Optical properties of experimental prostate tumors in vivo

On the in vivo photochemical rate parameters for PDT reactive oxygen species modeling

Photosensitizer photochemical parameters are crucial data in accurate dosimetry for photodynamic therapy (PDT) based on photochemical modeling. Progress has been made in the last few decades in determining the photochemical properties of commonly used photosensitizers (PS), but mostly in solution or in vitro. Recent developments allow for the estimation of some of these photochemical parameters in vivo. This review will cover the currently available in vivo photochemical properties of photosensitizers as well as the techniques for measuring those parameters. Furthermore, photochemical parameters that are independent of environmental factors or are universal for different photosensitizers will be examined. Most photosensitizers discussed in this review are of the type II (singlet oxygen) photooxidation category, although type I photosensitizers that involve other reactive oxygen species (ROS) will be discussed as well. The compilation of these parameters will be essential for ROS modeling of PDT.

Determination of optical properties in heterogeneous turbid media using a cylindrical diffusing fiber

For interstitial photodynamic therapy (PDT), cylindrical diffusing fibers (CDFs) are often used to deliver light. This study examines the feasibility and accuracy of using CDFs to characterize the absorption (μ(a)) and reduced scattering (μ'(s)) coefficients of heterogeneous turbid media. Measurements were performed in tissue-simulating phantoms with μ(a) between 0.1 and 1 cm(-1) and μ'(s) between 3 and 10 cm(-1) with CDFs 2 to 4 cm in length. Optical properties were determined by fitting the measured light fluence rate profiles at a fixed distance from the CDF axis using a heterogeneous kernel model in which the cylindrical diffusing fiber is treated as a series of point sources. The resulting optical properties were compared with independent measurement using a point source method. In a homogenous medium, we are able to determine the absorption coefficient μ(a) using a value of μ'(s) determined a priori (uniform fit) or μ'(s) obtained by fitting (variable fit) with standard (maximum) deviations of 6% (18%) and 18% (44%), respectively. However, the CDF method is found to be insensitive to variations in μ'(s), thus requiring a complementary method such as using a point source for determination of μ'(s). The error for determining μ(a) decreases in very heterogeneous turbid media because of the local absorption extremes. The data acquisition time for obtaining the one-dimensional optical properties distribution is less than 8 s. This method can result in dramatically improved accuracy of light fluence rate calculation for CDFs for prostate PDT in vivo when the same model and geometry is used for forward calculations using the extrapolated tissue optical properties.

Optoacoustic imaging of the prostate: development toward image-guided biopsy

Optoacoustic (OA) tomography has demonstrated utility in identifying blood-rich malignancies in breast tissue. We describe the development and characterization of a laser OA imaging system for the prostate (LOIS-P). The system consists of a fiber-coupled Q-switched laser operating at 757 nm, a commercial 128-channel ultrasonic probe, a digital signal processor, and software that uses the filtered radial back-projection algorithm for image reconstruction. The system is used to reconstruct OA images of a blood-rich lesion induced in vivo in a canine prostate. OA images obtained in vivo are compared to images acquired using ultrasound, the current gold standard for guiding biopsy of the prostate. Although key structural features such as the urethra could be identified with both imaging techniques, a bloody lesion representing a highly vascularized tumor could only be clearly identified in OA images. The advantages and limitations of both forward and backward illumination modes are also evaluated by collecting OA images of phantoms simulating blood vessels within tissue. System resolution is estimated to be 0.2 mm in the radial direction of the acoustic array. The minimum detectable pressure signal is 1.83 Pa. Our results encourage further development toward a dual-modality OA/ultrasonic system for prostate imaging and image-guided biopsy.

Trans-rectal ultrasound-coupled near-infrared optical tomography of the prostate, part II: experimental demonstration

We demonstrate trans-rectal optical tomography of the prostate using an endo-rectal near-infrared (NIR) applicator integrated with a transrectal ultrasound (TRUS) probe. The endo-rectal NIR applicator incorporated a design presented in our previously reported work. A continuous-wave NIR optical tomography system is combined with a commercial US scanner to form the dual-modality imager. Sagittal transrectal imaging is performed concurrently by endo-rectal NIR and TRUS. The TRUS ensures accurate positioning of the NIR applicator as well as guides NIR image reconstruction using the spatial prior of the target. The use of a condom, which is standard for TRUS, is found to have minimal effect on trans-rectal NIR imaging. Tests on avian tissues validates that NIR imaging can recover the absorption contrast of a target, and its accuracy is improved when the TRUS spatial prior is incorporated. Trans-rectal NIR/US imaging of a healthy canine prostate in situ is reported.

