Pilot study on light dosimetry for endobronchial photodynamic therapy

Clinical Practice of Photodynamic Therapy for Non-Small Cell Lung Cancer in Different Scenarios: Who Is the Better Candidate?

BACKGROUND

Photodynamic therapy (PDT) is a relatively safe and highly selectivity antitumor treatment, which might be increasingly used as a supplement to conventional therapies. A clinical overview and detailed comparison of how to select patients and lesions for PDT in different scenarios are urgently needed to provide a basis for clinical treatment.

SUMMARY

This review demonstrates the highlights and obstacles of applying PDT for lung cancer and underlines points worth considering when planning to initiate PDT. The aim was to make out the appropriate selection and help PDT develop efficacy and precision through a better understanding of its clinical use.

KEY MESSAGES

Increasing evidence supports the feasibility and safety of PDT in the treatment of non-small cell lung cancer. It is important to recognize the factors that influence the efficacy of PDT to develop individualized management strategies and implement well-designed procedures. These important issues should be worth considering in the present and further research.

Revisiting photodynamic therapy dosimetry: reductionist & surrogate approaches to facilitate clinical success

Photodynamic therapy (PDT) can be a highly complex treatment, with many parameters influencing treatment efficacy. The extent to which dosimetry is used to monitor and standardize treatment delivery varies widely, ranging from measurement of a single surrogate marker to comprehensive approaches that aim to measure or estimate as many relevant parameters as possible. Today, most clinical PDT treatments are still administered with little more than application of a prescribed drug dose and timed light delivery, and thus the role of patient-specific dosimetry has not reached widespread clinical adoption. This disconnect is at least partly due to the inherent conflict between the need to measure and understand multiple parameters in vivo in order to optimize treatment, and the need for expedience in the clinic and in the regulatory and commercialization process. Thus, a methodical approach to selecting primary dosimetry metrics is required at each stage of translation of a treatment procedure, moving from complex measurements to understand PDT mechanisms in pre-clinical and early phase I trials, towards the identification and application of essential dose-limiting and/or surrogate measurements in phase II/III trials. If successful, identifying the essential and/or reliable surrogate dosimetry measurements should help facilitate increased adoption of clinical PDT. In this paper, examples of essential dosimetry points and surrogate dosimetry tools that may be implemented in phase II/III trials are discussed. For example, the treatment efficacy as limited by light penetration in interstitial PDT may be predicted by the amount of contrast uptake in CT, and so this could be utilized as a surrogate dosimetry measurement to prescribe light doses based upon pre-treatment contrast. Success of clinical ALA-based skin lesion treatment is predicted almost uniquely by the explicit or implicit measurements of photosensitizer and photobleaching, yet the individualization of treatment based upon each patients measured bleaching needs to be attempted. In the case of ALA, lack of PpIX is more likely an indicator that alternative PpIX production methods must be implemented. Parsimonious dosimetry, using surrogate measurements that are clinically acceptable, might strategically help to advance PDT in a medical world that is increasingly cost and time sensitive. Careful attention to methodologies that can identify and advance the most critical dosimetric measurements, either direct or surrogate, are needed to ensure successful incorporation of PDT into niche clinical procedures.

Photodynamic nanomedicine in the treatment of solid tumors: perspectives and challenges

Photodynamic therapy (PDT) is a promising treatment strategy where activation of photosensitizer drugs with specific wavelengths of light results in energy transfer cascades that ultimately yield cytotoxic reactive oxygen species which can render apoptotic and necrotic cell death. Without light the photosensitizer drugs are minimally toxic and the photoactivating light itself is non-ionizing. Therefore, harnessing this mechanism in tumors provides a safe and novel way to selectively eradicate tumor with reduced systemic toxicity and side effects on healthy tissues. For successful PDT of solid tumors, it is necessary to ensure tumor-selective delivery of the photosensitizers, as well as, the photoactivating light and to establish dosimetric correlation of light and drug parameters to PDT-induced tumor response. To this end, the nanomedicine approach provides a promising way towards enhanced control of photosensitizer biodistribution and tumor-selective delivery. In addition, refinement of nanoparticle designs can also allow incorporation of imaging agents, light delivery components and dosimetric components. This review aims at describing the current state-of-the-art regarding nanomedicine strategies in PDT, with a comprehensive narrative of the research that has been carried out in vitro and in vivo, with a discussion of the nanoformulation design aspects and a perspective on the promise and challenges of PDT regarding successful translation into clinical application.

