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Small circular light fields (<= 2 cm diameter) are sometimes used for photodynamic therapy of skin and recurrent breast cancers on the chest wall. These fields have lateral dimensions comparable to the effective mean free path of photons in the turbid medium, which causes reduced light fluence rate compared to that of a broad beam of uniform incident irradiance. We have compared Monte-Carlo simulation with in-vivo dosimetry for circular fields (R = 0.25, 0.35, 0.5, 0.75, 1, 2, 3, and 8 cm) in a liquid phantom composed of intralipid and ink (μs' = 4 20-1/cm and μa = 0.1 cm-1) for wavelengths between 532 and 730 nm. We used anisotropy g = 0.9 and the index of refraction n = 1.4 for all Monte-Carlo simulations. The measured light fluence rate agrees with Monte-Carlo simulation to within 10%, with the measured value lower than that of the Monte-Carlo simulation on tissue surface. The ratio of the peak fluence rates between a circular beam and a broad beam under tissue is 0.58 - 0.96 or 0.84 - 1.00 for R between 0.5 - 2 cm and mueff = 1.1 or 2.0 cm-1, respectively. The ratio of peak fluence rate and incident irradiance for the broad beam is 5.9 and 6.4 for μeff = 1.1 and 2.0 cm-1, respectively. The optical penetration depth delta varies from 0.34 - 0.48 cm for R between 0.5 and 2 cm, with the corresponding δ= 0.51 cm for a broad beam. The ratio of fluence rate and incident irradiance above tissue surface is 1.4 - 1.8 or 1.9 - 2.2 for R between 0.5 - 2 cm and mueff = 1.1 or 2.0 1/cm, respectively. At depth of 0.2 cm inside tissue, Off-axis ratio OAR, defined as the ratio of fluence rate at off-axis distance r to that on the central axis, varies between 0.91 - 0.54 or 0.93 - 0.52 for off-axis distances r between 0.6 and 1.0 cm and μeff = 1.1 or 2.0 cm-1, respectively. In conclusion, in-vivo light dosimetry agrees with Monte-Carlo simulation for small field dosimetry provided the isotropic detector is corrected for the blind spot. The light fluence rates for small circular fields are substantially lower than that of the broad beam of the same incident irradiance.
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