Trans-rectal ultrasound-coupled near-infrared optical tomography of the prostate, part I: simulation

We investigate the feasibility of trans-rectal optical tomography of the prostate using an endo-rectal near-infrared (NIR) applicator that is to be integrated with a trans-rectal ultrasound (TRUS) probe. Integration with TRUS ensures accurate endo-rectal positioning of the NIR applicator and the utility of using TRUS spatial prior information to guide NIR image reconstruction. The prostate NIR image reconstruction is challenging even with the use of spatial prior owing to the anatomic complexity of the imaging domain. A hierarchical reconstruction algorithm is developed that implements cascaded initial-guesses for nested domains. This hierarchical image reconstruction method is then applied to evaluating a number of NIR applicator designs for integration with a sagittal TRUS transducer. A NIR applicator configuration feasible for instrumentation development is proposed that contains one linear array of optodes on each lateral side of the sagittal TRUS transducer. The performance of this NIR applicator is characterized for the recovery of single tumor mimicking lesion as well as dual targets in the prostate. The results suggest a strong feasibility of transrectal prostate imaging by use of the endo-rectal NIR/US probe.

Photodynamic therapy for treatment of solid tumors--potential and technical challenges

Photodynamic therapy (PDT) involves the administration of photosensitizer followed by local illumination with visible light of specific wavelength(s). In the presence of oxygen molecules, the light illumination of photosensitizer can lead to a series of photochemical reactions and consequently the generation of cytotoxic species. The quantity and location of PDT-induced cytotoxic species determine the nature and consequence of PDT. Much progress has been seen in both basic research and clinical application in recent years. Although the majority of approved PDT clinical protocols have primarily been used for the treatment of superficial lesions of both malignant and non-malignant diseases, interstitial PDT for the ablation of deep-seated solid tumors are now being investigated worldwide. The complexity of the geometry and non-homogeneity of solid tumor pose a great challenge on the implementation of minimally invasive interstitial PDT and the estimation of PDT dosimetry. This review will discuss the recent progress and technical challenges of various forms of interstitial PDT for the treatment of parenchymal and/or stromal tissues of solid tumors.

Prostate PDT dosimetry

We provide a review of the current state of dosimetry in prostate photodynamic therapy (PDT). PDT of the human prostate has been performed with a number of different photosensitizers and with a variety of dosimetry schemes. The simplest clinical light dose prescription is to quantify the total light energy emitted per length (J/cm) of cylindrical diffusing fibers (CDF) for patients treated with a defined photosensitizer injection per body weight. However, this approach does not take into account the light scattering by tissue and usually underestimates the local light fluence rate, and consequently the fluence. Techniques have been developed to characterize tissue optical properties and light fluence rates in vivo using interstitial measurements during prostate PDT. Optical methods have been developed to characterize tissue absorption and scattering spectra, which in turn provide information about tissue oxygenation and drug concentration. Fluorescence techniques can be used to quantify drug concentrations and photobleaching rates of photosensitizers.

Photodynamic therapy for prostate cancer: One urologist's perspective

Photodynamic therapy (PDT) has slowly found its place in the treatment of human disease. Currently, photodynamic therapy is being explored as a treatment option for localized prostate cancer. PDT for the treatment of prostate cancer will require ablation of both malignant and non-malignant glandular epithelium. Ablation of both malignant and normal epithelium adds a new treatment dimension since traditionally PDT has not targeted normal epithelial tissue. PDT for prostate cancer as currently envisioned will present challenges in terms of in situ monitoring of light, drug concentration, [Formula: see text] levels and biologic endpoints. The introduction of vascular-targeted photosensitizers fundamentally alters the traditional axioms for successful PDT treatment by obviating the need for "selective" tumor localization. Should clinical trials demonstrate the utility of this approach, patients with organ-confined disease will benefit.