Switchable bactericidal effects from novel silica-coated silver nanoparticles mediated by light irradiation

Here we report on the triggering of antibacterial activity by a new type of silver nanoparticle coated with porous silica, Ag@silica, irradiated at their surface plasmon resonant frequency. The nanoparticles are able to bind readily to the surface of bacterial cells, although this does not affect bacterial growth since the silica shell largely attenuates the intrinsic toxicity of silver. However, upon simultaneous exposure to light corresponding to the absorption band of the nanoparticles, bacterial death is enhanced selectively on the irradiated zone. Because of the low power density used for the treatments, we discard thermal effects as the cause of cell killing. Instead, we propose that the increase in toxicity is due to the enhanced electromagnetic field in the proximity of the nanoparticles, which indirectly, most likely through induced photochemical reactions, is able to cause cell death.

Photodynamic therapy of cerebral glioma – A review Part II – Clinical studies

Photodynamic therapy (PDT) is a binary treatment modality that has been used to treat malignant brain tumours for 25 years. The treatment involves the selective uptake of a photosensitizer (PS) by the tumour cells followed by irradiation of the tumour with light of the appropriate wavelength to excite and activate the PS resulting in selective tumour destruction and is a potentially valuable adjunct to surgical excision and other conventional therapies. PDT has undergone extensive laboratory studies and clinical trials with a variety of PS and tumour models. These are discussed with reference mainly to clinical studies involving the PDT of brain tumours.

A randomised, double-blind, placebo-controlled trial of photodynamic therapy using 5-aminolaevulinic acid for the treatment of cervical intraepithelial neoplasia

Photodynamic therapy (PDT) using topical 5-aminolaevulinic acid (ALA) has been used to treat histologically confirmed cervical intraepithelial neoplasia (CIN-I and -I/II) in a randomised, double-blind, placebo-controlled protocol. Fluorescence microscopy revealed that topical application of 3% ALA in Intrasite Gel to the cervix for 3 hr resulted in the accumulation of protoporphyrin IX in the cervical epithelium. Treatment of CIN with ALA-PDT was well tolerated, with only 3/12 patients in the PDT arm (0/13 in the placebo arm) reporting any discomfort during illumination. Histologic examination of the treated tissue following loop excision 3 months post-PDT indicated that 33% of patients had no evidence of CIN, 42% had no change in the grade of their disease, whilst 25% exhibited an apparent progression of disease. In the control group, the respective figures were 31%, 38% and 31%. There was no significant difference in response between the groups receiving ALA-PDT and those receiving placebo treatment.

Online electrochemical monitoring of nitric oxide during photodynamic therapy

Photodynamic therapy (PDT), as a novel treatment modality, is based on the use of a photosensitizing agent with an excitation light source for the treatment of various malignancies. Its effect is mediated through reactive oxygen species and nitric oxide (NO), which are shown to be present in apoptosis. Individual differences among patients and even in different areas of the same tumor in one patient may cause a major problem with PDT: dose calculation during application of the light. An electrochemical sensor is proposed for online monitoring of NO generation as a solution of this problem. 5-Aminolevulinic acid (ALA) was administered as the photosensitizer in rat cerebellum. An amperometric sensor, selective to NO, was designed and tested both in vitro and in vivo during PDT. ALA-mediated PDT resulted in rapid generation of NO, starting as early as the application of light on the tissue. Simultaneous amperometric recordings have been carried out for 5 min during PDT. The progressive increase in NO concentration peaked at 1.10 min and then the response current began to decrease until it reached a plateau at around 70% of its peak value. This study, for the first time, electrochemically demonstrates the generation of NO during PDT. Rapid and stable responses obtained by the experimental setup confirmed that this method could be used as an online monitoring system for PDT-mediated apoptosis.

Monte Carlo simulations for EndoBronchial Photodynamic Therapy: the influence of variations in optical and geometrical properties and of realistic and eccentric light sources

BACKGROUND AND OBJECTIVE

Light dosimetry for endobronchial photodynamic therapy is not very advanced to date. This study investigates the dependency of the fluence rate distribution in the bronchial wall on several parameters.

