Optical Properties of Human Prostate at 732 nm Measured In Vivo During Motexafin Lutetium–mediated Photodynamic Therapy¶

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Abstract
Characterization of the tissue light penetration in prostate photodynamic therapy (PDT) is important to plan the arrangement and weighting of light sources so that sufficient light fluence is delivered to the treatment volume. The optical properties (absorption [μa], transport scattering [μs′] and effective attenuation [μeff] coefficients) of 13 patients with locally recurrent prostate cancer were measured in situ using interstitial isotropic detectors. Measurements were made at 732 nm before and after motexafin lutetium (MLu)–mediated PDT in four quadrants. Optical properties were derived by applying the diffusion theory to the fluence rates measured at several distances (0.5–5 cm) from a point source. μa and μs′ varied between 0.07 and 1.62 cm−1 (mean 0.37 ± 0.24 cm−1) and 1.1 and 44 cm−1 (mean 14 ± 11 cm−1), respectively. μa was proportional to the concentration of MLu measured by an ex vivo fluorescence assay. We have observed, on average, a reduction of the MLu concentration after PDT, presumably due to the PDT consumption of MLu. μeff varied between 0.91 and 6.7 cm−1 (mean 2.9 ± 0.7 cm−1), corresponding to an optical penetration depth (δ = 1/μeff) of 0.1–1.1 cm (mean 0.4 ± 0.1 cm). The mean penetration depth at 732 nm in human prostate is at least two times smaller than that found in normal canine prostates, which can be explained by a four times increase of the mean value of μs′ in human prostates. The mean light fluence rate per unit source strength at 0.5 cm from a point source was 1.5 ± 1.1 cm−2, excluding situations when bleeding occurs. The total number of measurements was N = 121 for all mean quantities listed above. This study showed significant inter- and intraprostatic differences in the optical properties, suggesting that a real-time dosimetry measurement and feedback system for monitoring light fluences during treatment should be considered for future PDT studies.