of genomic subtype. In the group that started with started with taxanes or anthracyclines, for Luminal A or Luminal B subtype, postT1 could be used as the ideal time point during chemotherapy(Fig. 2). For Basal-like or HER2-enriched subtype, postT2 as the ideal time point during chemotherapy (Fig. 3). In the group that started with taxanes and anthracyclines, for HER2, Luminal B or Basal-like subtype, postT1 could be used as the ideal time point during the chemotherapy; for Luminal A subtype, postT2 as the ideal time point during the chemotherapy.

Fig. 2

DCE-MR images of a patient who suffered from breast carcinoma and received the chemotherapy started with taxanes, DWI and ADC images in pCR group. Red color represent high ADC value, green color represent mediate ADC value, and blue color represent low ADC value. a, d DCE-MR images at baseline and postT1, There was an irregular mass in the left breast and confirmed to be breast carcinoma with Luminal A subtype. After one cycle of NAC, the tumor didnʼt have no significant decrease in diameter. b, e DW images at baseline and postT1, The images showed how the whole volume of interest (VOI) was placed within the tumor area manually. c, f ADC maps at baseline and postT1, ADC values were 0.8162 × 10− 3 mm2/s at baseline and 1.4756 × 10− 3 mm2/s at postT1, ADC value varied significantly as early as postT1

Fig. 3

DCE-MR images of a patient who suffered from breast carcinoma and received the chemotherapy started with anthracyclines, DWI and ADC images in non-pCR group. Red color represent high ADC value, green color represent mediate ADC value, and blue color represent low ADC value. a, d DCE-MR images at baseline and postT2, There was an irregular mass in the right breast and confirmed to be breast carcinoma with Basal-like subtype. After two cycles of NAC, the tumor size didnʼt have no significant decrease in diameter. b, e DW images at baseline and postT2, The images showed how the whole volume of interest (VOI) was placed within the tumor area manually. c, f ADC maps at baseline and postT2, ADC values were 0.9345 × 10− 3 mm2/s at baseline and 1.3320 × 10− 3 mm2/s at postT2, ADC value varied significantly as early as postT2

The prediction performance of imaging parameters during chemotherapy

For all the subtypes, the AUC of ROC for △ADC from baseline to the ideal time point during chemotherapy was higher than that of ADC value at each time point(p < 0.05)(Table 6). In the group that started with taxanes, the highest AUC of ROC for △ADC (=0.865) was seen in Luminal B subtype, the cut-off value was 0.5746 × 10− 3 mm2/s, which yielded a sensitivity of 89.4% and a specificity of 83.4%. In the group that started with anthracyclines, the highest AUC of ROC for △ADC (=0.845) was seen in Luminal A subtype, and the cut-off value was 0.5589 × 10− 3 mm2/s, which yielded a sensitivity of 87.3% and a specificity of 73.4%. In the group that started with anthracyclines and taxanes, the highest AUC of ROC for △ADC (=0.879) was seen in Basal-like subtype, and the cut-off value was 0.5854 × 10− 3 mm2/s, which yielded a sensitivity of 89.9% and a specificity of 82.6%.

Table 6

Area under the curve from the ROC analysis of pCR prediction using different MRI measures

AUC	ADC at baseline	ADC at the ideal time point during chemotherapy	△ADC from baseline to the ideal time point during chemotherapy
Started with taxanes
Luminal A	0.556(0.513~ 0.612)	0.598(0.546~ 0.636)	0.678(0.598~ 0.749)
Luminal B	0.558(0.525~ 0.623)	0.602(0.567~ 0.645)	0.865(0.748~ 0.930)
Basal-like	0.543(0.503~ 0.605)	0.589(0.558~ 0.649)	0.723(0.614~ 0.843)
HER2-enriched	0.537(0.521~ 0.619)	0.593(0.549~ 0.638)	0.745(0.678~ 0.832)
Started with anthracyclines
Luminal A	0.545(0.521~ 0.598)	0.587(0.529~ 0.620)	0.845(0.769~ 0.920)
Luminal B	0.598(0.534~ 0.628)	0.612(0.528~ 0.656)	0.723(0.678~ 0.789)
Basal-like	0.612(0.567~ 0.654)	0.621(0.557~ 0.678)	0.756(0.698~ 0.845)
HER2-enriched	0.578(0.543~ 0.626)	0.614(0.551~ 0.636)	0.734(0.658~ 0.798)
Started with anthracyclines and taxanes
Luminal A	0.534(0.509~ 0.589)	0.567(0.546~ 0.600)	0.738(0.645~ 0.798)
Luminal B	0.545(0.502~ 0.620)	0.587(0.538~ 0.621)	0.756(0.655~ 0.809)
Basal-like	0.578(0.527~ 0.600)	0.602(0.567~ 0.629)	0.879(0.789~ 0.923)
HER2-enriched	0.602(0.526~ 0.645)	0.623(0.569~ 0.667)	0.783(0.698~ 0.823)

