Adaptation, Commissioning, and Evaluation of a 3D Treatment Planning System for High-Resolution Small-Animal Irradiation

Abstract

Although spatially precise systems are now available for small-animal irradiations, there are currently limited software tools available for treatment planning for such irradiations. We report on the adaptation, commissioning, and evaluation of a 3-dimensional treatment planning system for use with a small-animal irradiation system. The 225-kV X-ray beam of the X-RAD 225Cx microirradiator (Precision X-Ray) was commissioned using both ion-chamber and radiochromic film for 10 different collimators ranging in field size from 1 mm in diameter to 40 × 40 mm(2) A clinical 3-dimensional treatment planning system (Metropolis) developed at our institution was adapted to small-animal irradiation by making it compatible with the dimensions of mice and rats, modeling the microirradiator beam orientations and collimators, and incorporating the measured beam data for dose calculation. Dose calculations in Metropolis were verified by comparison with measurements in phantoms. Treatment plans for irradiation of a tumor-bearing mouse were generated with both the Metropolis and the vendor-supplied software. The calculated beam-on times and the plan evaluation tools were compared. The dose rate at the central axis ranges from 74 to 365 cGy/min depending on the collimator size. Doses calculated with Metropolis agreed with phantom measurements within 3% for all collimators. The beam-on times calculated by Metropolis and the vendor-supplied software agreed within 1% at the isocenter. The modified 3-dimensional treatment planning system provides better visualization of the relationship between the X-ray beams and the small-animal anatomy as well as more complete dosimetric information on target tissues and organs at risk. It thereby enhances the potential of image-guided microirradiator systems for evaluation of dose-response relationships and for preclinical experimentation generally.

Keywords: 3D dose distribution; commissioning; radiation therapy; small-animal irradiator; treatment planning system.

Figure 1

The interface of the vendor-supplied software, in which the beam-on time can be calculated based on the depth dose for each beam to a treatment isocenter. Note that the longer axial dimension of the tumor (as shown in Figure 2) required the use of the 10-mm circular collimator to assure adequate coverage, even though it unavoidably resulted in including normal tissues in the transaxial directions.

Figure 2

The interface of Metropolis: (A) segmentation of structures including tumor using contouring tools and (B) beam's eye-view (BEV).

Figure 3

Dose rate on central axis of the field at varying depths in the solid-water phantom measured by EBT3 film for all 10 collimators. The symbol “ϕ” refers to the diameter of circular collimators.

Figure 4

Horizontal dose profiles measured with EBT3 films at a 20-mm depth in the solid-water phantom for the 10 different collimators (see Supplemental Figures 11-13 for individual profiles for each collimator). The symbol “ϕ” refers to the diameter of circular collimators.

Figure 5

Comparison of depth-dependent dose rate measured with 3 different dosimeters for a 40 × 40-mm² collimator.

Figure 6

Plan evaluation in Metropolis with 3-dimensional (3D) isodose distribution (color wash) and dose-volume histograms (DVHs) for 2 orthogonal beams of 10-mm diameter. Red, blue, and pink curves indicate tumor, lung, and body surface in the transverse, coronal, and sagittal view images.

Figure 7

Comparison of target coverage shown in beam's eye-view (BEV; left column), 3-dimensional (3D) isodose distribution in color-wash overlaid on transverse computed tomography (CT) image (middle column), and dose-volume histogram (DVH; right column) for 3 different plans: (A) 2 vertical beams with ϕ15 mm collimator, (B) 2 angled beams with ϕ15 mm collimator for the critical structure (spinal cord), and (C) 2 angled beams with 20 × 20 mm² collimator to cover whole targets.