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

Abstract

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 IMRTcredentialing 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 9 1.25 mm 2. 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 q NaI = 2.062 g/cm 3). 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 9 40 mm 2 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 AE0.024 Gy and relative dose error of AE4.2% 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...

An investigation of kV mini-GRID spatially fractionated radiation therapy: dosimetry and preclinical trial

“…GRIDs were designed for the X-RAD 225Cx small animal irradiator that has been well characterized in our department (Precision x-ray, N. Banfield, CT) (Newton et al 2011, Rankine et al 2013, Bache et al 2015, Cramer et al 2015, Yoon et al 2020. GRIDs were precision-milled from 3 mm thick lead sheets, which blocked over 95% of the primary beam.…”

Improving the efficiency of small animal 3D‐printed compensator IMRT with beamlet intensity total variation regularization

“…This was achieved while limiting the TV, resulting in compensator modulation patterns that have shallow physical gradients relative to previous design methods. 18,19 Additionally, the TVR-IMRT method of 3D-printed compensator design was shown to provide an advantage over previous methods by reducing the sum of thicknesses used in the fluence map modulation region, resulting in lower cost and shorter print times, as well as reducing the total beam on time for each exposure, while preserving or improving the dose distribution in the target as confirmed by DVH and gamma analysis. The composite dose delivery of the TVR-IMRT plan passing the gamma analysis with 2%/0.3 mm criteria, compared with the IMRT plan only passing the 3%/0.5 mm criteria is further evidence that TVR-IMRT produces a compensator that is both more likely to print accurately and more robust to small positioning errors.…”

“…As shown with the plans generated by both IMRT and TVR‐IMRT remaining close to one another for all the plan dose evaluation metrics that we used. This was achieved while limiting the TV, resulting in compensator modulation patterns that have shallow physical gradients relative to previous design methods 18,19 …”

“…Another approach is compensator-based methods that utilize 3D-printed patterns through which the beam is delivered at specified gantry angles.To achieve their attenuation,a shaped polylactic acid (PLA) compensator can either be packed with sodium iodide powder, or the compensator itself can be printed out of PLA impregnated with copper or tungsten. 18,19 Some advantages of the compensator method are the submillimeter resolution achievable by modern 3D printers, a reduction in the number of segments that need to be delivered to achieve a conformal dose, and the absence of additional mechanical parts that require maintenance and quality control. 13,19 However, 3D-printed compensators are specific to each plan and beam angle; thus, a new set needs to be made for each plan, consuming additional material and printing time.…”

Intensity Modulated High Dose Rate (HDR) Brachytherapy Using Patient Specific 3D Metal Printed Applicators: Proof of Concept

“…Current research has been limited to making 3D printed models using PLA or other similar plastics. There is increasing interest in incorporating metallic materials in 3D prints, however that has been limited to using either metallic powders or ball bearings (18,19). Going forward, we will verify the dosimetric properties and the reproducibility of the manufacturing process.…”