Note: The data in the parentheses were 95% confidence intervals

Comparison of pathologic/histologic characteristics of tumor between pCR and non-pCR group

The pathologic/histologic characteristics of tumor for pCR and non-pCR group were summarized in Table 7. Microvessel density (MVD) was higher in pCR than in non-pCR group (p = 0.04). There was a higher percentage of stroma-poor tumors in pCR than in non-pCR group (p = 0.03). There was no difference in dominant cell type between two groups (p = 0.07). There was a higher percentage of central fibrosis in non-pCR than in pCR group (p = 0.04).

Table 7

The difference in pathological/histological characteristics between pCR and non-pCR group

Parameters	pCR	Non-pCR	P-value
Microvessel densitya	35.1 ± 6.67	22.6 ± 6.14	0.04
Percentage of stroma(n)			0.03
Stroma-rich	13(32.5%)	60(58.8%)
Stroma-poor	27(67.5%)	42(41.2%)
Dominant cell type(n)			0.07
Fibroblast	13(32.5%)	40(39.2%)
Collegan	20(50.0%)	42(41.2%)
Lymphocyte	7(17.5%)	20(19.6%)
Central fibrosis(n)			0.04
Absent	25(62.5%)	40(39.2%)
Present	15(37.5%)	62(60.8%)

Note: aData are mean values±standard deviations; pCR: pathologic complete response

Discussion

To our best knowledge, this is the first clinical study on exploration of the optimal time window of predicting the response to NAC for locally advanced breast carcinoma in light of DWI, which provides an important guidance for the appropriate adjustment of treatment regimens as early as possible in those patients who don’t have a satisfactory response to chemotherapy.

The main advantage of DWI is that this technique can be used to quantitatively measure extra-cellular water diffusion within the tumor area, on which the dynamic change of tumor micro-environment is monitored. However, DWI has some technical limitations, such as ghosting, insufficient fat suppression and insufficient signal noise ratio (SNR) [18]. In order to improve the imaging quality or maximally reduce the imaging artifact, the following measures were taken. First, all the patients were instructed to keep gentle breathing during MR examination. Second, 3.0 T MR scanner was used in order to improve SNR of images, while 3.0 T MR scanner also has some disadvantages, such as increased magnetic susceptibility artifact and eddy current related distortion [19]. Therefore, we used the narrow FOV and volume homogenization block matching the size of the unilateral breast in order to improve the homogeneity of the local magnetic field. Rosenkrantz et al. [19] demonstrated that ADC reproducibility was moderate at both 1.5 T and 3.0 T, and there was no significant difference in measurement of ADC value between 1.5 T and 3.0 T. Therefore, field strength may have no significant influence on the quantitative measurement of ADC value.

There exists a controversy on the value of baseline ADC in predicting the response to NAC for breast carcinoma. Some studies [20, 21] didn’t find a significant correlation between baseline ADC and pathologic response to chemotherapy, while other studies [22, 23] suggested that breast cancer with lower baseline ADC value had better treatment efficacy. This study showed that, for all the subtypes, baseline ADC value was significantly lower in pCR than in non-pCR group. According to several previous studies [24-26], the possible explanation is that, for non-PCR group, there are more necrosis and greater destruction of normal vasculature, which results in higher ADC value because of free diffusion or an increase of diffusing molecules. Because that the differences in baseline ADC value between pCR and non-pCR group were found for all the subtypes, the time point prior to treatment may be considered as the ideal time point of DWI examination, which allows the clinician to predict the response to NAC before treatment and thus to adjust the regimens appropriately as early as possible. However, according to ROC analysis, the predicting performance of baseline ADC value is greatly lower than that of ADC or △ADC during the chemotherapy for each genomic subtype. Therefore, it is not adequate to evaluate tumor response only on the basis of baseline ADC value.

This study showed that, compared with ADC, △ADC was a more sensitive parameter for predicting the response to NAC. According to our results, the differences in ADC value between two groups were found only at the minority of time points, while the difference in △ADC between two groups were found at the majority of time points. Similarly, Iwasa et al. [6] investigated the feasibility of DWI in evaluating the early response to NAC for breast carcinoma, and found that ADC value didn’t correlate with response rate, but △ADC had a significant correlation with response rate. Therefore, △ADC is more valuable and accurate in predicting the early response to NAC. More specifically, a significant △ADC indicates the chemo-sensitivity, while a minute △ADC indicates a less satisfactory response or even no response.

