Radiation dose perturbation in the presence of permanent vascular-access injection ports.

Injection ports are commonly used to maintain permanent vascular access in patients with malignant disease. The injection ports allow blood sampling as well as infusion or injection of chemotherapeutic agents directly into the circulation. The access catheters are usually placed in the subclavian vein with the injection port being implanted in the intraclavicular area. These injection ports are entirely self-contained underneath the skin and may have been placed in an area which subsequently requires radiation treatment. A comprehensive study of the perturbation effects of the injection ports when placed in the path of the beam was carried out. All dose measurements were performed using stainless steel and titanium injection ports. The radiation beams were 6 MV and 10 MV X rays and 6, 9, 12, 15, and 18 MeV electrons. The data indicate that the presence of injection ports significantly alters the radiation dose and dose uniformity. For example, the dose underlying a stainless steel port is reduced by 47% when 18 MeV electrons are used. This paper presents the dosimetric data, discusses the clinical significance of the results, and provides recommendations for design modifications of the ports.

A study of effective attenuation coefficient for calculating tissue compensator thickness.

Dose uniformity throughout the treatment volume is essential to precision radiation therapy. Tissue compensators are often used as a means to eliminate dose nonuniformity resulting from surface contour irregularities. This paper evaluates the accuracy of using an effective attenuation coefficient for calculating the thickness of missing tissue. This coefficient is found to vary strongly with thickness of missing tissue when the initial depth is situated in the buildup region. The use of a single attenuation coefficient produces errors as high as 54% in the calculated compensator thickness when 10-MV x rays are used. At depths greater than the depth of maximum dose, the attenuation coefficient remains a function of field size, not the initial depth.

Treatment of cancer of the pancreas by precision high dose (PHD) external photon beam and intraoperative electron beam therapy (IOEBT).

Twenty-five patients with a diagnosis of unresectable adenocarcinoma of the pancreas were explored in the Clement O. Miniger (COMROC) IOEBT operating amphitheater at the Medical College of Ohio. Seventeen were treated with IOEBT (20-30 Gy, 15 or 18 meV electrons) PHD external beam radiation therapy (40-60 Gy, 1.8 Gy per fraction) plus appropriate operative biliary and gastrointestinal bypass procedures. No intraoperative complications were observed. Two patients died of causes that may have been treatment-related. Two patients developed abdominocutaneous fistulae. Pain was ameliorated in eleven of twelve patients. Jaundice was relieved in all patients. Four of ten patients with weight loss showed a reversal of that trend. Patient survival was not significantly different from that of patients treated with high-dose precision therapy alone.

Treatment of cancer of the pancreas by intraoperative electron beam therapy: physical and biological aspects.

Radiation therapy has had a significant and an expanded role in the management of cancer of the pancreas during the last decade. In particular, for locally advanced disease, radiation therapy has improved the median survival of patients to 1 year. Intraoperative electron beam therapy has been applied to unresectable and resectable pancreatic cancer in an attempt to enhance local control of disease and to improve patient survival. This paper presents a survey of the role of radiation therapy in treatment of cancer of the pancreas, provides information on the radiobiological aspects of this treatment modality and details the physical and dosimetric characteristics of intraoperative radiation therapy with electrons. Presented are the design specifics of an applicator system, central axis beam data, applicator parameters, dose distribution data, shielding, treatment planning and means of verification. Emphasis is placed on the collaboration and cooperation necessary for all members of the intraoperative radiation therapy team including surgeons, radiation therapists, medical physicists, anesthesiologists, technologists, and nurses.

Radiation contamination and leakage assessment of intraoperative electron applicators.

In intraoperative radiation therapy, it is critical to reduce the radiation contamination outside the useful field by as much as physically feasible. Additionally, a uniform dose is clinically desirable across the tumor volume. A study of the Medical College of Ohio applicators indicates that the radiation contamination outside the field can be as high as 18% of the central axis dose. The effects of the photon collimator setting on the magnitude and energy of the radiation contamination are discussed and means are presented for reducing this unwanted radiation. The dose nonuniformity across the field is found to be virtually independent of the photon collimator setting and is shown to be mostly due to the transparent applicator wall. The clinical significance of the findings is discussed.

Modified mesh panel for radiation therapy treatment table.

The design and construction of a mesh panel for radiation therapy treatment tables is described. With the new design, sagging is virtually eliminated. The panels have been successfully used for a variety of patient sizes and weights.

A method for calculating megavoltage x-ray dose and dose parameters.

Limitations of the most commonly used dose calculation methodologies as apply to medium and high-energy x rays are discussed. A method is presented whereby the dose to a point in medium is divided into several components with expressions given for various components. The depth of transient-charged particle equilibrium is chosen as the reference point. Equations are derived for calculating and relating the dose parameters including the percent depth dose, the tissue-reference ratio, and the scatter-reference ratio. Particular emphasis is placed on the accuracy and adequacy of the technique in the region of buildup. The applicability of the present method to megavoltage photons is established for a number of x-ray beams in the range 4-45 MeV maximum energy.

Choice of reference point in megavoltage dosimetry.

This work considers the problems associated in megavoltage x-ray dosimetry concepts with the current techniques used for normalizing the various dose parameters. The depth of transient charged particle equilibrium (CPE) is introduced as the optimum choice of the normalization point. Properties of this depth are discussed and its uniqueness as a measure of beam quality is illustrated.

A comprehensive study of LiF TL response to high energy photons and electrons.

Effects of beam quality, cavity size, TLD state, and medium on the response of thermoluminescent dosimeters were studied. LiF TL-700 powder, extruded rods, and ribbons were irradiated with Co-60 gamma rays, 4, 6, 8, 33, and 45 -MV x rays, and 3, 5, 10, 15, 20, 25, 30, 35, 40, and 45-MeV electrons. Relative TL responses were assessed in water and Lucite, and the results compared. The results of this study were compared with those of previously published studies, and certain discrepancies resolved.

Clinical use of thermoluminescent dosimeters in supervoltage x-ray therapy.

A practical method for routine patient dose monitoring when single, multiple, or adjacent fields are irradiated with supervoltage x rays is described. The system uses litium fluoride TL-700 extruded rods in small gelatin capsules. The mode of calculation is described and the experimental data for 33- and 45-MeV energy betatron x rays are presented. An example is given to illustrate the use of this technique (experimental uncertainty, 3%).

Electron beam treatment-planning system.

Some of the physical parameters necessary in electron beam treatment planning are investigated for 5- to 45-MeV electrons extracted from a Brown-Boveri betatron. The percent depth ionization (PDI) curves are compared with the empirical relation given by Laughlin's equation, and an extension of Laughlin's equation is described. The modified absorption coefficient (MAC) method is introduced to correct the isodose distributions for the presence of inhomogeneities, such as lung, with the actual density of lung and the location of the inhomogeneity taken into consideration. Experimental data from measurements made in a water-cork phantom are presented, and the results are compared with the various calculated methods.