Photodynamic therapy: tumor volume limitation and tumor response for murine fibrosarcoma

The clonogenic survival rates and tumor growths following photodynamic therapy (PDT) were studied as a function of tumor volume for RIF-1 using C3H mice as an animal model. The clonogenic survival rates showed a saturation level approaching a 99.8% survival rate for large volumes (above 1,900 mm3), possibly indicating poor light utilization of 630 nm because of the limited tissue penetration and/or tumor hypoxia. With a small tumor volume [less than 150 mm3], the survival rate (less than 0.3%) showed an exponential increase with volume. The survival rates (less than 0.03%) corresponding to a volume lower than 50 mm3 gave complete tumor eradication. On the other hand, partial response and recurrence were noticed with volumes larger than 90 mm3, which had more than approximately 0.1% survival rates. After PDT, all tumors responded with immediate swelling and skin discoloration, followed by necrosis, tumor delay, and regrowth. Regrowth rate and tumor cell doubling time were slower than the control growths and decreased further following superficial irradiation with a second interstitial illumination. The regrowth rates were slow for small initial tumor volumes and fast for large initial tumor volumes. Also, the delay period was longer for the small initial tumor volumes in comparison to a shorter period for the medium tumor volumes and very low probability of delay for the large volumes.