In order to avoid the influence on the ADC measurement by different chemotherapy regimens, the optimal time window of DWI examination during the chemotherapy was explored for each chemotherapy regimen. This study found that the optimal time window for the prediction of tumor response varied across different subtypes for every chemotherapy regimen. For example, in the group that started with taxanes or anthracyclines, for Luminal A or Luminal B subtype, postT1 could be used as the ideal time point during chemotherapy. For Basal-like or HER2-enriched subtype, postT2 could be considered as the ideal time point during chemotherapy. The differences in the optimal time window across four genomic subtypes might be due to high heterogeneity of breast carcinoma. For example, successful chemotherapy causes cytotoxic tumor cell death, which results in a decrease in the proportion of immature microvessel density, but the degree of decrease in microvascular structures varies across different subtypes [27-29]. Bedair et al. [30] compared the potency of ADC value at different time points in predicting the early response to NAC for breast cancer, and found that the difference in percentage increase of ADC value between responders and non-responders achieved a significance after 3 cycles of chemotherapy. Bedair et al. selected only three time points (before start of chemotherapy, after completion of three cycles and at the end of chemotherapy), while our study selected seven time points before, during and after chemotherapy. Consequently, we believe that the optimal time window established by us is more reasonable and accurate.

O’Flynn et al. [31] investigated the value of △ADC in predicting pathologic response to chemotherapy for all the subtypes, and found that the area under ROC curve of △ADC from pre-treatment to after two cycles of chemotherapy for predicting responders was 0.69, which is lower than the result acquired by us. For example, in the group that started with taxanes, for Luminal B subtype, the AUC of ROC for △ADC in discriminating pCR and non-pCR was 0.865. The discrepancy between two studies may be due to the difference in patient constitution, MR device, method of imaging analysis or selection of time points. Especially, the selection of different time points may be the most important reason. It was worthwhile to note that the sensitivity and specificity at the cut-off value for each subtype were not so excellent, therefore, it is still a challenge that these cut-off values are used to discriminate pCR from non-pCR in clinical settings. Nevertheless, the results acquired by us provide useful information on the prediction of pathologic response to NAC, on which individual chemotherapy regimens can be adjusted or optimized more rapidly, and allows patients to receive the most appropriate treatment.

To better understand the value of DWI parameters in predicting the response to NAC, we also investigated the value of some pathologic characteristics in predicting the response to NAC. This study showed that MVD, tumor stroma ratio and central fibrosis were important factors for predicting the response to NAC. The tumors that had high MVD, tumor stroma ratio or little central fibrosis were prone to be sensitive to NAC, which was similar to several previous studies [32, 33]. However, it is worthwhile to note that these histologic/pathologic characteristics of tumor are obtained after surgery following the completion of NAC, and not enough to represent the status of histologic/pathologic characteristics during the course of chemotherapy.

Minarikova et al. [3] explored the predictive value of multiple imaging parameters that acquired from DWI and DCE-MRI at different time points during chemotherapy in breast cancer, and found that the measurement of tumor size served as a better predictor than ADC values. In contrast, ADC can reflect the micro-environment of tumor after chemotherapy, and could be used as an important supplement to mid-therapy diameter changes. Therefore, the combination of multi-parametric MRI can improve the accuracy and reliability of prediction of response to NAC. In the future, it is necessary to make the further study to explore the optimal MRI acquisition and evaluation method, as well as the ideal time point of predicting the response to NAC using DCE-MRI combined with DWI.

There are several limitations to this study. First, not all the patients received MR examination after 4 cycles of NAC, which might result in evaluation bias at the time points after 4 cycles. Second, according to our results, the time point prior to treatment could be used as the ideal time point of DWI examination, but the potency of ADC value at this time point was greatly lower that of ADC value or △ADC during chemotherapy for each subtype, therefore, the ideal time point during chemotherapy was explored, which was an important supplement to the time point prior to treatment. Finally, our study was limited by the small sample size with differing rates of pCR within the tumor subtypes, and further studies with a large number of patients are needed to confirm our preliminary results.

Conclusions

In summary, the time point prior to treatment can be considered as the optimal time point regardless of genomic subtype. For each chemotherapy regimen, the optimal time point during chemotherapy varies across different genomic subtypes during chemotherapy. Compared with ADC value at each time point, △ADC is a more reliable sensitive parameter for predicting tumor response. The results acquired by us provide useful information on the prediction of pathologic response to NAC, which promises to serve as a useful guidance for the adjustment of individual treatment regimens more rapidly.