Endoscopic, rapid near-infrared optical tomography

This is believed to be the first demonstration of near-infrared (NIR) optical tomography employed at the endoscope scale and at a rapid sampling speed that allows translation to in vivo use. A spread-spectral-encoding technique based on a broadband light source and linear-to-circular fiber bundling was used to provide endoscopic probing of many source-detector fibers for tomography as well as parallel sampling of all source-detector pairs for rapid imaging. Endoscopic NIR tomography at an 8 Hz frame rate was achieved in phantoms and tissue specimens with a 12 mm probe housing eight sources and eight detectors. This novel approach provides the key feasibility studies to allow this blood-based contrast imaging technology to be attempted in detection of cancer in internal organs via endoscopic interrogation.

Optical properties of human prostate at 732 nm measured in mediated photodynamic therapy

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 [mu(a)], transport scattering [mu(s)'] and effective attenuation [mu(eff)] 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. mu(a) and mu(s)' 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. mu(a) 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. mu(eff) varied between 0.91 and 6.7 cm(-1) (mean 2.9 +/- 0.7 cm(-1)), corresponding to an optical penetration depth (delta = 1/micro(eff)) 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 mu(s)' 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.

Techniques for delivery and monitoring of TOOKAD (WST09)-mediated photodynamic therapy of the prostate: Clinical experience and practicalities

Photodynamic therapy of solid organs requires sufficient PDT dose throughout the target tissue while minimizing the dose to proximal normal structures. This requires treatment planning for position and power of the multiple delivery channels, complemented by on-line monitoring during treatment of light delivery, drug concentration and oxygen levels. We describe our experience in implementing this approach in Phase I/II clinical trials of the Pd-bacteriophephorbide photosensitizer TOOKAD (WST09)-mediated PDT of recurrent prostate cancer following radiation failure. We present several techniques for delivery and monitoring of photodynamic therapy, including beam splitters for light delivery to multiple delivery fibers, multi-channel light dosimetry devices for monitoring the fluence rate in the prostate and surrounding organs, methods of measuring the tissue optical properties in situ, and optical spectroscopy for monitoring drug pharmacokinetics of TOOKAD in whole blood samples and in situ in the prostate. Since TOOKAD is a vascular-targeted agent, the design and implementation of the techniques are different than for cellular-targeted agents. Further development of these delivery and monitoring techniques will permit full on-line monitoring of the treatment that will enable real-time, patient-specific and optimized delivery of PDT.

Determination of the distribution of light, optical properties, drug concentration, and tissue oxygenation in-vivo in human prostate during motexafin lutetium-mediated photodynamic therapy

It is desirable to quantify the distribution of the light fluence rate, the optical properties, the drug concentration, and the tissue oxygenation for photodynamic therapy (PDT) of prostate cancer. We have developed an integrated system to determine these quantities before and after PDT treatment using motorized probes. The optical properties (absorption (micro(a)), transport scattering (micro(s'), and effective attenuation (micro(eff)) coefficients) of cancerous human prostate were measured in-vivo using interstitial isotropic detectors. Measurements were made at 732 nm before and after motexafin lutetium (MLu) mediated PDT at different locations along each catheter. The light fluence rate distribution was also measured along the catheters during PDT. Diffuse absorption spectroscopy measurement using a white light source allows extrapolation of the distribution of oxygen saturation StO2, total blood volume ([Hb]t), and MLu concentration. The distribution of drug concentration was also studied using fluorescence from a single optical fiber, and was found to be in good agreement with the values determined by absorption spectroscopy. This study shows significant inter- and intra-prostatic variations in the tissue optical properties and MLu drug distribution, suggesting that a real-time dosimetry measurement and feedback system for monitoring these values during treatment should be considered in future PDT studies.

Effect of Tumor Host Microenvironment on Photodynamic Therapy in a Rat Prostate Tumor Model

Purpose: Tumor host microenvironment plays an important role in tumor growth, metastasis, and response to cancer therapy. In this study, the influence of tumor host environment on tumor pathophysiology, photosensitizer distribution, and photodynamic therapy (PDT) treatment effect was examined in the metastatic at lymph node and lung (MatLyLu) rat prostate tumor.

