We developed a method which enabled correlation of in vivo detected lymph nodes from different primary solid malignancies with pathology. The workflow was technically feasible in prostate, rectal, periampullary, and head-and-neck cancer patients. The intermediate step of the 7T ex vivo MRI scan of resected specimens allowed in principle correlation on a nodal level, but at least on a nodal station level between the in vivo MRI images and histology. For each tumor location, different approaches were required to maintain anatomical orientation of the resected specimen and correctly match the results to the pathological evaluation. In esophageal, periampullary, and rectal cancer, lymph nodes were dissected en-bloc with the primary tumor. Therefore, lymph nodes could be matched using corresponding anatomical landmarks to the ex vivo MRI. In head-and-neck and prostate cancer, lymphadenectomy and primary tumor dissection were performed separately. Hence the lymphadenectomy specimens were vacuum sealed in plastic bags in order to maintain orientation and easy positioning of the specimen within the scanner.
Comparison with literature
Few attempts have been made to match individual lymph nodes visible on MRI with their precise histologic matching after lymphadenectomy. For rectal cancer, Park et al proposed a method where ex vivo ultrasound was used to match in vivo MRI with histopathology [9]. The use of identical scanning parameters for both in vivo MRI and ex vivo ultrasound enabled successful matching on a nodal level in 91% of the lymph nodes. However, only lymph nodes >3 mm were depicted with MRI and matched with histopathology. A recent study applied ultrahigh field ex vivo MRI of TME specimens in 22 rectal cancer patients and demonstrated that the mean size of the lymph nodes found on ex vivo MRI was below 3 mm [12]. Luciani et al. [10] studied 1.5T ex vivo MRI for precise node-to-node correlation in female breast cancer patients, but 20% of nodes found with pathology were not depicted with ex vivo MRI. A likely explanation is that the used magnetic field strength was only 1.5T, MR sequences were 2-dimensional and slice thickness was 2-3 mm, easily missing the smaller sized nodes. By comparison in the present study, ex vivo MRI was performed with 3D sequences and a partition/slice thickness of 0.29 mm at a field strength of 7T, most likely improving nodal yield on MRI. Korteweg et al examined healthy axillary lymphoid tissue of two deceased females on a clinical 7T ex vivo MRI system and detected lymph nodes <1 mm, corroborating our results [3]. Both studies [10, 3] proposed a framework for exact matching of radio-pathological findings by pinning the dissected specimen to an MR compatible grid containing an explanatory absciss and ordinate [10] or vertical and horizontal reference lines [3]. For each lymph node, the in-plane position was assessed and correlated to the MR images. Matching was successful in 80% [10] and 88% [3] of the lymph nodes, although this percentage strongly depends on the total amount of identified lymph nodes.
Strengths and limitations
We developed a workflow to match individual lymph nodes detected on in vivo MRI to the resection specimen by incorporating 7T ex vivo MRI to the protocol. This method can be used in studies validating MRI, such as USPIO-enhanced MRI, for detection of lymph node metastases on a nodal level with histopathology as gold standard. The method was illustrated in prostate, rectal, periampullary, esophageal, and head-and-neck cancer. The results demonstrate that various approaches enabled maintenance of orientation in both large, small and anatomically complex resection specimens. Therefore, the presented workflow can easily be adapted to other organs. High resolution ex vivo MRI images provide direct information on the location of lymph nodes in resection specimens and is of immediate help to harvest nodes in general, or suspicious nodes in particular. In case of a short time interval between USPIO-enhanced MRI and surgery, maintenance of signal intensity on the ex vivo MR-images represents the absence of USPIO deposits and thus a lymph node suspicious for harboring a metastasis. This functional information can be beneficial for node-to-node correlation. Radiologic differentiation between a lymph node and a blood vessel was formerly potentially difficult. However, using 3D high-resolution MR sequences enabling visualization of nodal structures in the transverse, sagittal and coronal plane, this is no longer an issue.
Some potential limitations should also be mentioned. Node-to-node correlation for all lymph nodes is challenging. Differences in spatial resolution between in and ex vivo exams can lead to differences in detection of lymph nodes, particularly the small ones. Additionally, there could be a difference between the resected volume and a volume defined as resected on in vivo images, which also can lead to differences in the amount of evaluated lymph nodes. Also, the fixation process needed for the pathology processing causes volume change of the specimen, which leads to a change of configuration of the specimen and landmarks. Neck dissection and PLND specimens, for example, are relatively small but harbor many lymph nodes and generally contain only a few anatomical landmarks. Therefore, it is of paramount importance that the anatomical orientation of the specimen is well documented during dissection. In this way a reliable node-to-node correlation can be achieved or in case of clusters of small lymph nodes a per nodal station correlation can be performed. In addition, the ex vivo MRI requires a considerable investment of time which is not always desirable in clinical practice. Likewise, not every institution has an 7T preclinical MR system available for ex vivo measurements. Therefore, the method we developed is particularly suitable for studies validating imaging techniques for nodal assessment.
Clinical implications
In various types of cancer, detection of small lymph node metastases is urgently needed. Both over- and undertreatment in oncological therapeutic regimens result in unnecessary morbidity and mortality. Treatment needs to be tailored on a patient level to overcome this problem. Thus, knowledge regarding merely the presence (N+) or absence (N0) of nodal metastases is not sufficient anymore. Clinicians need to be informed about the number, size and exact location of metastatic deposits which is a prerequisite to target therapies. Individual treatment strategies consist of local tumor excision or ablation without lymphadenectomy in N0 patients, reducing unnecessary morbidity in this group of patients. In N+ cancer patients the exact N-staging can lead to additional treatment options for local lymph node metastases such as surgical resection, ablation, or image guided radiotherapy. Currently, a variety of promising non-invasive diagnostic imaging tests are developed to detect lymph node metastases, e.g. hybrid PET-MRI, USPIO-enhanced MRI and targeted fluorescence imaging [17, 4]. Using our described workflow with 7T ex vivo MRI as an intermediate step to guide histopathological work-up, new techniques can be validated on a node-to-node level with histopathology. The detailed 7T ex vivo MRI of the resection specimen aids in finding the smallest lymph nodes that otherwise might be missed with routine pathological evaluation [12], supporting the validation of in vivo imaging techniques.