STUDY DESIGN/MATERIALS AND METHODS

A Monte Carlo model is employed for the illumination of a cylindrical cavity by a linear diffuser to compute the fluence rate distribution in the tissue. The influence of optical and geometrical properties (e.g., the absorption coefficient of the bronchial mucosa and the diameter of the treated lumen) have been investigated, as well as the consequences of varying output characteristics of the diffusers. The optical properties used are those of ex vivo pig bronchial mucosa.

RESULTS

With on-axis linear diffusers that can be modelled as a row of isotropic point sources, a constant fluence rate buildup factor can be employed for varying diffuser lengths and lumen diameters. Extreme off-axis placement of the diffuser causes a highly variable, considerable increase in the maximum fluence rate as well as a highly asymmetrical fluence rate profile on the circumference of the illuminated lumen. The fluence rate profiles resulting from illumination with realistic diffusers can be evaluated by implementing the measured radiance profiles of these diffusers in the model. The changes in fluence rate caused by variations in the optical properties of the bronchial mucosa could be accounted for by diffusion theory. This relationship can be used to extrapolate the ex vivo results to the clinical situation.

CONCLUSION

A set of practical rules of thumb is presented that can help to estimate fluence rate distributions in clinical practice.

An interstitial light assembly for photodynamic therapy in prostatic carcinoma

OBJECTIVE

To develop an interstitial laser light delivery system using multiple optical fibres for photodynamic therapy (PDT) in the treatment of prostate cancer.

PATIENTS AND METHODS

A laser beam was divided equally with a 1 x 4 fibre splitter to deliver PDT simultaneously through four 2-cm long, flexible cylindrical optical diffusers. Biplanar transrectal ultrasonography (TRUS) and a template were used to position the optical fibres percutaneously. In vivo measurements of light penetration depth (1/micro[eff] ) in prostate tissue were made in seven patients, using a sheathed isoprobe to measure light fluence rates at varying radial distances from the diffuser. The prostate was fixed with stabilization needles to minimize displacement during needle placement.

RESULTS

The mean (sd, range) micro(eff) in the prostates of the seven patients was 0.35 (0.07, 0.22-0.44) mm-1, which produced closely parallel slopes of light attenuation. However, there was up to a 10-fold variation in absolute light levels at the same diffuser-detector separation distances amongst the seven patients, probably caused by blood pooling around the diffuser light source. A similar problem around the isoprobe detector was overcome by sheathing the probe in clear plastic tubing. By stabilizing the prostate, the optical fibre positioning was precise to within 2 mm.

CONCLUSION

Although this light delivery and TRUS assembly were developed for clinical PDT in the prostate, the same instrumentation can be used reliably for in vivo light-penetration studies. Haemorrhage was unpredictable and highlighted one of the main problems which needs to be overcome.

Short- and long-term normal tissue damage with photodynamic therapy in pig trachea: a fluence-response pilot study comparing Photofrin and mTHPC

The damage to normal pig bronchial mucosa caused by photodynamic therapy (PDT) using mTHPC and Photofrin as photosensitizers was evaluated. An endobronchial applicator was used to deliver the light with a linear diffuser and to measure the light fluence in situ. The applied fluences were varied, based on existing clinical protocols. A fluence finding experiment with short-term (1-2 days) response as an end point showed considerable damage to the mucosa with the use of Photofrin (fluences 50-275 J cm(-2), drug dose 2 mg kg(-1)) with oedema and blood vessel damage as most important features. In the short-term mTHPC experiment the damage found was slight (fluences 12.5-50 J cm(-2), drug dose 0.15 mg kg(-1)). For both sensitizers, atrophy and acute inflammation of the epithelium and the submucosal glands was observed. The damage was confined to the mucosa and submucosa leaving the cartilage intact. A long-term response experiment showed that fluences of 50 J cm(-2) for mTHPC and 65 J cm(-2) for Photofrin-treated animals caused damage that recovered within 14 days, with sporadic slight fibrosis and occasional inflammation of the submucosal glands. Limited data on the pharmacokinetics of mTHPC show that drug levels in the trachea are similar at 6 and 20 days post injection, indicating a broad time window for treatment. The importance of in situ light dosimetry was stressed by the inter-animal variations in fluence rate for comparable illumination conditions.

