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Port wine stains is a congenital vascular malformation disease, which occurs mostly on the face and neck. Photodynamic therapy is an effective treatment for port wine stains, which utilizes photodynamic reaction between light, photosensitizer and oxygen in the tissue to produce phototoxic substance singlet oxygen to destroy the lesion. However, the irradiance dose is simply described by the irradiance on a specific plane instead of the irradiance on the lesion with curve shape. The clinician has little knowledge about the effective irradiance value on the lesion, which limits the understanding between the effect and the dosage. To get the effective irradiance on the lesion, a two-steps algorithm is proposed in this paper. Firstly, the point cloud data of lesion are constructed, and the normal vector of each point of the point cloud is calculated based on the least square method. Secondly, according to the first and second laws of irradiance, the effective irradiance on the lesion can be obtained when the parameters of the light source are available. Several typical port wine stains lesions are used to verify the difference between the irradiance on the plane and the lesion. The algorithm can not only help the clinician to make the preoperative planning of photodynamic therapy, but also provide a method for the study on the relationship between the effective irradiance dose on the lesions and the treatment effect of photodynamic therapy for port wine stains.
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