A new technique called optical frequency domain imaging (OFDI) provides amazing images of tumors (especially their blood vessels) in situ.

3D tumor vasculature with OFDI
“(a) OFDI images of representative control and treated tumors 5 d after initiation of antiangiogenic VEGFR-2. The lymphatic
vascular networks are also presented (blue) for both tumors. (b) Quantification of tumor volume and vascular geometry and
morphology in response to VEGFR-2 blockade.”

Vakoc, B., Lanning, R., Tyrrell, J., Padera, T., Bartlett, L., Stylianopoulos, T., Munn, L., Tearney, G., Fukumura, D., Jain, R., & Bouma, B. (2009). Three-dimensional microscopy of the tumor microenvironment in vivo using optical frequency domain imaging Nature Medicine DOI: 10.1038/nm.1971