RC 816 - Evaluating lymph node involvement: an impossible task?
1. To understand diagnostic imaging difficulties in the evaluation of nodal involvement.
2. To understand the complementary information obtained with CT, MRI and PET.
3. To learn about advanced imaging techniques (CT - dual-energy; MRI - DWI, and PET) for evaluating nodal involvement.
4. To recognise pitfalls in evaluating nodal involvement using CT, MRI and PET.
1. To understand the role of local nodal staging and its importance for management and prognosis.
2. To become familiar with the current imaging criteria for assessment of nodal metastases.
3. To understand the diagnostic performance of cross-sectional imaging.
Lymph nodes are often the first site of metastasis in many of the common cancers. Presence or absence of regional lymph node metastasis is a critical component of the UICC/AJCC TNM classification, now in its 8th edition. Nodal(N) staging has therapeutic significance in a number of cancers in determining operable vs inoperable disease, and use of neo-adjuvant or adjuvant therapy. For most malignancies, CT and MRI (and ultrasound) remain the main imaging modalities for nodal staging. Differentiation of metastatic lymph nodes from normal or benign reactive nodes relies on various parameters afforded by CT/MRI; size is most widely used, and the only parameter used for nodal classification in the RECIST v1.1 criteria. In RECIST, 10mm is used as cutoff for non-pathological nodes, though in the chest and abdomen location-specific threshold sizes have been described. However, using size criteria alone has limited diagnostic accuracy when compared with the reference standard of histology; in rectal cancer, there is significant overlap in the sizes of benign and malignant nodes. Thus, various other imaging parameters, including attenuation, signal, homogeneity, border regularity, location, number, clustering and contrast enhancement, are also used for nodal classification in routine clinical practice. While each feature individually offers relatively poor diagnostic accuracy, with possible pitfalls such as necrotic nodes in infection, combinations of multiple imaging parameters can lead to improved sensitivity and specificity for diagnosing malignant nodes. Techniques including computer-aided algorithms have been explored to provide more quantitative nodal assessment. Nevertheless, nodal staging on CT and MRI remains a challenge.
1. To understand the principle of DWI of nodes.
2. To learn about the appearances of malignant nodes on diffusion-weighted MRI.
3. To become familiar with node-specific enhanced MRI.
Up-to-date lymph node staging is based on size and shape criteria only; however, micrometastases can also be present in normal-sized lymph nodes and nodes can be enlarged due to inflammatory changes. New contrast agents in MRI such as ultrasmall particles of iron oxide (USPIO) have substantially improved the diagnostic accuracy of lymph node staging. Unfortunately, USPIO are not commercially available on the market. DW-MRI is a noninvasive method that provides tissue microstructural information, and several studies mainly in the pelvis have shown promising results for lymph node detection and differentiation between benign and malignant nodes. These studies reported sensitivities of 79-100% and specificities of 74-93% using the underlying ADC-value; lower ADCs were reported in malignant nodes as compared to benign ones. On the other hand, it has been shown that there is a considerable overlap between ADC values of benign and malignant nodes. A recent prospective study in 120 patients with bladder and prostate cancer and normal-sized pelvic LNs on conventional cross-sectional imaging compared DW-MRI to histopathology based on extended pelvic lymph node dissection. It has been shown that the combination of DW-MRI findings and meticulous analysis of morphological findings allowed to detect malignant lymph nodes even in normal-sized nodes. The combination of USPIO with DW-MRI might further facilitate and improve lymph node staging in the future, provided that USPIOs will become available for clinical use. Initial promising results on the use of ferumoxytol in this context have already been publishes recently.
1. To learn the typical appearance on nodal metastatic disease on FDG.
2. To recognise the pitfalls for interpretation.
3. To become familiar with new radiotracers, including choline and PSMA PET, for the demonstration of nodal disease.
PET/CT imaging using 18-fluoro-deoxyglucose (FDG), a glucose analogue, has an established role in oncology for the staging and response assessment of a variety of tumours. For the assessment of nodal involvement visual analysis and semi-quantitative SUV analysis are utilised. However, there are no reliable absolute SUV cutoffs to differentiate between benign and malignant lymph nodes. It is very important to be familiar with the typical patterns of spread of the specific cancer being assessed as this also influences the likelihood of disease involvement. Glucose metabolism is not specific for malignancy and false positives can occur with inflammation, infection and other processes such as a sarcoid-like reaction to malignancy. Further pitfalls are that some well-differentiated malignancies have only low-level glucose metabolism and the limited spatial resolution of PET means involvement of small nodes may be missed or the level of metabolic activity may be underestimated in small nodes. New radiotracers which target more specific pathways such as C-11/ F-18 fluorocholine which target cell membrane metabolism and Ga-68 prostate-specific membrane antigen (PMSA) which targets a cell surface protein are gaining increasing use in prostate cancer imaging and the Ga-68 Dota-peptide tracers for somatostatin receptor imaging of neuroendocrine tumours.