Photodynamic therapy (PDT) is currently being developed as a new therapeutic modality with minimal side effects in the treatment of many different types of cancers [1]. PDT requires three essential components, which includes a photosensitizing agent, activation light, and molecular oxygen [2, 3]. Light is one of the governing components for PDT, and the only component that can be controlled externally [4]. The understanding of light penetration in a tumor is critical for an efficient PDT protocol. A well-designed PDT protocol provides minimal microvascular damage and efficient killing of malignant tumor cells [5–7]. The depth of light penetration in a tumor depends on the microvascular nature of the tumor, amount of solid and liquid contents present, and the wavelength of the light source. The first two factors depend on the nature of the tumor tissue and therefore the optimal light dose is not standard for all tumors. Furthermore the liquid content of a tumor changes with the tumor temperature due to vasodilatation, and this in turn affects the depth of light penetration.

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