MR-CBCT image-guided system for radiotherapy of orthotopic rat prostate tumors

<div><p>Multi-modality image-guided radiotherapy is the standard of care in contemporary cancer management; however, it is not common in preclinical settings due to both hardware and software limitations. Soft tissue lesions, such as orthotopic prostate tumors, are difficult to identify using cone beam computed tomography (CBCT) imaging alone. In this study, we characterized a research magnetic resonance (MR) scanner for preclinical studies and created a protocol for combined MR-CBCT image-guided small animal radiotherapy. Two in-house dual-modality, MR and CBCT compatible, phantoms were designed and manufactured using 3D printing technology. The phantoms were used for quality assurance tests and to facilitate end-to-end testing for combined preclinical MR and CBCT based treatment planning. MR and CBCT images of the phantoms were acquired utilizing a Varian 4.7 T scanner and XRad-225Cx irradiator, respectively. The geometry distortion was assessed by comparing MR images to phantom blueprints and CBCT. The corrected MR scans were co-registered with CBCT and subsequently used for treatment planning. The fidelity of 3D printed phantoms compared to the blueprint design yielded favorable agreement as verified with the CBCT measurements. The geometric distortion, which varied between -5% and 11% throughout the scanning volume, was substantially reduced to within 0.4% after correction. The distortion free MR images were co-registered with the corresponding CBCT images and imported into a commercial treatment planning software SmART Plan. The planning target volume (PTV) was on average 19% smaller when contoured on the corrected MR-CBCT images relative to raw images without distortion correction. An MR-CBCT based preclinical workflow was successfully designed and implemented for small animal radiotherapy. Combined MR-CBCT image-guided radiotherapy for preclinical research potentially delivers enhanced relevance to human radiotherapy for various disease sites. This novel protocol is wide-ranging and not limited to the orthotopic prostate tumor study presented in the study.</p></div

PTV contours displayed on the distortion corrected fused MR-CBCT image.

<p>Cyan represents PTV before correction, while pink depicts PTV after correction.</p

Grid size measurements before and after correction.

<p>Grid size measurements before and after correction.</p

Two preclinical MR-CBCT imaging phantoms.

<p>(a) Calibration phantom design (b) Geometry phantom design (c) 3D printed geometry phantom (d) Geometry phantom assembly. The cylindrical phantom was designed to fit within imaging and treatment cradles. The fillable compartments allow inclusion of materials such as water, air, silicone and glass to generate contrast.</p

Characterization of magnetic field distortion at 13 mm, 56 mm and 70 mm.

<p>The cyan arrows represent the deformation vector fields (DVF). The red circles correspond to the original outline of the phantom in the MR images before correction, while the green circles correspond to the CBCT outline of the phantom.</p

Experimental setup and Varian 4.7 T MR scanner.

<p>(a) The animal was anesthetized using isoflurane and oxygen mixture for MR measurements and radiation treatment. (b) The animal’s vitals (respiratory and pulse oximetry) and temperature were monitored. (c) The animal was covered by a water-heated blanket to keep the temperature stable during MR measurement. (d) Varian 4.7 T MR scanner used in the study.</p

CBCT and the corresponding distortion corrected MR images.

<p>(a) CBCT and (b) MR of the contouring and contrast modules. (c) CBCT and (d) MR of the grid module.</p

In-house designed MR-CBCT compatible cradle.

<p>(a) computer-aided cradle design, (b) Preclinical MR scanning cradle, (c) PXI 225Cx CBCT simulation cradle.</p

CBCT and MR geometry phantom images before and after applied correction.

<p>Arrows point to noticeable geometric distortion.</p

Tumor delineation blind test.

<p>Tumor delineation blind test.</p