Specific issues in small animal dosimetry and irradiator calibration

Abstract

Purpose: In response to the increased risk of radiological terrorist attack, a network of Centers for Medical Countermeasures against Radiation (CMCR) has been established in the United States, focusing on evaluating animal model responses to uniform, relatively homogenous whole- or partial-body radiation exposures at relatively high dose rates. The success of such studies is dependent not only on robust animal models but on accurate and reproducible dosimetry within and across CMCR. To address this issue, the Education and Training Core of the Duke University School of Medicine CMCR organised a one-day workshop on small animal dosimetry. Topics included accuracy in animal dosimetry accuracy, characteristics and differences of cesium-137 and X-ray irradiators, methods for dose measurement, and design of experimental irradiation geometries for uniform dose distributions. This paper summarises the information presented and discussed.

Conclusions: Without ensuring accurate and reproducible dosimetry the development and assessment of the efficacy of putative countermeasures will not prove successful. Radiation physics support is needed, but is often the weakest link in the small animal dosimetry chain. We recommend: (i) A user training program for new irradiator users, (ii) subsequent training updates, and (iii) the establishment of a national small animal dosimetry center for all CMCR members.

Figure 1

Diagnostic and megavoltage photon beam percent depth dose curves (from Bourland 2006, used with permission).

Figure 2

Moving source gamma irradiator, vertical source orientation. A linear γ source moves from the shielded source vault (A) and (B) (bottom to top) to the shielded irradiation chamber (cavity). The source is enclosed by the tube seen in (B). A rotating platter can be mounted on sample driveshaft locations (#1, #2, and #3) to provide rotation for dose homogeneity. A timer mechanism controls the irradiation time, ending with the return of the sources to the shielded source vault.

Figure 3

Tissue-equivalent rodent phantoms for irradiator dosimetry. (A) Rat and mouse phantoms with holes for dosimeter placement. (B) Phantom dimensions relative to animal. (C) Rotation platter used in irradiator (Figure 2) for dosimetry as well as experiment.

Figure 4

Jig design: Rotation film stage assembly was designed to attach to a rotational platform provided by the manufacturer. A sandwich of two 2 mm thick Plexiglas plates was used to provide structural support and to establish proper charged particle equilibrium for the film. Two Plexiglas plate sizes were designed to be interchangeable with the base assembly for positions 1, 2, or 3 (Figure 2C): (A) small 10 cm wide plates for measurements at position 1 closest to the cesium-137 source and (B) large 30 cm wide plates for isodose curve measurements at positions 2 and 3.

Figure 5

Left column (A and D): Radiochromic film dose distributions are shown in colour wash for positions 1 and 2 (Figure 2C). Middle column (B and E): Dose distributions were converted to isodose curves. Right column (C and F): Manufacturer’s isodose curves. Comparison of measured and manufacturer’s isodose curve shows good agreement and verifies proper irradiator performance. The vertical axis (y-axis) represents the distance from the floor of the irradiator housing.

Figure 6

Vertical and horizontal line dose profiles were obtained along the dotted lines in Figure 5A and D, and plotted with the corresponding manufacturer provided data for positions 1 – 2, respectively. Comparison of measured and manufacturer’s line dose profiles shows good agreement and verifies proper irradiator performance.