Gerhard J. Mueller

Comparing the P3-approximation with diffusion theory and with Monte Carlo calculations of light propagation in a slab geometry

The energy fluence rate in a slab of tissue has been calculated for a monodirectional and unpola- rized incident light beam of infinite diameter perpendicular to the tissue surface. Published values for the optical constants of human dermis are used and the angular dependence of scattering is de- scribed by the Henyey-Greenstein function. Results are given for index matched and index mis- matched situations. The P3-approximation to the transport equation appears to be a considerable improvement over the P1 or diffusion approximation. Another improvement is obtained when a delta function is added to the scattering function. In the diffusion model this is called the delta- Eddington approximation. In the P3-approximation this addition yields a near perfect agreement with results of Monte Carlo calculations. The improvements are particularly apparent at the boundary where the incident beam enters the tissue. The relevance of these results for dosimetry in Photodynamic Therapy is discussed.

Optics of tissue

The optical properties of biological media are in general very complex and they are characterized by local inhomogeneties in the light velocity and absorption. The local variations in the optical ve- locity are due to differences in the optical polarizability between the cells and their surroundings as well as between the various parts of each individual cell.

Spot-size dependence of laser retinal dosimetry

Laser development in opthalmology is ever more using extended sources or optics producing large retinal images. Such applications require a retinal dosimetry for minimizing the possiblity of damage. Some biological data show that present limit values for large image sizes are not safe. A formulation is proposed allowing one to specify an appropriate retinal dosimetry suitable for the use for very large images.

Quantities and units of optical radiation and their measurement

During the SPIE Institue meeting a lot of consideration was given to the Black Box Model, the concept that one has during medical application of laser radiation, a box which contains the biological tissue with all its parameters and properties, very often including even environmental influences. Laser radiation is fed into this box as an input and the output is the medical result. The discussions showed that too much is put into the black box, so that the proper description of the effects taking place in the biological tissue, the basis for their understanding, is not possible. It is therefore important to specify as far and as exactly as possible the parameters which may influence the result of a medical treatment and to use internationally recognized terms and units which are unambiguous, to describe them.

Dosimetry for photo-ablation technique

With short intensive laser pulses a thin layer can be removed from high absorbing non-transparent tissue in a manner which may be compared with a small explosion. In the field of laser medicine this effect is called photoablation.

The need for dosimetry in medical treatment

Prof. Milner has nicely introduced the interrelationship of laser radiation parameters, environmen- tal parameters and tissue parameters which influence the biological tissue result. His analogy of a black box is quite appropriate.

A survey of interaction mechanisms, dosimetry and safety concepts in YAG laser ophthalmic photodisruption

We here review a series of basic concepts of laser-ocular media interaction, useful to establish do- simetry and safety criteria in the use of Nd:YAG nanosecond and picosecond photodisruptors. The role of the laser parameters and of the irradiation geometry is discussed in relation to the effective- ness of the disruption procedure and to the associated potential risks. An overview of temporal and spatial dynamics of plasmas in liquids is presented. Emphasis is also given to the risk of intraocular lens rupture during capsulotomy and vitreal surgery. The significance of the data acquired on mo- dels in assessing the dosimetry of laser photodisruptors is discussed. Finally, general safety con- cepts in the use of these instruments are presented.

How to define dosimetry for laser treatment the black box approach

Using lasers in medicine, we have on one side the laser as a source of electromagnetic radiation in the visible, UV and IR region and on the other side the human absorbing tissue as a target hit from the laser radiation.

Applied dosimetric measurements

Until recently dosimetry in photobiology and phototherapy was restricted to measure the physical parameters of the non-ionizing optical radiation source used, and the biological parameters of the tissue were not taken really into account. After discussing the role of tissue equivalent phantoms in dosimetry some unconventional ideas are presented which, when realized, may lead to dosimetry that renders values expressing not only the physical parameters of the non-ionizing optical radia- tion but also the tissue dependent responses.

Dosimetric concepts for optical radiation

Unlike penetrating ionizing radiation, optical radiation is generally absorbed very superficially. Ex- cept for a narrow band of visible and near-infrared (IR-A) radiation from approximately 400-1400 nm, skin and other biological tissues are nearly opaque to optical radiation. For this reason, volumetric or mass based concepts of absorbed dose are of little value. Additionally, the abosorbed radiant energy is conducted out of the absorbing site and for this reason thermal effects depend largely upon the size and location of the absorbing site as well as exposure and exposure rate. Con- cepts of exposure dose are therefore most useful and practical.