A semianalytical method for the design of a linac x-ray beam flattening filter.

The purpose of this study was to design an improved flattening filter for a Therac 20 medical linear accelerator. Profiles of the 18-MV x-ray beam produced by this accelerator measured along the diagonal of a 40 X 40 cm field at a depth of 5 cm were measured, and it was found that there were regions near the corners of the field where the dose was 109% of the central axis dose. An iterative algorithm for designing flattening filters was developed which required, as input, precise measurements of the following data: the unflattened primary beam profile, the fraction of the beam due to contamination radiation arising from interactions of primary photons with the flattening filter and the collimator assemblies, and the attenuation of the primary photons in water and lead as a function of angle from the central axis of the beam. A new flattening filter was designed and profiles of the beam were measured at a number of depths. These measurements showed that the beam was flattened to within +/- 1% out to 24 cm along the diagonal of a 40 X 40 cm field at a depth of 5 cm.

Total attenuation coefficients and scattering phase functions of tissues and phantom materials at 633 nm.

Measurements have been made of the total attenuation coefficient sigma t and the scattering phase function, S(theta), of 632.8 nm of light for a number of animal model tissues, blood, and inert scattering and absorbing media. Polystyrene microspheres of known size and refractive index, for which sigma t and S(theta) can be calculated by Mie theory, were used to test the experimental methods. The purpose of the study was to define typical ranges for these optical properties of tissues, as a contribution to the development of experimental and theoretical methods of light dosimetry in tissue, particularly related to photodynamic therapy of solid tumors. The results demonstrate that, for the representative tissues studied, the total attenuation coefficients are of the order of 10-100 mm-1, and that the scattering is highly forward peaked, with average cosine of scatter in the range 0.6-0.97.

Photodynamic therapy with chloroaluminum sulfonated phthalocyanine in the rat window chamber.

The efficacy of photodynamic therapy tumor destruction is dependent upon both the interruption of the tumor vasculature and the resultant production of unstable oxygen species causing cellular oxidation and death. Chloroaluminum sulfonated phthalocyanine (CASP) is a recently developed photosensitizer. In order to study the direct vascular effects of CASP on a non-tumor system, a rat window chamber was utilized. Twelve rats were implanted with the window chamber, and were divided into two groups of six. Three rats served as controls for each group (receiving light alone, CASP alone, or no treatment). The remaining 6 rats received 10 mg/kg CASP intravenously 4 days after chamber placement. Photoactivation with light was performed 24 hours after injection (power density 200 mW/cm2, irradiance 100 J/cm2, lambda = 675 nm). Utilizing integrating sphere measurements and image analysis, marked vascular changes in the form of initial vasospasm followed by vaso-constriction and loss of chamber neovascularization were noted in the CASP-PDT group. The control groups exhibited no significant changes. Manipulation of the chamber vasculature at strategic time-points may translate into improved response rates for photodynamic therapy in a tumor model.

Monte Carlo modeling of light propagation in highly scattering tissues--II: Comparison with measurements in phantoms.

Measurements of the fluence-depth distributions and of the diffuse reflectance of 633 nm light have been made in liquid media with optical properties similar to soft tissues. The results are compared with predictions of Monte Carlo computer calculations in order to test the adequacy of Monte Carlo modeling of light transport in tissue. Except at extremely high albedo, the experimental data and the Monte Carlo results agree well for the depth dependence of the fluence as a function of incident light beam diameter and optical absorption and scattering, and for the dependence of the diffuse reflectance on the albedo. The absolute experimental values for the fluence must be renormalized by a factor which varies with the albedo in order to match the model values, and the possible sources of this discrepancy are discussed.

Monte Carlo modeling of light propagation in highly scattering tissue--I: Model predictions and comparison with diffusion theory.

Using optical interaction coefficients typical of mammalian soft tissues in the red and near infrared regions of the spectrum, calculations of fluence-depth distributions, effective penetration depths and diffuse reflectance from two models of radiative transfer, diffusion theory, and Monte Carlo simulation are compared for a semi-infinite medium. The predictions from diffusion theory are shown to be increasingly inaccurate as the albedo tends to zero and/or the average cosine of scatter tends to unity.

