The use of storage phosphors for portal imaging in radiation therapy: therapists' perception of image quality.

Two drawbacks in quality of portal radiographs in radiation therapy are their low contrast and low spatial resolution. These are due to the low differential absorption of body tissues at therapeutic energies and to a relatively large radiation source. We used an experimental, high-contrast sensitivity storage phosphor imaging system (Eastman Kodak Co.) to produce portal images. The system consists of a storage phosphor detector, a high-contrast sensitivity laser scanner (12 bit), an image processing module, and a laser printer (12 bit). Patients undergoing radiation therapy treatments had both a conventional portal image and a storage phosphor image taken. Both were displayed side-by-side and were evaluated independently by three radiotherapists according to quality of information to verify the treatment field. Each of the three radiotherapists rated the storage phosphor images to be better (p less than 0.001) than the conventional images. However, rated improvements of low-contrast storage phosphor images of the pelvis and abdomen (40) were significantly lower than those of high-contrast (head, neck, and chest) images (53).

Dosimetric evaluation of a variable field electron beam applicator.

A commercially available variable electron beam applicator designed for the Phillips SL-75/20 linear accelerator was tested for possible clinical use. The basic design of this collimator consists of four movable L-shaped stainless steel blades forming the corners. In addition, four smaller fixed blades of the same material and thickness fill in the sides. Film placed in polystyrene phantoms were exposed in all experiments. Field size, field flatness, beam homogeneity and penumbra measurements, all indicated that the applicator is acceptable for clinical use. However, for use with large field sizes and/or high energy electron beams some minor modifications are suggested.

Photon contamination in 8-20-MeV electron beams from a linear accelerator.

The amount of x-ray contamination near the surface of a phantom irradiated with electron beams was measured directly. Measurements were done to ascertain if photon contamination in the beam contributes a higher dose to the more superficial layers of an irradiated medium than indicated by conventional methods. A 1.4-kG magnetic field was used to deflect the electron beams generated by a Philips SL/75-20 linear accelerator. The electron energies studied were 8, 10, 12, 14, 17, and 20 Mev. After sweeping the electron beam, a significant amount of photon contamination was measured in all cases. The characteristic qualities of the photon contamination were measured directly in a water tank. They were found to agree with those of bremsstrahlung spectra generated in a thin target with a virtual source at the location of the scattering foil.

Fast and slow neutrons in an 18-MV photon beam from a Philips SL/75-20 linear accelerator.

Fast- and slow-neutron contamination in an 18-MV photon beam from a Philips SL/75-20 linear accelerator has been measured. Aluminum and indium foils were activated to determine fast- and slow-neutron fluence, which were largely independent of field sizes. Measured fast-neutron fluences were typically 13.9 X 10(4) and 4.4 X 10(4) neutrons/cm2/rad of x ray inside and 5 cm outside the field, respectively. Slow-neutron fluences, 1.3 X 10(4) neutrons/cm2/rad of x ray, remained relatively constant inside and outside the field. The reported results are about three times higher than neutron fluences recently reported with a betatron operated at the same energy.

Orthogonal fields: variations in dose vs. gap size for treatment of the central nervous system.

Variations in dose at the junction at the lower end of two lateral skull portals and the upper end of a posterior spinal portal used for irradiation of the central nervous system (CNS) were studied using 60Co and 6 MV linear accelerator radiation. The relationship between dose and gap size was measured via film dosimetry. Small variations in gap size resulted in large variations in dose (+/- 40%) at the junction. These variations in dose were more pronounced for the 6 MV accelerator than for the 60Co, but the volume of high and low dose tended to be smaller with the 6 MV linear accelerator.

Computer program for central axis dose calculations for high energy photons.

A computer program is described for central axis dose calculations for 60Co and 6 MV photons. Fixed distance or isocentric calculations for regular or irregular fields can be performed. Corrections are made for wedges and blocking trays. Tissue-air ratios and percentage depth doses are calculated from scatter-air ratios eliminating the need for empirical formulas. Calculated tissue-air ratios and percentage depth doses for elongated fields agree to within 2 per cent with measured data, eliminating the need for tables of equivalent squares. Calculated values for irregular fields also agree to within 2 per cent with measured data.

A treatment accessory for CNS irradiation in children.

A treatment accessory for use in CNS radiotherapy of small children enables the head and spinal fields to be treated while the child lies supine. Children are not moved during therapy which minimizes the problem of gaps between the head and spinal fields.