A new treatment regime is presenter using a low concentration formulation at a low dosage of hydrophobic photosensitizers (PS) that shows improved pharmacokinetics and an effective method for photodynamic therapy ("PDT"). The new formulation has better pharmacological effect compared to standard photosensitizer formulation with standard dosage. It was found that PDT treatments using the disclosed low concentration formulations provide for more accurate, more efficient and more convenient dosing. It was found that the inventive formulation； (1) reduces the time for a therapeutically effective level of photosensitizer to accumulate in diseased tissue； and, (2) reduces the time for achieving a sufficient ratio of photosensitizer in diseased tissue vs. healthy tissue. As a result, the formulation of the invention reduces the time interval between PS application/administration and irradiation (the drug-light interval or "DLI") and can provide for a "same day" PDT treatment option. The inventive formulation can be used for PDT treatment regimes where photosensitizers are administered in at least one preselected dose, including a low concentration therapy for PDT. In particular, when meta-(Tetrahydroxyphenyl)Chlorin (m-THPC) is the photosensitizer, then a concentration of 0.8 mg/ml to 0.04 mg/ml in a mixture of pure propylene glycol and ethanol in a 3:2 volume ratio accumulates in diseased tissue and differentiates between diseased tissue and normal tissue sufficiently quickly for 'one day' or overnight administration and activation treatment procedures to be possible.