Photodynamic therapy

Photodynamic therapy involves administration of a tumor-localizing photosensitizing agent, which may require metabolic synthesis (i.e., a prodrug), followed by activation of the agent by light of a specific wavelength. This therapy results in a sequence of photochemical and photobiologic processes that cause irreversible photodamage to tumor tissues. Results from preclinical and clinical studies conducted worldwide over a 25-year period have established photodynamic therapy as a useful treatment approach for some cancers. Since 1993, regulatory approval for photodynamic therapy involving use of a partially purified, commercially available hematoporphyrin derivative compound (Photofrin) in patients with early and advanced stage cancer of the lung, digestive tract, and genitourinary tract has been obtained in Canada, The Netherlands, France, Germany, Japan, and the United States. We have attempted to conduct and present a comprehensive review of this rapidly expanding field. Mechanisms of subcellular and tumor localization of photosensitizing agents, as well as of molecular, cellular, and tumor responses associated with photodynamic therapy, are discussed. Technical issues regarding light dosimetry are also considered.

Enhanced photodynamic effects using fractionated laser light

Photodynamic eradication of tumour cells depends on the presence of a photosensitizer and light delivery to the cells. The present study investigated the influence of fractionated light (on-off mode) on cell killing as documented by a colony-forming assay. Photosensitizers were m-THPC (ethanol soluble, Foscan) and m-THPC-MD (water soluble, both from Scotia Pharmaceuticals, Guildford, UK). Fractionated laser light at a wavelength of 652 nm with a light duration of 0.05 s was more effective than continuous illumination at the same power density for both photosensitizers. We propose that fractionated laser light is more toxic due to short phases of recovery during the dark intervals, probably resulting in more singlet oxygen under these conditions. By use of Foscan, for example, and fractionated laser light, a similar effect is expected for the treatment of solid tumours. In this case we expect improvements in photodynamic therapy (PDT) for patients by lowering the concentrations of photosensitizer and/or by reducing the applied light dose.

Applicator for light delivery and in situ light dosimetry during endobronchial photodynamic therapy: First measurements in humans

This paper presents a design of an applicator for light delivery and light dosimetry during endobronchial photodynamic therapy (EB-PDT). The design incorporates a linear diffuser that is fixed in the centre of the lumen by a steel spring basket that does not block air flow. An isotropic light detector is included in this design, to measure the light fluence actually delivered to the bronchial mucosa surface. The applicator is designed for use with common bronchoscopy equipment, and can be used with bronchoscopes with a large biopsy channel ( approximately 3 mm). The first clinical measurements were performed and caused no additional discomfort to the (nonphotosensitized) patients. The data showed considerable inter-patient variability of the light fluence rate measured as a result of fixed output power of the diffuser. This fact and the expected strong dependence of the fluence rate on the lumen diameter stress the importance of in situ fluence rate measurement for a proper evaluation of the relationship between light fluence and the biological response of EB-PDT.

Performance of a light applicator for photodynamic therapy in the oral cavity: calculations and measurements

Photodynamic therapy (PDT) is an experimental therapy for the treatment of superficial cancer using laser light. In PDT a uniform light distribution is usually required for an optimal therapeutic effect. To irradiate part of the oral cavity uniformly for PDT, two prototype applicators were built, each for a different lower jaw. The applicators made use of the light transmitted through the wall of a cavity of diffusely reflecting material. The radiant exitance of the applicators was measured at 632.8 nm. For the radiant exitance of M of the two applicators, a uniformity ratio, UR = Mmax/Mmin < 2 was found, which was considered reasonable for clinical applications. Calculations predict that the UR can be improved by decreasing the thickness of the cavity wall.

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.

Photophysical and photobiological processes in the photodynamic therapy of tumours

