Subcutaneous and intravenous delivery of diagnostic agents to the lymphatic system: applications in lymphoscintigraphy and indirect lymphography.

Lymph node status is important in the staging of many malignancies. Although tissue characterization by histologic analysis of biopsy samples may improve staging, noninvasive staging is more acceptable to both patients and clinicians. Several imaging techniques may serve this goal. Modern noninvasive techniques such as computed tomography and magnetic resonance detect lymph node abnormality by nodal enlargement, but that does not always imply malignant involvement. On the other hand, many nodes are infiltrated or replaced by tumour without change in size. This becomes a serious diagnostic defect by these modalities. Consequently, attention has been focused to develop contrast agents and radiolabelled complexes for better cancer detection as well as characterization of individual tumours in lymph nodes. For delivery of such materials to regional lymph nodes one can take advantage from the distinct physiological function of the lymphatic capillaries. The thin-walled and fenestrated lymphatic microvessel is easily penetrated by particulate and macromolecular agents after injection into the extracellular space. Once inside the vessel, materials that are transported with the lymph either specifically target certain nodal elements (e.g. neoplastic cells) or become cleared by macrophages located in the lymph nodes. Indeed, interstitial delivery of diagnostic agents have been of benefit in determining regional spread of cancer and assessing lymphatic function either by lymphoscintigraphy or indirect lymphography. On the other hand, development of contrast materials that can reach lymph nodes after a single intravenous injection is highly desirable because of the large number of lymph nodes in the body and access being difficult to most of them. Today, a number of contrast agents exist that can reach a vast array of lymph nodes in the body, particularly those that are not readily accessible for histologic evaluation, after a single intravascular injection to help distinguish between normal and tumour-bearing nodes or reactive and metastatic nodes with magnetic resonance. In this article we critically examine advantages and limitations of both subcutaneous and intravenous routes of injection for the delivery of diagnostic agents to the lymphatic system.