Abstract
We thank Priola et al. (1) for the interest in our article (2) and for the insightful comments. We appreciate the opportunity to underline that the purpose of our study was to evaluate diffusion-weighted magnetic resonance imaging (DW-MRI) for early treatment response (mean 2 days after initiation of treatment), and not to compare the usefulness of positron emission tomography/computed tomography (PET/CT) with DW-MRI the first days after onset of treatment. PET/CT was performed at a different time compared to DW-MRI in order to avoid contamination by post-therapeutic inflammatory changes, and as such to serve as gold standard for lymphomatous lymph nodes (LLN). As pointed out, PET/CT may, like DW-MRI, not be useful for evaluation of treatment response the first days after onset of treatment. In our study we demonstrated that DW-MRI performed a few days after onset of chemotherapy cannot replace PET/CT performed more than 2 weeks after onset.
Our results showed no significant change in apparent diffusion coefficient (ADC) of the selected LLN from 1 to 4 days after initiation of the first cycle of chemotherapy compared to pretreatment values despite significant decrease in SUVmax indicating clinical response (mean 19 days after first administration of treatment). The selection of individual LLN was based on CT criteria and not on baseline FDG uptake or baseline DW-MRI; however, CT findings of all the selected nodes strongly suggested active disease. A recent study (3) reported that lesions with low baseline ADC values exhibited a significant higher ADC increase at follow-up, and by selecting individual LLN presenting low ADC values at baseline one might speculate that our study might have demonstrated significant change in ADC. In patients with lymphoma it is a challenge to establish whether moderately enlarged lymph nodes are affected. Because most pathological enlarged lymph nodes in daily radiological practice are only moderately enlarged, and this represents a dilemma, we decided to focus on these nodes. In our study mean pretreatment short-axis diameter (SAD) of the selected LLN was 20 mm. Changes in ADC values might correlate with change in tumor volumes in patients with diffuse large B-cell lymphoma (DLBCL) (4), but our present study focused on moderately enlarged lymph nodes and consequently the largest LLN were excluded. If the largest individual LLN had been chosen for measurement, the ADC change might have been significant. A major challenge for DW-MRI is the lack of standardization of acquisition, measurement and analysis (5). Although histogram analysis may increase the predictive value of ADC measurements of malignant tissue intermixed with benign tissue (5) as in Hodgkin’s lymphoma (HL), the method is time-consuming. We wanted to use a method that was simple enough for daily practice and therefore decided to obtain mean ADC of individual LLN on a single slice.