Photodynamic therapy (
PDT) can lead to the creation of heterogeneous, response-limiting
hypoxia during illumination, which may be controlled in part through illumination fluence rate. In the present report we consider (1) regional differences in
hypoxia, vascular response, and cell kill as a function of
tumor depth and (2) the role of fluence rate as a mediator of depth-dependent regional intratumor heterogeneity. Intradermal RIF murine
tumors were treated with
Photofrin PDT using surface illumination at an irradiance of 75 or 38 mW cm(-2). Regional heterogeneity in
tumor response was examined through comparison of effects in the surface vs. base of
tumors, i.e. along a plane parallel to the skin surface and perpendicular to the incident illumination. 75 mW cm(-2)
PDT created significantly greater
hypoxia in
tumor bases relative to their surfaces. Increased
hypoxia in the
tumor base could not be attributed to regional differences in
Photofrin concentration nor effects of fluence rate distribution on photochemical oxygen consumption, but significant depth-dependent heterogeneity in vascular responses and cytotoxic response were detected. At a lower fluence rate of 38 mW cm(-2), no detectable regional differences in
hypoxia or cytotoxic responses were apparent, and heterogeneity in vascular response was significantly less than that during 75 mW cm(-2)
PDT. This research suggests that the benefits of low-fluence-rate
PDT are mediated in part by a reduction in intratumor heterogeneity in hypoxic, vascular and cytotoxic responses.