Experimental Design: MatLyLu tumors implanted in different host environment [i.e., orthotopically (in the prostate) or s.c.] were compared for difference in vessel density, average vessel size, vascular permeability, tumor vascular endothelial growth factor production, and tumor oxygenation. Uptake of photosensitizer verteporfin in tumors in both sites was determined by fluorescence microscopy. To compare tumor response to PDT, both orthotopic and s.c. MatLyLu tumors were given the same doses of verteporfin and laser light treatment, and PDT-induced tumor necrotic area was measured histologically.

Results: Orthotopic MatLyLu tumors were found to grow faster, have higher vessel density and more permeable vasculature, have higher vascular endothelial growth factor protein levels, and have lower tumor hypoxic fraction than the s.c. tumors. Uptake of photosensitizer verteporfin in the orthotopic tumor was higher than in the s.c. tumors at 15 minutes after injection (1 mg/kg, i.v.), and became similar at 3 hours after injection. For the vascular targeting PDT treatment (0.25 mg/kg verteporfin, 50 J/cm2 at 50 mW/cm2, 15 minutes drug-light interval), there was no significant difference in PDT-induced tumor necrotic area between the orthotopic and s.c. tumors, with 85% to 90% necrosis in both types of tumors. However, tumor necrosis induced by the cellular targeting PDT (1 mg/kg verteporfin, 50 J/cm2 at 50 mW/cm2, 3 hours drug-light interval) was significantly different in the orthotopic (64%) versus the s.c. (29%) tumors.

Conclusions: Tumor host environment can significantly affect photosensitizer verteporfin distribution and PDT treatment effect. Verteporfin-PDT regimen targeting tumor cells is more sensitive to such influence than the vascular targeting PDT. Our study showed the importance of tumor host environment in determining tumor physiologic properties and tumor response to PDT. To obtain clinically relevant information, orthotopic tumor model should be used in the experimental studies.

Light dosimetry for multiple cylindrical diffusing sources for use in photodynamic therapy

Since prostatic carcinoma is usually multifocal within the prostate, effective photodynamic therapy (PDT) of prostatic carcinoma is expected to require the photochemical destruction of the entire organ. Accurate light dosimetry will be essential to avoid damage to proximal sensitive tissue such as the rectum. The prostate will be illuminated using interstitial cylindrical fibreoptic light sources and, because of the limited transparency of prostate tissue, these sources will be mounted in a parallel array analogous to the source array used in brachytherapy. Both source spacing and the light delivered to each source will control light dosimetry from a parallel array of fibreoptic sources implanted into tissue. Clinical PDT will require dose planning in order to determine the position and illumination of each source prior to treatment, but unfortunately few methods of predicting light fluence from cylindrical interstitial sources currently exist. In this paper, a novel light fluence model is used to predict tissue transillumination resulting from cylindrical interstitial sources. The cylindrical source is modelled as a finite array of infinitesimal small sources using Christian Huygens' famous single-slit diffraction model. We show that this source model when combined with a robust derivation of fluence in a spherical geometry using diffusion theory, accurately predicts fluence levels from a single cylindrical source in a variety of media. This method is found to retain its accuracy near the sources. With a simple extension, this fluence model is used to predict the light fluence levels from an array of three sources and the predicted fluence is found to compare favourably with experimental data.

Photodynamic therapy: a new approach to prostate cancer

Photodynamic therapy (PDT) is based on the concept that light irradiation can change an inert substance into an active one. In urology, hematoporphyrin derivative (HpD) and Photofrin (Axcan Scandipharm Inc., Birmingham, AL) are used most commonly as photosensitizing agents predominantly for the treatment of transitional cell carcinoma of the bladder. To investigate the basics for PDT of prostate cancer, several studies were performed on the optical characteristics of prostate tissue and prostate carcinoma tissue in vitro and in vivo and on the penetration depths of different laser wavelengths. Initial experimental studies to treat prostate cancer with PDT using HpD were done on Dunning tumors in rats. Combined with interstitial applicators, photodynamic therapy seems to have a great potential in the treatment of prostate carcinoma. However, it is an experimental treatment and even a preliminary evaluation will be possible only after the conclusion of clinical studies with the corresponding long-term results.

