Realtime light dosimetry software tools for interstitial photodynamic therapy of the human prostate

Abstract

Photodynamic therapy (PDT) for the treatment of prostate cancer has been demonstrated to be a safe treatment option capable of inducing tissue destruction and decreasing prostate specific antigen (PSA) levels. However, prostate-PDT results in large intra- and interpatient variations in treatment response, possibly due to biological variations in tissue composition and short-term response to the therapeutic irradiation. Within our group, an instrument for interstitial PDT on prostate tissue has been developed that combines therapeutic light delivery and monitoring of light transmission via numerous bare-ended optical fibers. Here, we present algorithms that utilize data on the light distribution within the target tissue to provide realtime treatment feedback based on a light dose threshold model for PDT. This realtime dosimetry module is implemented to individualize the light dose and compensate for any treatment-induced variations in light attenuation. More specifically, based on the light transmission signals between treatment fibers, spatially resolved spectroscopy is utilized to assess the effective attenuation coefficient of the tissue. These data constitute input to a block-Cimmino optimization algorithm, employed to calculate individual fiber irradiation times provided the requirement to deliver a predetermined light dose to the target tissue while sparing surrounding sensitive organs. By repeatedly monitoring the light transmission signals during the entire treatment session, optical properties and individual fiber irradiation times are updated in realtime. The functionality of the algorithms is tested on diffuse light distribution data simulated by means of the finite element method (FEM). The feasibility of utilizing spatially resolved spectroscopy within heterogeneous media such as the prostate gland is discussed. Furthermore, we demonstrate the ability of the block-Cimmino algorithm to discriminate between target tissue and organs at risk (OAR). Finally, the realtime dosimetry module is evaluated for treatment scenarios displaying spatially and temporally varying light attenuation levels within the target tissue. We conclude that the realtime dosimetry module makes it possible to deliver a certain light dose to the target tissue despite spatial and temporal variations of the target tissue optical properties at the therapeutic wavelength.

The role of photodynamic therapy (PDT) physics

“…The group in Lund, Sweden is developing a multiple channel system of point sources that can be used as either light sources or detectors. 107 A personal computer controls the position and duration of the point sources to optimize light fluence distribution. Another group in Toronto, Canada has commercialized a four-channel light source with computerized power control in each channel.…”

Towards Conformal Interstitial Light Therapies: Modelling Parameters, Dose Definitions and Computational Implementation

“…et al, 2009). Hence, it is often replaced by diffusion theory, when homogeneous light-tissue interaction coefficients are assumed (Altschuler et al, 2005;Rendon, 2008), or diffusion theory in combination with the finite element method (Davidson et al, 2009;Johansson et al, 2007). Unfortunately, neither approach provides the flexibility desired for treatment planning.…”

Treatment planning using tailored and standard cylindrical light diffusers for photodynamic therapy of the prostate

“…Other groups have previously proposed using the Cimmino feasibility algorithm (Censor et al 1988) to optimize the diffuser outputs (Altschuler et al 2005, Johansson et al 2007. However, De Pierro and Iusem (1985) have shown that, for the inconsistent case, the Cimmino algorithm minimizes (2.8) rather than (2.5), making the comparison of different diffuser positions in the outer loops inconsistent.…”

“…Recently, Johansson et al (2007) have tested their on-line dosimetry system by simulating (based on finite element methods) heterogeneous changes in the optical properties by adding small random changes in the absorption coefficient over and otherwise constant field, and calculating new irradiation times for their treatment fibres using a Cimmino algorithm. In agreement with our results, they find that prostate coverage can be nearly recovered to nominal values.…”

“…Computer-based optimization can improve planning efficiency and potentially find better solutions, yet only a few groups have investigated such strategies. Altschuler et al (2005) developed an optimization strategy that employs cylindrical diffusers, while the algorithm of Johansson et al (2007) assumes cut end fibres. These groups have implemented similar optimization strategies, where a search over diffuser positions is combined with a Cimmino linear feasibility algorithm (Censor et al 1988) to find the output powers (or irradiation times) for the diffusers.…”