The effect of different local temperature increase in normal and tumorous tissue, influenced by red light commonly used in photodynamic therapy, on tumor growth rate was reanalyzed, taking into account the tumor tissue penetration depth of the light used. The rear part of the tumor is definitely not receiving the same light dose, and consequently the same amount of heat energy, as the front part. The effect is taken into account by using a mathematical model of tumor growth for the rear part of the tumor. Positive experimental results of hyperthermia due to the energy deposited by the red light, are more encouraging after such a correction than previously reported.