Photodynamic therapy with Pd-Bacteriopheophorbide (TOOKAD): successful in vivo treatment of human prostatic small cell carcinoma xenografts

Small cell carcinoma of the prostate (SCCP), although relatively rare, is the most aggressive variant of prostate cancer, currently with no successful treatment. It was therefore tempting to evaluate the response of this violent malignancy and its bone lesions to Pd-Bacteriopheophorbide (TOOKAD)-based photodynamic therapy (PDT), already proven by us to efficiently eradicate other aggressive non-epithelial solid tumors. TOOKAD is a novel bacteriochlorophyll-derived, second-generation photosensitizer recently, developed by us for the treatment of bulky tumors. This photosensitizer is endowed with strong light absorbance (epsilon(0) approximately 10(5) mol(-1) cm(-1)) in the near infrared region (lambda=763nm), allowing deep tissue penetration. The TOOKAD-PDT protocol targets the tumor vasculature leading to inflammation, hypoxia, necrosis and tumor eradication. The sensitizer clears rapidly from the circulation within a few hours and does not accumulate in tissues, which is compatible with the treatment of localized tumor and isolated metastases. Briefly, male CD1-nude mice were grafted with the human SCCP (WISH-PC2) in 3 relevant anatomic locations: subcutaneous (representing tumor mass), intraosseous (representing bone metastases) and orthotopically within the murine prostate microenvironment. The PDT protocol consisted of i.v. administration of TOOKAD (4 mg/kg), followed by immediate illumination (650-800 nm) from a xenon light source or a diode laser emitting at 770 nm. Controls included untreated animals or animals treated with light or TOOKAD alone. Tumor volume, human plasma chromogranin A levels, animal well being and survival were used as end points. In addition, histopathology and immunohistochemistry were used to define the tumor response. Subcutaneous tumors exhibited complete healing within 28-40 days, reaching an overall long-term cure rate of 69%, followed for 90 days after PDT. Intratibial WISH-PC2 lesions responded with complete tumor elimination in 50% of the treated mice at 70-90 days after PDT as documented histologically. The response of the orthotopic model was also analyzed histologically with similar results. The study with this model suggests that TOOKAD-based PDT can reach large tumors and is a feasible, efficient and well-tolerated approach for minimally invasive treatment of local and disseminated SCCP.

In vivo optical properties of normal canine prostate at 732 nm using motexafin lutetium-mediated photodynamic therapy

The optical properties (absorption [mu(a)], transport scattering [mu('s)] and effective attenuation [mu(eff)] coefficients) of normal canine prostate were measured in vivo using interstitial isotropic detectors. Measurements were made at 732 nm before, during and after motexafin lutetium (MLu)-mediated photodynamic therapy (PDT). They were derived by applying the diffusion theory to the in vivo peak fluence rates measured at several distances (3, 6, 9, 12 and 15 mm) from the central axis of a 2.5 cm cylindrical diffusing fiber (CDF). Mu(a) and mu('s) varied between 0.03-0.58 and 1.0-20 cm(-1), respectively. Mu(a) was proportional to the concentration of MLu.Mu(eff) varied between 0.33 and 4.9 cm(-1) (mean 1.3 +/- 1.1 cm(-1)), corresponding to an optical penetration depth (8 = 1/(mu(eff)) of 0.5-3 cm (mean 1.3 +/- 0.8 cm). The mean light fluence rate at 0.5 cm from the CDF was 126 +/- 48 mW/cm2 (N = 22) when the total power from the fiber was 375 mW (150 mW/cm). 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 advocated for future PDT studies. However, no significant changes were observed before, during and after PDT within a single treatment site.

Interstitial photodynamic therapy in subcutaneously implanted urologic tumors in rats after intravenous administration of 5-aminolevulinic acid

