A precision 3D conformal treatment technique in rats: Application to whole-brain radiotherapy with hippocampal avoidance

A method for generating intensity-modulated radiation therapy fields for small animal irradiators utilizing 3D-printed compensator molds

PURPOSE

The purpose of this study was to investigate the feasibility of using fused deposition modeling (FDM) three-dimensional (3D) printer to generate radiation compensators for high-resolution (~1 mm) intensity-modulated radiation therapy (IMRT) for small animal radiation treatment. We propose a novel method incorporating 3D-printed compensator molds filled with NaI powder.

METHODS

The inverse planning module of the computational environment for radiotherapy research (CERR) software was adapted to simulate the XRAD-225Cx irradiator, both geometry and kV beam quality (the latter using a phase space file provided for XRAD-225Cx). A nine-field IMRT treatment was created for a scaled-down version of the imaging and radiation oncology core (IROC) Head and Neck IMRT credentialing test, recreated on a 2.2-cm-diameter cylindrical phantom. Optimized fluence maps comprising nine fields and a total of 2564 beamlets were calculated at resolution of 1.25 × 1.25 mm² . A hollow compensator mold was created (using in-house software and algorithm) for each field using 3D printing with polylactic acid (PLA) filaments. The molds were then packed with sodium iodide powder (NaI, measured density ρ_NaI  = 2.062 g/cm³ ). The mounted compensator mold thickness was limited to 13.8 mm due to clearance issues with couch collision. At treatment delivery, each compensator was manually mounted to a customized block tray attached to the reference 40 × 40 mm² collimator. Compensator reproducibility among three repeated 3D-printed molds was measured with Radiochromic EBT2 film. The two-dimensional (2D) dose distributions of the nine fields were compared to calculated 2D doses from CERR using gamma comparisons with distance-to-agreement criteria of 0.5-0.25 mm and dose difference criteria of 3-5%.

RESULTS

Good reproducibility of 3D-printed compensator manufacture was observed with mean error of ±0.024 Gy and relative dose error of ±4.2% within the modulated part of the beam. Within the limit of 13.8 mm compensator height, a maximum radiation blocking efficiency of 91.5% was achieved. Per field, about 45.5 g of NaI powder was used. Gamma analysis on each of the nine delivered IMRT fields using radiochromic films resulted in eight of nine treatment fields with >90% pass rate with 5%/0.5 mm tolerances. However, low gamma passing rate of 49-66% (3%/0.25 mm to 5%/0.5 mm) was noted in one field, attributed to fabrication errors resulting in over-filling the mold. The nine-field treatment plan was delivered in under 30 min with no mechanical or collisional issues.

CONCLUSIONS

We show the feasibility of high spatial resolution IMRT treatment on a small animal irradiator utilizing 3D-printed compensator shells packed with NaI powder. Using the PLA mold with NaI powder was attractive due to the ease of 3D printing a PLA mold at high geometric resolution and the well-balanced attenuation properties of NaI powders that prevented the mold from becoming too bulky. IMRT fields with 1.25-mm resolution are capable with significant fluence modulation with relative dose accuracy of ±4.2%.

Applications of 3D printing in small animal magnetic resonance imaging

Three-dimensional (3D) printing has significantly impacted the quality, efficiency, and reproducibility of preclinical magnetic resonance imaging. It has vastly expanded the ability to produce MR-compatible parts that readily permit customization of animal handling, achieve consistent positioning of anatomy and RF coils promptly, and accelerate throughput. It permits the rapid and cost-effective creation of parts customized to a specific imaging study, animal species, animal weight, or even one unique animal, not routinely used in preclinical research. We illustrate the power of this technology by describing five preclinical studies and specific solutions enabled by different 3D printing processes and materials. We describe fixtures, assemblies, and devices that were created to ensure the safety of anesthetized lemurs during an MR examination of their brain or to facilitate localized, contrast-enhanced measurements of white blood cell concentration in a mouse model of pancreatitis. We illustrate expansive use of 3D printing to build a customized birdcage coil and components of a ventilator to enable imaging of pulmonary gas exchange in rats using hyperpolarizedXe 129 . Finally, we present applications of 3D printing to create high-quality, dual RF coils to accelerate brain connectivity mapping in mouse brain specimens and to increase the throughput of brain tumor examinations in a mouse model of pituitary adenoma.

Implementation and evaluation of respiratory gating in small animal radiotherapy

Major advance was done in preclinical radiotherapy thanks to the development of image guided micro-irradiator. Nevertheless, some applications still can benefit of improvements, such as the irradiation of mobile tumors. This preclinical radiotherapy case presents increased difficulties compared to clinical practice because of the waveform of small animals breathing cycle, its frequency and amplitude. To answer this issue, we developed a specific beam shutter and implemented respiratory gating on the X-RAD 225Cx preclinical irradiator. In the first step of this study, the shutter was accurately characterized. Opening and closing speed of 1.28 and 0.33 mm ms^-1 were respectively measured, and a transmission of 0.7% of the beam was measured with the shutter fully closed. Beam-on times were also determined for various gating parameters and highlighted a difference of 57 ms between the beam delivery duration and the gate width. This discrepancy was compensated during the respiratory monitoring adjustment. In a second step, a respiratory protocol was evaluated with two vertical beams of 2.5 and 5 mm diameters, for motion amplitudes ranging from 0.5 to 4 mm. This evaluation demonstrated the effectiveness of our set up to perform motion compensation for amplitude as small as 0.5 mm despite a dose gradient of 1.47 cGy mm^-1 observed with the 5 mm irradiation field, due to the shutter opening and closing durations. We also investigated the efficiency of a scintillating fiber dosimeter, adapted to small beams and providing real-time dose rate measurements. This detector showed very good performances to detect motion in small irradiation fields and would be very suitable to monitor the number of delivered gates until the planned delivered dose is achieved. This study presented a new respiratory gating set up and showed that very efficient motion compensation could be achieved in small animal radiotherapy.