Photodynamic therapy (PDT) is an innovative and attractive modality for the treatment of small and superficial tumours. PDT, as a multimodality treatment procedure, requires both a selective photosensitizer and a powerful light source which matches the absorption spectrum of the photosensitizer. Quadra Logic's Photofrin, a purified haematoporphyrin derivative, is so far the only sensitizer approved for phase III and IV clinical trials. The major drawbacks of this product are the lack of chemical homogeneity and stability, skin phototoxicity, unfavourable physicochemical properties and low selectivity with regard to uptake and retention by tumour vs. normal cells. Second-generation photosensitizers, including the phthalocyanines, show an increased photodynamic efficiency in the treatment of animal tumours and reduced phototoxic side effects. At the time of writing of this article, there were more than half a dozen new sensitizers in or about to start clinical trials. Most available data suggest a common mechanism of action. Following excitation of photosensitizers to long-lived excited singlet and/ or triplet states, the tumour is destroyed either by reactive singlet oxygen species (type II mechanism) and/or radical products (type I mechanism) generated in an energy transfer reaction. The major biological targets of the radicals produced and of singlet oxygen are well known today. Nucleic acids, enzymes and cellular membranes are rapidly attacked and cause the release of a wide variety of pathophysiologically highly reactive products, such as prostaglandins, thromboxanes and leukotrienes. Activation of the complement system and infiltration of immunologically active blood cells into the tumorous region enhance the damaging effect of these aggressive intermediates and ultimately initiate tumour necrosis. The purpose of this review article is to summarize the up-to-date knowledge on the mechanisms responsible for the induction of tumour necrotic reactions.

Photodynamic therapy in lung cancer. A review

Photodynamic therapy (PDT) in lung cancer was introduced in 1980 to treat tumours located in the major airways. After systemic injection of photosensitizers, tumour illumination is performed using a laser fibre to transmit light of a specific wavelength. PDT can be performed under local anaesthesia using the flexible fibreoptic bronchoscope. Skin photosensitivity is the most important treatment morbidity caused by the prolonged cutaneous retention of photosensitizer molecules. Ample data have shown that PDT is effective in obtaining tumour necrosis, but the skin photosensitivity issue limits its palliative potential. Moreover, competing bronchoscopic techniques, such as electrosurgery, Nd-YAG laser and brachytherapy, are available and seem to be equally palliative for the debulking of intraluminal obstructive lung tumours. The curative potential of PDT in patients with roentgenologically occult lung cancer is the most interesting aspect of this treatment modality. A significant number of patients with lung cancer have limited pulmonary function. A normal tissue sparing treatment such as PDT may provide an alternative, as patients may also have subsequent multiple lung cancer primaries. Since early lung cancer detection is now becoming feasible, PDT may be applied to treat roentgenologically occult tumours with curative intent. This may optimize treatment efficacy in the near future.

Detection of early stages of carcinogenesis in adenomas of murine lung by 5-aminolevulinic acid-induced protoporphyrin IX fluorescence

Administration of the heme precursor 5-aminolevulinic acid (ALA) leads to the selective accumulation of the photosensitizer protoporphyrin IX (PpIX) in certain types of normal and abnormal tissues. This phenomenon has been exploited clinically for detection and treatment of a variety of malignant and nonmalignant lesions. The present preclinical study examined the specificity of ALA-induced porphyrin fluorescence in chemically induced murine lung tumors in vivo. During the early stages of tumorigenesis, ALA-induced PpIX fluorescence developed in hyperplastic tissues in the lung and later in early lung tumor foci. In early tumor foci, maximum PpIX fluorescence occurred 2 h after the administration of ALA and returned to background levels after 4 h. There was approximately a 20-fold difference in PpIX fluorescence intensity between tumor foci and the adjacent normal tissue. The specificity of ALA-induced fluorescence for hyperplastic tissues and benign tumors in lung during tumorigenesis suggests a possible use for this fluorochrome in the detection of premalignant alterations in the lung by fluorescence endoscopy. Two non-small cell lung cancer cell lines developed ALA-induced PpIX fluorescence in vitro. These lines exhibited a light-dose-dependent phototoxic response to ALA photodynamic therapy (PDT) in vitro. Because PpIX is a clinically effective photosensitizer for a wide variety of malignancies, these results support the possible use of ALA-induced PpIX PDT for lung cancer.

Calibration of isotropic light dosimetry probes based on scattering bulbs in clear media

Miniature light detectors with isotropic response (isotropic light probes) permit quantitative measurement of light energy fluence rates in turbid media such as biological tissues. These isotropic probes are, for example, applied in photodynamic therapy to correlate light fluence in tissue with (tumour) tissue response, in vitro and in vivo. After description of its construction, two methods of calibration of an isotropic probe in air are discussed, in collimated and in diffuse light. The probe was first calibrated in air in collimated light, after which its response to diffuse light was checked in a flat and in a spherical geometry. Subsequently, the probe's response to collimated light in clear media, for example, water or glycerine which have refractive indices larger than that of air, has been established experimentally. The diffusion approximation to the transport equation in a simple spherical geometry has been used to calculate the probe's response as a function of the refractive index of clear media. The extent of agreement between theory and experiment indicates that the physical mechanisms are understood and indirectly validates the theoretical models.

