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Biomedical subjects

R D Larsen

Publications and source records attributed to R D Larsen.

At least 55 records · Page 3Linked to original sources
PubMed1984

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.

Breast Neoplasms↗
PubMed1983

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.

Hodgkin Disease↗
PubMed1975

The use of wedge filters to improve dose distribution with the partial rotation technique.

Rotational arcs covering less than 360 degrees are frequently used in order to spare critical structures in radiation therapy; however, this results in a lack of dose uniformity in the target volume. Wedge filters can be used to restore dose uniformity in the treatment volume when limited arcs are employed. The authors emphasize the use of small rotational arcs (less than 90 degrees) in treating eccentrically located tumors such as those of the spinal cord and describe a simple model which can be used to calculate the wedge angle as a function of the arc angle.

Dose-Response Relationship, Radiation↗
PubMedDate not supplied

Calculations for beam-flattening filters for high-energy x-ray machines.

A flattening filter is an important component in a medical accelerator to modify the photonbeam properties. To simplify the calculations of the flattening-filter profile, we have developed a computer program which sums primary and scatter and then makes iterations in the primary component to produce a desired total-dose profile. The program can account, to first order, for radial spectral changes by using an effective primary attenuation coefficient which varies with radius. Calculations made to model the Clinac-4 dose profiles using the measured variation of half-value layer with radius show good agreement with the measured data. It is shown that the variation of quality within the beam impairs the flatness that can be achieved over a range of depths. Since perfect flatness cannot be achieved for small and large fields with one flattening filter, one may choose a primary profile which is a compromise over a range of field sizes and depths. A compromise profile for a 4-MV beam is discussed.

Elementary Particles↗
PubMedDate not supplied

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.

Humans↗