Photodynamic therapy (PDT) may be an attractive option for treatment of early stage prostate cancer. Aminolevulinic acid (ALA) acts as a prodrug leading to a selective accumulation of a photosensitizer, protoporphyrin IX (PpIX), in epithelial cells. We investigated the efficacy of ALA-mediated PDT for rat R3327-H prostate cancer, compared with the AY-27 bladder tumor. Rats bearing either AY-27 or R3327-H tumors were randomized to different groups when their tumors reached approximately 1000 mm3. At the day of PDT, animals were administered 500 mg/kg ALA intravenously 4 hours prior to laser therapy. The argon-pumped dye laser light (630 nm) was coupled to multiple quartz fibers with cylindrical diffusing tips, which were inserted into the tumor in icosahedral pattern. Light exposure was varied to yield doses of 1000 to 3000 J/tumor. Animals bearing R3327-H tumors were imaged with 99mTc-HMPAO scintigraphy to evaluate tumor perfusion changes induced by PDT. There was a light-dose dependent tumor response in both tumor models. The mean time for R3327-H tumor to re-grow to 4 x treatment volume was 79.7 days in the control group (light only), 159 days in 1000 J group, and 169 days in 2000 J group (P < 0.05). Tumors treated with 3000 J were clinically cured (P < 0.01). Likewise, for AY-27 tumors, the average time to re-grow to 4 x treatment volume was 13.7 days in the control group, 179.3, 183.3, and 185.7 days in groups of 1000, 1500, and 2000 J (P < 0.05), respectively. Tumors treated with 3000 J were clinically cured (P < 0.01). 99mTc-HMPAO scintigraphy demonstrated a mild perfusion impairment following PDT. Interstitial PDT with ALA/PpIX is equally effective in treating prostate cancer and TCC in these heterotopic rat models.

Light dosimetry using the P3 approximation

In earlier work, we demonstrated that radiance, calculated using the P3 approximation in a plane wave geometry, could be used to accurately predict the optical parameters of an Intralipid/methylene blue phantom. Plane wave geometry is impractical for clinical use but the results of this work encouraged us to further develop the P3 approximation for a spherical geometry, described in this paper. Radiance predicted by this model for a defined Intralipid/methylene blue phantom was compared with radiance measured in this phantom. The results demonstrate that the spherical derivation of the P3 approximation will reproducibly predict optical parameters of a tissue phantom as effectively as the slab geometry derivation of the P3 approximation. In a similar protocol, the P3 approximation was used to estimate the optical parameters of ex vivo human prostate. Radiance in this case was measured in the prostate samples using an after loading technique. Three prostate samples tested were found to be surprisingly optically homogeneous. The after loading protocol described in this paper could form the basis of a minimally invasive and effective clinical method to optically characterize human prostate.

In vivo light transmission spectra in EMT6/Ed murine tumors and Dunning R3327 rat prostate tumors during photodynamic therapy

BACKGROUND AND OBJECTIVE

Variations in the optical coefficients in tissue and the photosensitizer during photodynamic therapy (PDT) will require adjustment of the light dose during the course of therapy. We have studied the dynamics using light transmission spectra for two different tumor models when tetrasulfonated aluminum phthalocyanine (AlPcS4) was used as photosensitizer.

STUDY DESIGN/MATERIALS AND METHODS

Spectra were measured noninvasively in the EMT6/Ed murine tumor model, and with interstitially implanted source and probe fibers in the Dunning R3327-AT rat tumor model. Measurements were performed in the range 600-840 nm, using a tunable dye laser, a diode laser, and a Ti:Sapphire laser. AlPcS4 has absorption in the range 600-700 nm with an absorption peak at 670 nm in saline.

RESULTS

The in vivo spectrum of AlPcS4 both in the EMT6/Ed tumor model and the Dunning R3327-AT tumor model differs from the spectrum of AlPcS4 in saline. The absorption at 670 nm was reduced, whereas the absorption at 640 nm increased. Exposure of phototherapeutic levels of light caused reduced light absorption by the photosensitizer and further spectral shift.

CONCLUSION

We found that the AIPcS4 absorption spectrum changes in a biological environment, and we also observed increased light transmission at the treatment wavelength during PDT in both tumor models. Instability in the absorption spectrum of the photosensitizer may influence the effectiveness of PDT.

Radiance modelling using the P3 approximation

Light dosimetry is an essential component of effective photodynamic therapy (PDT) of tumours. Present PDT light dosimetry techniques rely on fluence-based models and measurements. However, in a previous paper by Barajas et al, radiance-based light dosimetry was explored as an alternative approach. Although successful in demonstrating the use of Monte Carlo (MC) simulations of radiance in tissue optical characterization, the MC proved time consuming and impractical for clinical applications. It was proposed that an analytical solution to the transport equation for radiance would be desirable as this would facilitate and increase the speed of tissue characterization. It has been found that the P3 approximation is one such potential solution. Radiance and fluence expressions based on the P3 approximation were used to optically characterize an Intralipid-based tissue phantom of varying concentration of scatterer (Intralipid) and absorber (methylene blue) using a plane wave illuminated, semi-infinite medium geometry. The results obtained compare favourably with the Grosjean approximation of fluence (a modified diffusion theory) using the same optical parameters (mu(a), mu(s), g). The results illustrate that radiance-based light dosimetry is a viable alternative approach to tissue characterization and dosimetry. It is potentially useful for clinical applications because of the limited number of invasive measurements needed and the speed at which the tissue can be characterized.

