Potential therapeutic misadministration due to inappropriate electron beam field shaping.

Lead or cerrobend blocking strips are used to shape electron treatment fields when an appropriate custom insert is not available. For the Varian 2100C accelerator, the structural supports of the electron applicators impede the free placement of these field-shaping strips on the open custom insert frame while placement at the top of the applicator is unimpeded. We have investigated the dosimetric ramifications of placing field shaping strips at the top level of the 15x15 applicator for 6, 9, and 16 MeV electrons. Our results demonstrate as much as a 30% dose decrease and 2 cm penumbral increase when this is done compared to field shaping at the insert level. The magnitude of this dosimetric error qualifies as a therapeutic misadministration in many states depending on how many treatments are delivered in this manner. Based on this finding, we recommend that routine use of lead strip blocking be discouraged in favor of custom inserts due to the potential for inappropriate placement on some linear accelerators.

Code of practice for brachytherapy physics: report of the AAPM Radiation Therapy Committee Task Group No. 56. American Association of Physicists in Medicine.

Recommendations of the American Association of Physicists in Medicine (AAPM) for the practice of brachytherapy physics are presented. These guidelines were prepared by a task group of the AAPM Radiation Therapy Committee and have been reviewed and approved by the AAPM Science Council.

Optimization of arc angles in rotational therapy.

Data is presented that will aid in optimizing the arc angles used for rotational therapy for a 6 Mv x-ray beam. Computer calculations of isodose distributions were carried out and the Ap and Lateral dimensions of the 95 and 100% isodose line were tabularized as a function of the field width (6-14 cm), distance between isocenter and patient midline (0-3 cm), and bilateral 120 or 100 degrees arc angle. The 300 degrees rotation cases we studied were generally found to be inferior to 120 degrees or 100 degrees bilateral arc rotations. 120 degrees arcs are best when the target center is within 2 cm of the patient's Ap center. As that separation increases, the bilateral arc angle should be decreased or wedges should be rotated through 300 degrees to avoid maximum doses greater than 105%. The data tables can be used as a starting point for computer calculations to select the field width and the arc angle for a particular target location and size. Of note is the degree to which the dimensions of the 100% isodose line decreases for small field widths and large isocenter to patient midline distances.

Variability of heating patterns in animals by magnetic induction hyperthermia.

In a test of electromagnetic induction hyperthermia to deep viscera of a live dog model, we found that heating was not uniform to any depth, but was quite variable. In general, there was a thermal gradient between peripheral and central portions of the transposed spleen of about 1 degree C. Though heat generation within the abdomen was not uniform, its temperature pattern in the alive animal resulted in significant heating of that part of the organ that had been surgically placed at the center of the animal. This heating could not be explained by perfusion with regionally heated core blood. Our results indicate that extensive investigations in living systems and complex dynamic phantoms will be necessary before individual patient response can be predicted.

The pharmacologic manipulation of blood flow in hyperthermia therapy.

Many human tumors treated by hyperthermia do not reach therapeutic temperatures (42 degrees C). The explanation for this difference may be that some tumors react to thermal stress in a manner similar to normal tissues; ie, they increase blood flow during hyperthermia in order to dissipate the heat. Higher temperatures might be achieved in these heat-resistant tumors by administering vasoconstrictive agents in an effort to reduce blood flow. In this preliminary study, we determined the extent to which pharmacologic inhibition of local blood flow might allow higher temperatures to develop in normal muscles exposed to localized radiofrequency hyperthermia. We found that the local muscle temperature rise could be increased by at least 90% in two dogs and six rabbits with the use of a local vasoconstrictive drug.

Single- and double-plane iridium-192 interstitial implants: implantation guidelines and dosimetry.

Computerized dosimetric studies of single- and double-plane iridium-192 (Ir-192) planar implants were performed. With respect to dose homogeneity, we found that the optimal source and ribbon separation for single-plane implants was 1.0 cm. For double-plane implants, the preferred ribbon and plane separation was 1.5 cm, maintaining a 1-cm separation for the sources. Using these separations, standard dose rate curves for single- and double-plane Ir-192 implants were generated by computer calculations. These standard curves are useful for quickly and fairly accurately estimating the dose from any size planar implant, without requiring more time-consuming individual computer dosimetry. We believe that the curves will prove to be of practical clinical value to physicists and radiotherapists.

Blood flow in human tumors during hyperthermia therapy: demonstration of vasoregulation and an applicable physiological model.

A quantitative assessment of the effect of localized magnetic-loop hyperthermia on blood flow was performed in 12 human tumors using the 133Xe clearance method. Because blood flow in these tumors changed in response to needle injection, a physiologically based, one-compartment model was developed that included both a hyperemic and a steady-state component. In six tumors, changes in blood flow induced by heat were also observed. The ability of tumor vessels to respond dynamically to stress and the degree of response may be predictive of tumor heating capacity and subsequent therapeutic response.