The present study investigated the ability of pretreatment ADC to predict pCR in breast cancer patients undergoing neoadjuvant chemotherapy. The prediction of treatment outcome could be very important for clinical care. In fact, accurate prediction of treatment response using imaging could help to individualize treatment and to avoid ineffective chemotherapy in patients.
A key finding of the present study was the dependence of the molecular subtype for the association between pCR and pretreatment ADC values. In fact, pCR is the best outcome for neoadjuvant chemotherapy in breast cancer patients [18, 19]. As reported previously, it is an important prognostic factor for both disease-free survival and overall survival in patients with breast cancer [18, 19]. So far, patients with pCR of breast cancer have an improved 5-year disease-free survival rate of 87% and a 5-year overall survival rate of 89% in comparison to patients without pCR [20].
Previously, a meta analysis including 13 studies with 575 patients identified that the pooled sensitivity and specificity of MRI in prediction of pCR was 0.88 (95% CI, 0.78; 0.94) and 0.69 (95% CI, 0.51; 0.83), respectively [21]. The included studies used morphological MRI data including T2 weighted and dynamic contrast enhanced (DCE-MRI) images but not DWI. In another meta analysis investigating only studies with DCE-MRI, a pooled sensitivity of 0.80 (95% CI, 0.70, 0.88) and a specificity of 0.84 (95% CI; 0.79, 0.88) was identified [22]. While the sensitivity can be considered as sufficient for clinical routine, there is still lack of specificity. Presumably, the addition of another diagnostic MRI sequence might improve diagnostic results.
The present study provides new data regarding the subtypes as well as multivariable analyses, which were not as clearly stated before. It was shown that ADC values have the highest statistical association with pCR prediction in the HER2 enriched subtype, which might be caused by the different treatment regimes for this subtype including the addition of trastuzumab. However, the exact underlying reasons for this behaviour remain elusive.
There are several reasons why ADC values might be able to predict pCR. Ideally, neoadjuvant chemotherapy reduces tumor cell count completely. Biologically aggressive tumors particularly benefit from neoadjuvant chemotherapy, as tumors are more vulnerable to chemotherapy when in a proliferation state. ADC correlates inversely with cell count and tumor aggressiveness [6, 9, 23]. The direct inverse associations between ADC values and proliferation potential, quantified by Ki-67 index was shown in several analyses, including for breast cancer patients [24,25,26,27].
It has already been shown that the ADC is a valuable imaging parameter to discriminate benign from malignant tumors, as the proposed threshold of 1.0 × 10− 3 mm2/s was identified in a large meta analysis based upon 13,847 lesions [9]. In another recent study, ADC values were also capable of reducing biopsies in BIRADS 4 lesions in up to 32.6% of cases [10].
One key fact is that ADC values increased during/after neoadjuvant chemotherapy as another important discriminating parameter to assess treatment response [14, 27]. Numerous studies confirmed this hypothesis including a large multicenter trial based in North America in a prospective setting [14]. Moreover, increase of ADC values during neoadjuvant chemotherapy was more useful than tumor size or volume change after therapy [28]. As such, most studies utilized the difference between the pretreatment ADC value and the ADC value after treatment to predict pCR.
However, a more important question is whether it is possible to predict the effect of neoadjuvant chemotherapy accurately based on pretreatment values. The reported data using pretreatment ADC are contradictory [13]. While some authors found an association between pretreatment ADC and pCR after neoadjuvant chemotherapy, others did not. Bedair et al. reported that responders had lower pretreatment ADC values (× 10− 3 mm2/s) in comparison to non-responders, namely 0.92 ± 0.02 and 1.20 ± 0.02, respectively (p < 0.001) [29]. Similar results were reported by Liu et al. based upon a large retrospective study with 176 patients [30]. In this study different cut-off values were also proposed in accordance with the molecular subtype. Thus, triple negative cancers had the highest ADC-cut off value with 1.43 × 10− 3 mm2/s, whereas Luminal B had the lowest with 1.33 × 10− 3 mm2/s [30]. One reason for the identified results for treatment response prediction could be seen in these reported inherent differences of ADC values according to the subtypes. Yet, there is definite need for further research in this regard.
However, in the study of Bufi et al. there were no relevant differences of pretreatment ADC values between responders and non-responders: 1.13 ± 0.19 vs 1.09 ± 0.19 (× 10− 3 mm2/s), respectively, concluding that pretreatment ADC values are not a useful imaging parameter to predict treatment response [31]. Of note, in this study, the most patients had the Luminal A subtype with 143 of 225 patients [31].
In short, the results of using pretreatment ADC values to reliably predict treatment response following neoadjuvant chemotherapy are still conflicting. One strength of our present analysis is the multivariable regression analysis to adjust for potential confounders, which was not performed previously. A recent study employed a multivariable regression analysis to predict pCR based upon 50 patients [32]. While clinical stage and T stage had high associations with pCR, for MRI findings, only the ADC value change below 15% after two cycles of chemotherapy was associated with pCR (OR= 9.865, 95%CI 1.024–95.021) [32].
Choi et al. investigated a novel ADC-parameter, called ADCdiff, which is the difference between the maximum and minimum ADC values [33]. With this approach, the ADCdiff was superior to ADCmean, ADCmax and ADCmin to predict pCR in this study based on 49 patients [33].
The present analysis based on a multicenter cohort showed that pretreatment ADC values are an independent parameter associated with pCR but the baseline ADC values of responders to NAC and non-responders overlapped in a relevant manner. This was also shown for the subgroups, which suggests that pretreatment ADC cannot be used as a reliable prognostic surrogate marker for pCR.
Our study used subgroup analyses to test for differences with regard to the immunohistochemical subtype. The correct classification of subtype is crucial due to differences in approach to treatment as well as prognostic implications.
One noteworthy study used 18F-labeled fluorodeoxyglucose positron emission tomography-computed tomography (PET-CT) to predict the response to trastuzumab or pertuzumab in HER2-positive breast cancer [34]. The primary endpoint was however not met and the area under the curve was only 0.72 (80% CI, 0.64 to 0.80) [34]. One can conclude that even FDG-PET-CT is not able to reliably predict treatment response in the HER-2-enriched subtype.
The present results are based upon large multi-center data, which identified distinctive differences of the associations between pretreatment ADC values and treatment response accordingly to the molecular subtype. Further prospective studies are needed to assess the discriminative power of pretreatment ADC values.
Our analysis has some limitations to address. First, the multi-center cohort was acquired in a retrospective manner with possible inherent bias. Second, the image reading was not performed in a centrally reading session. It was performed in each center by experienced radiologists. There might be some bias obtained by the different readers. However, it was shown that ADC is a reliable imaging biomarker with low interreader heterogeneity and high reproducibility [35, 36]. Third, the centers used different MRI scanners and different DWI sequences, which results in heterogeneity of the ADC values. Beyond that, an important point is that the diffusion time of the included DWI sequences differed between the centers, which could not be accounted for. Standardisation must be achieved to establish ADC values as an imaging biomarker into clinical routine. Fourth, pCR was defined using different classification criteria. This might result in bias but it represents daily clinical care, as these classifications are used in the different centers in clinical routine.