SU-E-T-273: Commissioning Motorized Jaws for a Micro-CT/RT

To design, construct and commission a set of computer-controlled motorized jaws for a micro-CT to perform conformal image-guided small animal radiotherapy.

We report on the design and evaluation of our custom-built motorized orthogonal jaws, which allows the delivery of off-axis rectangular fields, installed on a GE eXplore CT 120 preclinical imaging system. The jaws in the x direction are independently driven, while the y direction jaws are symmetric. All motors have backup encoders, verifying their positions. Measurements were performed using the large focal spot (1.0 mm) at 140 kVp, 50 mA, 25 ms pulse duration, with 220 ms between pulses. Additionally, a Monte Carlo beam model of the micro-CT and jaws has been created in BEAMnrc. Square beam profiles (2 × 2, 5 × 5, 10 × 10, 20 × 20 and 30 × 30 mm) were measured using EBT2 film in the center of a 40 × 40 × 40 mm solid water block. A calibrated Farmer ion chamber in a 50 × 20 × 70 mm solid water block was used to measure the output of three field sizes: 50 × 50, 40 × 40 and 30 × 30 mm. Both setups were simulated in DOSXYZnrc. Films were scanned on a flatbed scanner (Epson 1000XL) and converted to dose using a fitted calibration curve.

Relative output factors measured with the ion chamber agree with Monte Carlo simulations within 2%. Penumbra widths from film agree well with simulations. The averaged random error of positioning both jaws is estimated at less than 0.1 mm. Finer calibration points will improve the absolute jaw positioning errors.

The computer-controlled motorized jaws can be positioned reliably down to sub-millimeter resolutions. Upon completion of small field dosimetry (< 1 cm), this will be ready for conformal radiotherapy for small animals.