A totally integrated simulation technique for three-field breast treatment using a CT simulator.

A method was devised to simulate patients with breast cancer in the actual treatment position utilizing a diagnostic CT spiral scanner, 3-D Image Workstation for virtual simulation, and a laser coordinate system to transfer planning parameters to the patient's skin. It was desired to produce non-divergent tangential beams through the lung as well as a matched line for tangential and supraclavicular fields. The patients were immobilized in an Alpha CradleTM cast. Radio-opaque markers were placed on the superior, inferior, medial, and lateral margins of the field so as to afford appropriate initial field set-up approximations. The patient was scanned. The data set was then transferred to the workstation where an isocenter was chosen. The patient was marked. Virtual simulation was then performed. This method employed a half beam technique for the posterior edge of the tangential fields. Table rotation and blocking of the superior margin of the tangential fields were used to produce a vertical edge to match a supraclavicular field. Using a beam's eye view the lateral tangent was matched to the medial exit. A digitally reconstructed radiograph was created to define the tangent fields and place the supraclavicular block. Our initial experience with 50 patients verifies that this is a reproducible and accurate technique. Time required for immobilization and tangential field simulation is approximately 30 minutes. Data is available for 3-D treatment planning or 2-D treatment planning on a reconstructed transverse slice angled to match the collimator angle through the patient. Using a CT simulator for simulation of breast cancer affords accuracy of at least equal magnitude to conventional simulators as determined by beam films and ease of set-up. This technique also affords greater ease in changing treatment parameters without having to resimulate the patient.

Laminated primary ceiling barriers for medical accelerator rooms.

In this work the performance of a number of laminated shields used as the primary ceiling barrier for high-energy accelerator rooms was investigated. The neutron and photon dose equivalent rates were measured outside each shield. Based on the measured photon level and the calculated x-ray leakage level the dose equivalent rate due to photons produced by neutron interactions with the shield materials was estimated. Shielding parameters for polyethylene were established for photons and neutrons. It was found that the barriers designed using the techniques given in an NCRP Report suffered from excessive radiation leakage.

An adjustable collimator for stereotactic radiosurgery.

Stereotactic radiosurgery utilizing a linear accelerator is now carried out using small non-coplanar beams to produce a high-dose volume that is approximately spherical in shape. It is desirable to shape the high-dose region so that it is similar to the target volume thereby reducing the amount of healthy tissue irradiated by a high dose. An adjustable collimator has been developed that can be employed to control the target volume height and diameter. Based on film dosimetry and measurements made with a small parallel-plate ionization chamber it was determined that the beams produced by the collimator have properties that make them suitable for radiosurgery. The basic dosimetry parameters (output factors and tissue maximum ratios, TMR) needed for patient treatment were evaluated. It was found that the diameter and length of the high-dose region produced by the patient rotation method are related to the light-field size at 100 cm from the x-ray target. A more favourable dose distribution was found for the adjustable collimator as compared to treating the same volume by use of the two-target method.

Evaluation of neutron dose equivalent levels at the maze entrance of medical accelerator treatment rooms.

This paper evaluates the accuracy of Kersey's method of calculation of the dose level due to photoneutrons at the maze entrance of medical accelerators. In this study measurements and calculations of the neutron dose equivalent were made for 13 medical accelerator facilities. The group of accelerators was composed of four different models from two manufacturers. The maze length for the treatment rooms varied from 3.0 to 8.54 m and 10 of the mazes had a single 90-deg turn with the remaining having two 90-deg turns. Moderated activation detectors and a portable neutron remmeter were used for measurements in the treatment room and maze, respectively. It was found that the maximum disagreement between the measured and calculated values was a factor of 2.3 with the calculated value exceeding the measured value. The majority of the measured values were within 25% of the calculated levels. It was concluded that Kersey's method is suitable for use in designing medical accelerator mazes.

A patient rotator for stereotactic radiosurgery.

A new technique for stereotactic radiosurgery by use of a patient rotator is described. Using the rotator with a small collimated beam of 6 MV x-rays, a small well-defined region of the brain can be irradiated to a high dose with rapid fall off of the dose outside the target volume. Since the linear accelerator gantry does not move during therapy the possibility of a collision between the gantry and the patient or stereotactic equipment is eliminated. The system is also independent of the rotational stability of the linear accelerator gantry axis and turntable axis. Dose distributions measured in a Lucite head phantom with film exhibited properties well suited for radiotherapy. Tests carried out to evaluate the ability to irradiate a selected target point within the brain with the rotator system showed a maximum positional error of 1.0 and 2.0 mm for angiography and CT localisation respectively.