Q-switched neodymium:yttrium-aluminum-garnet laser treatment of lentigo maligna.

Lentigo maligna is a premalignant lesion of atypical melanocytes that typically arises on the head and neck of elderly patients. It is considered a melanoma in situ with a significant risk for transformation to invasive lentigo maligna melanoma. Surgery is the preferred method of treatment; however, because of the advanced age of the typical patient with lentigo maligna, the frequency of complicating medical problems, and the cosmetic or functional aspects of treatment, surgical excision is not always feasible. The purpose of this pilot study was to evaluate the efficacy and safety of Q-switched neodymium:yttrium-aluminum-garnet laser treatment of lentigo maligna. Eight patients were treated with 532 and/or 1064 nm wavelengths from the laser. All patients showed a response to laser therapy, and 2 patients treated with 1 treatment from each wavelength had complete eradication of the LM, with no evidence of recurrence in 42 months. Further study is warranted, but Q-switched neodymium:yttrium-aluminum-garnet laser is a promising alternative treatment for lentigo maligna.

A comparison of copper vapor and flashlamp pumped dye lasers in the treatment of facial telangiectasia.

METHODS: Twelve adult patients with bilateral facial telangiectasias were treated with two yellow light lasers. A section of each involved area was treated with a copper vapor laser and a similar section in the same patient was treated with the flashlamp pumped dye laser. RESULTS: Both lasers provided satisfactory clearance at 2 and 6 weeks. Treatment was not painless with either laser; however, no patient needed local or general anesthesia. The time taken to treat equivalent areas was similar for both lasers. Postoperative swelling was greater with the flashlamp pumped dye laser than with the copper vapor laser and the time required for healing was longer with the flash-lamp pumped dye laser. CONCLUSION: The larger purpuric macules produced post-operatively by the flashlamp pumped dye laser were less cosmetically acceptable to patients when compared with the thin linear crusting produced by the copper vapor laser. Scarring or textural changes were not seen with either laser.

Efficacy of corneal eye shields in protecting patients' eyes from laser irradiation.

BACKGROUND: The continuing development of new types and applications of lasers has appeared to surpass the development of specific eye protection for these lasers. There are a variety of eye shields on the market, but few are specifically designed for laser protection. OBJECTIVE: Our purpose was to test a variety of eye shields by two parameters, light transmission and temperature rise, and to determine from these measurements the most protective shield for patients. METHODS: We tested four plastic shields, one metal shield, and two sets of tanning goggles for temperature rise and light transmission when irradiated with a beam from a flashlamp-pumped, pulsed-dye laser. RESULTS: The temperature rise at the surface of the shield opposite the laser impacts was no more than 0.2 degree C in any case. White light was transmitted at significant levels through several of the shields, but yellow light transmittance was noted only through the green eye shield. CONCLUSION: Our measurements indicate that all except the green shield appeared safe from transmission of the 585-nm radiant energy. However, the optimal laser eye shield, in our opinion, would be a composite of several different shields' characteristics.

Optical properties of Intralipid: a phantom medium for light propagation studies.

Intralipid is an intravenous nutrient consisting of an emulsion of phospholipid micelles and water. Because Intralipid is turbid and has no strong absorption bands in the visible region of the electromagnetic spectrum, and is readily available and relatively inexpensive, it is often used as a tissue simulating phantom medium in light dosimetry experiments. In order to assist investigators requiring a controllable medium that over a finite range of wavelengths is optically equivalent to tissue, we have compiled previously published values of the optical interaction coefficients of Intralipid, most of which were measured at a wavelength of 633 nm. We have extended the measurements of the absorption and reduced scattering coefficients from 460 to 690 nm and the total attenuation coefficient from 500 to 890 nm. These measurements show that, for stock 10% Intralipid, the absorption coefficient varies from 0.015 to 0.001 cm-1 between 460 and 690 nm, the reduced scattering coefficient varies from 92 to 50 cm-1 between 460 and 690 nm, the total attenuation coefficient varies from 575 to 150 cm-1 between 500 and 890 nm, and the average cosine of scatter varies from 0.87 to 0.82 between 460 and 690 nm. With these data, we discuss the design of an optically tissue-equivalent phantom consisting of Intralipid and black India ink.