Laser dosimetry studies in the prostate

OBJECTIVE

To review the currently available data of photodynamic therapy (PDT) optical dosimetry for possible prostatic applications.

SUMMARY BACKGROUND DATA

PDT is a new cancer treatment modality often used as an alternative tumor treatment method. Recently, PDT has been suggested as an alternative therapy for prostatic carcinoma and BPH.

METHODS

PDT: utilizes light and a preadministered photosensitizer drug to achieve localized tumor control. This article reviews currently available data on optical dosimetry of PDT in both human and canine prostates.

RESULTS

At 630 nm, a common wavelength used for Photofrin PDT, results indicate that light penetration is similar in cancerous and normal prostatic tissue. Because of limited light penetration, multiple fiber irradiation is necessary if eradicating the entire prostate glad is the ultimate goal. The available data also show that dynamic changes occur in light fluence rate distribution during PDT irradiation.

CONCLUSIONS

PDT can be used to destroy prostatic tissue. Real-time optical dosimetry is necessary if accurate lesion volume control is desired.

Temporal and illumination-induced variations in the in vivo light transmission spectra of four photosensitizers in EMT6/Ed murine tumours

Temporal and illumination-induced variations in the in vivo light transmission spectrum of the photosensitizer will influence light dosimetry for photodynamic therapy (PDT). The present authors have studied the in vivo spectra of four photosensitizers in the EMT6/Ed murine tumour model in Balb/c mice. The following photosensitizers were used: bis(dimethylthexylsiloxy)silicon 2,3-naphthalocyanine (SiNc 8), benzoporphyrin-derivative monoacid ring A (BPD Verteporfin), Photofrin and ethanolamined hypocrellin B (HBEA-R2). Spectra were measured non-invasively in the EMT6/Ed murine tumour model in the spectral range 600-840 nm, using a diode laser, a dye laser and a Ti:sapphire laser. Red-shift and broadening of the SiNc 8 absorption band was observed at 790 nm, and a slight red-shift was observed in the BPD, HBEA-R2 and Photofrin in vivo absorption spectrum. Exposure to 300 J of light at the peak absorption wavelength caused complete photobleaching of BPD at 690 nm, and a reduced absorption by SiNc 8 at 780 nm, Photofrin at 626 nm, and HBEA-R2 at 656 nm.

Monte Carlo modelling of angular radiance in tissue phantoms and human prostate: PDT light dosimetry

Photodynamic therapy (PDT) is a promising technique for destroying tumours. Photosensitizing drugs presently available are not sufficiently tumour specific; hence, light dosimetry is required in order to control light exposure and thereby restrict cell kill to the target tissue to avoid damage to healthy tissue. Current light dosimetry methods rely on tissue optical characterization by fluence measurements at several points. Fluence-based tissue characterization is impractical for tumours in organs such as prostate where access by optical probes is limited and the tumours are highly optically inhomogeneous. This paper explores the potential of radiance-based light dosimetry as an alternative. Correlation is found between Monte Carlo simulation of radiance in a tissue phantom and radiance measurements made using a new radiance probe. Radiance is sensitive to variations in the tissue optical parameters, absorption coefficient mu(a), scattering coefficient mu(s), and anisotropy factor g, and therefore is potentially useful for tissue characterization. Radiance measurements have several advantages over fluence measurements. Radiance measurements provide more information from a single location, better spatial resolution of the tissue optical parameters, and higher sensitivity in discriminating between different media. However, the Monte Carlo method is too slow to be of practical value for tissue characterization by correlation of measured and simulated radiance. An analytical solution to the transport equation for radiance would be desirable as this would facilitate and increase the speed of tissue characterization.

