Figure 1. Measurement of permeability of the tumour vasculature to fluoroscein isothiocyanate (FITC)-albumin using intravital microscopy of tumours growing in dorsal skin flap "window" chambers. A time sequence (0–2 h) of FITC-albumin extravasation in the P22 rat tumour was detected by multiphoton fluorescence imaging, which provides images with no out of focus fluorescence for full quantification. "Patlak" plots of four random vessels are shown. K_I, the plasma clearance constant for albumin was calculated from the gradient of the straight line portion of the graph in ml blood per ml tissue per minute (ml•ml^–1•min^–1). The vascular permeability–surface area product (PS product) is equivalent to K_I for a high molecular weight compound whose uptake into tissue is permeability (rather than flow) limited and whose backflux into the blood is low.