Clinically shaped fields: relative outputs for Varian multileaf collimators and tray-mounted cerrobend blocks.

The purpose of this study was to determine the variation in relative output factors for fields shaped using a multileaf collimator (MLC) versus tray-mounted cerrobend blocks. A total of 21 different clinical fields from 16 patients were compared by casting cerrobend blocks from the same films that had generated the MLC shapes. Output measurements were made on a Varian 2300CD (26 1 cm leaves per side) in solid water for 6 and 20 MV at source-to-axis-distance (SAD = 100 cm) and at two depths, 5 cm (d5) and the clinically prescribed depth (d(clinical)). All measurements were taken under the central axis of the beam unless the shaped field was obviously skewed off axis or the beam was split. Several cases were repeated on a Varian 2100CD (40 1 cm leaves per side) MLC. For the 21 cases studied, the average overall MLC factor (MLC reading divided by the block reading) was found to be 0.9952 with a standard deviation of 0.0024. Repeat measurements performed on the 2100CD were within 0.2% of the MLC factors measured (for the same fields) on the 2300CD. When the data are broken down in terms of clinical applications (type of case at a clinically prescribed depth and energy), the resulting total clinical average is 0.9957 with a standard deviation of 0.0022. Both the overall average and the total clinical average MLC factors (0.995-0.996) agree with previous literature, which used abstract or generic field shapes. Further investigation of the data from this study may find a correlation between the MLC factor and the percent of open field.

Value of increasing film processing time to reduce radiation dose during mammography.

We systematically tested the effects on radiation dose and image quality of increasing the mammographic film processing time from the standard 90 sec to 3 min. Hurter and Driffield curves were obtained for a Kodak Min-R-OM1-SO177 screen-film combination processed with Kodak chemistry. Image contrast and radiation dose were measured for two tissue-equivalent breast phantoms. We also compared sequential pairs of mammograms, one processed at 90 sec and one at 3 min, from 44 patients on the basis of nine categories of image quality. Increased processing time reduced breast radiation dose by 30%, increased contrast by 11%, and produced slight overall gains in image quality. Simple modifications can convert a 90-sec processor to a 3-min unit. We recommend that implementation of extended processing be considered, especially by those centers that obtain a large number of screening mammograms. Three-minute film processing can reduce breast radiation dose by 30% and increase contrast by 11% without compromising image quality.

The effect of breast composition on absorbed dose and image contrast.

We have studied the effect of breast composition on the average whole breast dose, average glandular dose, and image contrast in mammography, using both computational and experimental methods. Three glandular/adipose compositions were considered: 30/70, 50/50, and 70/30 by weight, for both 3- and 5-cm breast thickness. Absorbed dose was found to increase with greater glandular content and this increase is more pronounced for thick breasts and softer beams. For typical screen-film x-ray beams, the average dose to a highly glandular breast is nearly twice the dose to a highly adipose breast and the average glandular dose about 40% higher. Dose was reduced when higher energy beams were employed. The use of a grid increased the dose by a factor of 2.0 to 2.6. Finally, the measured image contrast decreases with increasing breast glandularity, to a greater extent in small breasts and when low energy beams were employed.

Absorbed breast dose: dependence on radiographic modality and technique, and breast thickness.

The increased use of mammography has brought into focus the necessity for radiation dose reduction. In particular, the effect of radiographic technique on radiation dosimetry is not well documented. In this paper, the dependence of absorbed dose on radiographic modality, radiographic technique, and breast thickness is studied, with the following principal results: Breast thickness and incident half-value layer (HVL) are sufficient to characterize the normalized (mrad/incident roentgen) breast dose. The average breast dose depends on both beam HVL and kVp; the dependence on breast thickness is more pronounced for screen-film mammography, indicating the need for firm compression. Screen-film mammography shows substantial dose savings over xeromammography for thinner breasts imaged without a grid; this dose advantage disappears for thicker breasts and is generally reversed when a grid is used.