Light dosimetry in vivo

This paper starts with definitions of radiance, fluence (rate) and other quantities that are important with regard to in vivo light dosimetry. The light distribution in mammalian tissues can be estimated from model calculations using measured optical properties or from direct measurements of fluence rate using a suitable detector. A historical introduction is therefore followed by a brief discussion of tissue optical properties and of calculations using diffusion theory, the P3-approximation or Monte Carlo simulations. In particular the form of the scattering function is considered in relation to the fluence rate close to the tissue boundary, where light is incident. Non-invasive measurements of optical properties yield the absorption coefficient mu a and mu s(1 - g), where mu s is the scattering coefficient and g is the mean cosine of the scattering angle. An important question is whether this combination is sufficient, or whether g itself must be known. It appears that for strongly forward scattering, as in mammalian tissues, rather detailed knowledge of the scattering function is needed to reliably calculate the fluence rate close to the surface. Deeper in the tissue mu s (1 - g) is sufficient. The construction, calibration and use of fibre-optic probes for measurements of fluence rate in tissues or optical phantoms is discussed. At present, minimally invasive absolute fluence (rate) measurements seem to be possible with an accuracy of 10-20%. Examples are given of in vivo measurements in animal experiments and in humans during clinical treatments. Measurements in mammalian tissues, plant leaves and marine sediments are compared and similarities and differences pointed out. Most in vivo light fluence rate measurements have been concerned with photodynamic therapy (PDT): Optical properties of the same normal tissue may differ between patients. Tumours of the same histological type may even show different optical properties in a single patient. Treatment-induced changes of optical properties may also occur. Scattered light appears to contribute substantially to the light dose. All these phenomena emphasize the importance of in situ light measurements. Another important dosimetric parameter in PDT is the concentration and distribution of the photosensitizer. Apart from in vivo fluorescence monitoring, the photosensitizer part of in vivo PDT dosimetry is still in its infancy.

Anisotropy of radiance in tissue phantoms and Dunning R3327 rat tumors: radiance measurements with flat cleaved fiber probes

BACKGROUND AND OBJECTIVE

The goal of this study is to determine if flat cleaved fiber probes are appropriate for interstitial measurements of radiance in tissue. Flat cleaved probes have the advantage of high responsivity, and they are easy to insert into tissue. Owing to the non-isotropic response of flat cleaved probes, a calibration function is required, taking the anisotropy in the radiance in tissue into account.

STUDY DESIGN, MATERIALS AND METHODS

The method used to determine this function consists of radiance measurements in tissue, performed with a flat cleaved fiber probe mounted on a stereotactic stage for insertion into the tissue from different directions. Interstitial irradiation at 630 nm was delivered by a spherical source.

RESULTS

We found that the degree of anisotropy in the radiance decreases with increasing distance from the interstitially implanted source in two different tissue phantoms and in the Dunning R3327-AT and R3327-H rat tumor models.

CONCLUSION

A position-dependent calibration function is required for interstitially implanted flat cleaved fiber probes. An anisotropy function is presented, which modifies the measurements of radiance with a flat cleaved probe, to account for the change in anisotropy in the radiance. The anisotropy functions for the two tumor models differ substantially.

In vivo laser light distribution in human prostatic carcinoma

The extent of laser light diffusion within prostatic tumor is of major importance in the treatment of localized prostatic cancer with photodynamic therapy (PDT). The penetration of 633 nm. wavelength red light was studied in eleven patients with suspected prostatic cancer using a novel method suitable for in situ measurements. Light delivery and detector fiber, placed interstitially within the gland, determined light attenuation at different interfiber separations. Of 11 patients, 10 had bilateral and 1 had single lobe studies. The mean +/- the standard error of the mean attenuation coefficients (sigma eff) for benign and malignant prostate tissue were 0.35 +/- 0.02 mm-1 and 0.36 +/- 0.02 mm-1, respectively, indicating similar optical densities (p = .58). Patients with bilateral lobe involvement showed little intraglandular variation in sigma eff (p = 0.23). However, there was interpatient variation (sigma eff = 0.28 to 0.48 mm-1) reflecting biological differences which, though therapeutically important, were not statistically significant (p = 0.057). This study showed that treatment requires individualization and predicted that 4 cylindrical diffusers are expected to destroy 25 ml. of prostatic tumor with PDT.