Breast phantom for ion chamber dosimetry.

For studies of breast treatment techniques, a water phantom has been developed which allows high accuracy dose measurements using an ion chamber. The phantom is made from the torso of a female mannequin, which is supported so that it can be positioned either supine or prone. The back of the mannequin has been cut away to allow the phantom to be filled with water when positioned prone. The phantom can be simulated, planned, and treated in the manner of a patient. During "treatment" (prone rather than supine), ion chamber measurements can be made at any desired point within (or outside) the treatment volume. The ion chamber support system accurately assigns an (x, y, z) coordinate to each detector position within the phantom.

A test object for evaluation of portal films.

A test object has been designed for evaluation of the image quality of portal films in high energy photon radiation therapy. It consists of a pattern of notched polyvinyl chloride cylinders, fastened to a plastic sheet and immersed in water during exposure. In a pilot experiment, films produced with the test object were evaluated by a panel of observers. The results indicate that the use of the test object simulates the clinical application of portal films well. It is concluded that the test object can simplify studies of the efficacy of various methods to produce and view portal films.

Technique to match mantle and para-aortic fields.

A technique is described to match the mantle and para-aortic fields used in treatment of Hodgkin's disease, when the patient is treated alternately in supine and prone position. The approach is based on referencing the field edges to a point close to the vertebral column, where uncontrolled motion is minimal and where accurate matching is particularly important. Fiducial surface points are established in the simulation process to accomplish this objective. Dose distributions have been measured to study the combined effect of divergence differences, changes in body angulation and setup errors. Even with the most careful technique, the use of small cord blocks of 50% transmission is an advisable precaution for the posterior fields.

Design and characteristics of a facility for total-body and large-field irradiation.

A facility for total-body X ray irradiation has been built using two 4 MV linear accelerators, one supported from the ceiling and one placed in a floor pit. The maximum distance between the sources is 410 cm. The patient lies supine on a light, movable support with a stretched-canvas top, halfway between the sources where the field size is 80 X 220 cm2. A special flattening filter maintains the doserate variation in air within +/- 3% over the central 70 X 200 cm2 of the field, but some quality variations within the beam are noticeable. The doserate at 205 cm distance from the sources is variable between 0.05 and 0.8 Gy/min (half from each source). To permit treatment of large fields at higher doserates, the accelerators can be moved vertically to place the sources at 120 cm or 160 cm from the patient's midplane. For this purpose, independently movable collimators are provided and the flattening filter is designed to provide two options, one for the large total-body field and the other with less filtration covering a smaller solid angle. At 120 cm distance, each beam can provide a doserate of up to 1.1 Gy/min.

The effect of flattening filter design on quality variations within an 8-MV primary x-ray beam.

Single material flattening filters supplied by manufacturers of medical linear accelerators are designed to produce the desired primary dose profile while maintaining output at a maximum level. This design criterion tends to produce substantial quality variations within the primary beam. Quality variations, as expressed by half-value layer in brass and polystyrene, were measured for an 8-MV primary beam both unfiltered and with the flattening filter supplied by the manufacturer. Most of the quality variation was introduced by the filter. Two approaches were then used to reduce this quality variation, each at a cost of a 25% reduction in output. First, a hardening filter was added to the manufacturer's flattening filter. The second approach was to design a new composite flattening filter made from brass and lead. For both approaches, the increase in quality variation over the intrinsic (no filter) variation was reduced by one-half.

Determination of effective electron source size using multislit and pinhole cameras.

Two independent methods have been utilized for determination of effective source sizes for 6, 12, and 20 MeV electron beams generated by a Varian 2100C linear accelerator. First, a multislit camera has been constructed using parallel aluminum plates and plastic strip spacers, similar to the beam-spot camera for the photon source imaging. Second, pinhole imaging was performed using a lead plate with a small hole on the central axis of the beam. The plate thickness and the hole diameter varied with electron energy. The cameras were positioned directly at the opening of the movable photon collimator. The size of the source distribution from each camera was characterized by its full width at half-maximum (FWHM) value. The measured values of FWHM are different for each camera because of their different imaging principles. For the multislit camera, the measured FWHM values were (6.3 +/- 0.4) cm for the 6 MeV beam, (3.6 +/- 0.4) cm for 12 MeV, and (2.7 +/- 0.4) cm for 20 MeV. For the pinhole camera the measured values of FWHM were (7.9 +/- 0.6) cm for 6 MeV, (4.5 +/- 0.4) cm for 12 MeV, and (3.0 +/- 0.4) cm for the 20 MeV beam. Additionally, the effective source position was derived from output measurements at different values of the SSD, which were fitted to the inverse square law.

Evaluation of a beam-spot camera for megavoltage x rays.

A beam-spot camera for measurements of x-ray focal spots of accelerators has been evaluated. The device consists of closely packed 0.25-mm-thick lead and 0.25-mm-thick cardboard strips. It is placed on radiographic film with the lamellae parallel to the beam axis and an exposure made. The images were scanned with a microdensitometer. The results indicate that the broadening of the source intensity profile at half maximum is of the order 1 mm, which permits the use of the beam-spot camera for acceptance testing and quality control. Longer tails in the density profile limit the quantitative information that can be extracted from the images.

Comparison of calculated and measured heterogeneity correction factors for 125I, 137Cs, and 192Ir brachytherapy sources near localized heterogeneities.

The influence of tissue and applicator heterogeneities on brachytherapy dose distributions is not well understood, despite widespread use of shielded applicators in intracavitary therapy. Heterogeneity correction factors (HCF) have been measured using a silicon diode detector arising from bounded heterogeneities consisting of lead, steel, titanium, silver, aluminum, and air cylinders near brachytherapy sources of 125I, 137Cs, and 192Ir. In addition, transverse-axis dose distributions for the three sources in homogeneous water were measured for distances of 0.2 to 16.0 cm. For each point of measurement, relative diode readings were simulated by a Monte Carlo photon transport code utilizing accurate models of the source internal structure, the experimental measure geometry and the source-strength calibration geometry. Comparison of measured and calculated HCF's reveals excellent agreement (1%-3% average) over a wide range of materials, diameters, and thicknesses. In addition, Monte Carlo simulation not only accurately reproduced the relative transverse-axis dose distributions in homogeneous medium, but was able to predict the variation of diode response with photon energy with an accuracy of 3% over the range of 30-662 keV. Our measurements demonstrate that HCF's vary by as much as 60%-100% with distance and heterogeneity diameter for a fixed thickness. Finally, silicon diode measurements of HCF (denied as reading with heterogeneity/reading in homogeneous medium) is shown to lead to errors of 5%-30% for 137Cs and 192Ir sources in the presence of high-atomic number shielding materials. This paper concludes, that Monte Carlo simulation is a powerful, convenient and accurate tool for investigating the long-neglected area of brachytherapy heterogeneity corrections.

Daily check instrument for photon and electron beam quality assurance of medical linacs.

An instrument for daily beam checks of medical accelerators is described. Using three silicon diodes, the instrument provides information about calibration, beam symmetry, flatness, and energy variation for both photon and electron beams. Operating characteristics of the instrument and the electronic circuit design permit simple and reliable measurements.