Light distribution by linear diffusing sources for photodynamic therapy

The distribution of the light emitted by linear light diffusers commonly employed in photodynamic therapy (PDT) has been investigated. A device is presented which measures the angular distribution of the exiting light at each point of the diffuser. With these data the fluence rate in air or in a cavity at some distance from the diffuser can be predicted. The results show that the light is scattered from the diffuser predominantly in the forward direction. Experiments and calculations show that the fluence rate in air and in a cavity of scattering tissue at some distance from the diffuser has a maximum near the tip of the diffuser, instead of near the middle. However, the fluence rate resulting from an interstitial diffuser in a purely scattering tissue phantom shows a maximum in the bisecting plane of the diffuser as would be predicted when the diffuser emitted light isotropically. The scattering nature of the tissue is expected to cancel the anisotropy of the diffuser.

Use of sheet color filters for video-endoscopic observation during intraluminal photodynamic therapy

BACKGROUND AND OBJECTIVE

Photodynamic therapy (PDT) is currently evaluated in clinical studies for the treatment of bronchial and oesophageal tumors.

STUDY DESIGN/MATERIALS AND METHODS

Usually, a cylindrical diffuser is entered into the lumen via a flexible endoscope. Subsequently, the diffuser is positioned at the tumor location under direct- or video-endoscopic vision and manually kept in position by the clinician during the treatment. However, video endoscopes are saturated (overexposure) due to the intense light from the diffuser tip and scattered light from the tissue. This hinders continuous monitoring of the diffuser position.

RESULTS

A simple color filter sheet, suitable for use with endoscopes with removable CCD-video head, appeared to be very effective in improving video endoscopic monitoring during treatment.

CONCLUSION

The standard fiberoptic endoscopes with the accessory CCD video head are more suitable for PDT treatment monitoring than modern endoscopes with the integrated CCD camera.

Ex vivo light dosimetry and Monte Carlo simulations for endobronchial photodynamic therapy

The light distribution during photodynamic therapy of the bronchial tree has been estimated by measuring the fluence rate in ex vivo experiments on dissected pig bronchi. The trachea was illuminated (630 nm) with a cylindrical diffuser and the fluence rate was measured with a fibre optic isotropic probe. The experiment with the diffuser on the central axis was also simulated with Monte Carlo techniques using the optical properties that were determined with a double-integrating-sphere set-up. The results from ex vivo experiments and the Monte Carlo simulations were found to agree within the error of measurement (15%), indicating that the Monte Carlo technique can be used to estimate the light distribution for varying geometries and optical properties. The results showed that the light fluence rate in the mucosa of the tracheal tract may increase by a factor of six compared to the fluence rate in air (in the absence of tissue). This is due to the scattering properties of the tissue and the multiple reflections within the cavity. Further ex vivo experiments showed that the positioning of the diffuser is critical for the fluence rate in the lesion to be treated. When the position of the diffuser was changed from the central axis to near the lesion, the fluence rate in the mucosa increased significantly by several orders of magnitude as compared to the initial (central) illumination. The inter- and intraspecimen variations in this increase were large (+/- 35%) because of variations in optical and geometrical properties and light source positioning, respectively. These variations might cause under- or overdosage resulting in either insufficient tumour necrosis or excessive normal tissue damage.

Three-dimensional characterization of the light distribution from diffusing cylindrical optical-fiber tips

A rapid and sensitive technique for quantitatively examining the three-dimensional light distribution from optical-fiber diffusers for photodynamic therapy applications was developed. We have used this system to evaluate the longitudinal and azimuthal uniformity of the light output from cylindrical diffusing tips and have determined a standard detector configuration for assessing the optical quality of these diffusers. As an example of the sensitivity and the accuracy of this method, the effect of microbending of the fiber on the output intensity distribution, as well as the effect of the choice of laser used, was investigated and was seen to have a significant impact on the uniformity of these diffusers.