Extensive tumor invasion of draining LNs can alter or completely block normal lymphatic fluid passage, leading to changes of lymphatic drainage pathways and altered integrity of the lymphatic vasculature that can aid in the formation of local and secondary metastases. It has been postulated that tumor blockage of lymphatic drainage prevents inflow of lymph fluid into the draining LNs thus resulting in change of normal lymphatic drainage pathways (Leijte et al., 2009
). Although palpation may be an inaccurate technique tostage lymph node involvement in this study, it is the clinical standard as intraoperative palpation of suspicious LNs may reduce false negative rates of sentinel LNs in patients with breast cancer (Choi et al., 2008
). In fact, all suspicious metastatic LNs at 2 and 3 weeks post implantation were swollen and hard. For longitudinal imaging, tissue samples could not be collected at 2 weeks p.i. and hence palpation was the measure utilized. Nonetheless, based upon our technical studies of NIR fluorescence imaging presented herein, in vivo
NIR lymphatic imaging demonstrates transient changes in overall lymphatic function and architecture in response to tumor growth and metastasis. In these studies, it is noteworthy that the lymphatic flow indicated by ICG may not mimic migration of tumor cells to the draining basins, since ICG was intradermally injected into distant sites from the primary tumor (i.e., ~3cm from the tip of the tail in a tail tumor model and the base of the tail in a hindlimb tumor model). Yet it has been reported that all SLNs detected with injection of a mapping agent at the tumor site in melanoma patients were also visible with distant injection (Rettenbacher et al., 2001
). Therefore, functional NIR lymphatic imaging may potentially provide a better prognostic means to determine LN metastasis on the basis of architectural and functional changes. Whether images of either tumor model (i.e., tail and hindlimb tumor models) demonstrate lymphangiogenesis, the normal biological processes of co-opting otherwise non-functional lymphatic vessels in response to edema or elevated interstitial pressures, or a combination of both, requires further longitudinal imaging studies to understand the temporal relationship of key signaling molecules and receptors, cancer status of draining LNs, and alterations in lymphatic function and structure visualized in these feasibility studies.
In other studies from our laboratory (Rasmussen et al., 2009
), we have employed NIR fluorescence imaging to visualize lymphatic function and architecture in normal human subjects as well as aberrant dysfunction and architecture in subjects who have undergone nodal staging surgery and/or suffer from lymphatic disorders. Whether the functional and architectural changes reported herein and in our previously reported human studies occur in the regional lymphatics of patients with metastatic disease remains to be answered in ongoing work. Nonetheless, the animal work presented herein provides preclinical evidence for the use of NIR fluorescence imaging for evaluating the lymphatic response for possible clinical cancer diagnostics as well as for further study to understand the role of the lymphatics in cancer progression and immune responses to tumors.