Implementation and commissioning of an integrated micro-CT∕RT system with computerized independent jaw collimation.
OBJECTIVE
To design, construct, and commission a set of computer-controlled motorized jaws for a micro-CT∕RT system to perform conformal image-guided small animal radiotherapy.


METHODS
The authors designed and evaluated a system of custom-built motorized orthogonal jaws, which allows the delivery of off-axis rectangular fields 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 jaw positions. Mechanical performance of the jaws was characterized. Square beam profiles ranging from 2×2 to 60×60 mm2 were measured using EBT2 film in the center of a 70×70×22 mm3 solid water block. Similarly, absolute depth dose was measured in a solid water and EBT2 film stack 50×50×50 mm3. A calibrated Farmer ion chamber in a 70×70×20 mm3 solid water block was used to measure the output of three field sizes: 50×50, 40×40, and 30×30 mm2. Elliptical target plans were delivered to films to assess overall system performance. Respiratory-gated treatment was implemented on the system and initially proved using a simple sinusoidal motion phantom. All films were scanned on a flatbed scanner (Epson 1000XL) and converted to dose using a fitted calibration curve. A Monte Carlo beam model of the micro-CT with the jaws has been created using BEAMnrc for comparison with the measurements. An example image-guided partial lung irradiation in a rat is demonstrated.


RESULTS
The averaged random error of positioning each jaw is less than 0.1 mm. Relative output factors measured with the ion chamber agree with Monte Carlo simulations within 2%. Beam profiles and absolute depth dose curves measured from the films agree with simulations within measurement uncertainty. Respiratory-gated treatments applied to a phantom moving with a peak-to-peak amplitude of 5 mm showed improved beam penumbra (80%-20%) from 3.9 to 0.8 mm.


CONCLUSIONS
A set of computer-controlled motorized jaws for a micro-CT∕RT system were constructed with position reliably better than a tenth of a millimeter. The hardware system is ready for image-guided conformal radiotherapy for small animals with capability of respiratory-gated delivery.
Department of Medical Biophysics, The University of Western Ontario, 1151 Richmond Street, London, Ontario N6A 3K7, Canada.
Michael D
MD
Jensen
W Thomas
WT
Hrinivich
Jongho A
JA
Jung
David W
DW
Holdsworth
Maria
M
Drangova
Jeff
J
Chen
Eugene
E
Wong
D000818:Animals; D008168:Lung / Q000502:physiology / Q000528:radiation effects; D009010:Monte Carlo Method; D011874:Radiometry; D061089:Radiotherapy, Image-Guided / Q000295:instrumentation*; D051381:Rats; D012119:Respiration; D012984:Software; D018511:Systems Integration; D055114:X-Ray Microtomography / Q000295:instrumentation*