3D printed plastics for beam modulation in proton therapy

Evaluation of Fused Deposition Modeling Materials for 3D-Printed Container of Dosimetric Polymer Gel

Accurate dosimetric verification is becoming increasingly important in radiotherapy. Although polymer gel dosimetry may be useful for verifying complex 3D dose distributions, it has limitations for clinical application due to its strong reactivity with oxygen and other contaminants. Therefore, it is important that the material of the gel storage container blocks reaction with external contaminants. In this study, we tested the effect of air and the chemical permeability of various polymer-based 3D printing materials that can be used as gel containers. A methacrylic acid, gelatin, and tetrakis (hydroxymethyl) phosphonium chloride gel was used. Five types of printing materials that can be applied to the fused deposition modeling (FDM)-type 3D printer were compared: acrylonitrile butadiene styrene (ABS), co-polyester (CPE), polycarbonate (PC), polylactic acid (PLA), and polypropylene (PP) (reference: glass vial). The map of R2 (1/T2) relaxation rates for each material, obtained from magnetic resonance imaging scans, was analyzed. Additionally, response histograms and dose calibration curves from the R2 map were evaluated. The R2 distribution showed that CPE had sharper boundaries than the other materials, and the profile gradient of CPE was also closest to the reference vial. Histograms and dose calibration showed that CPE provided the most homogeneous and the highest relative response of 83.5%, with 8.6% root mean square error, compared with the reference vial. These results indicate that CPE is a reasonable material for the FDM-type 3D printing gel container.

Development, Monte Carlo simulations and experimental evaluation of a 3D range-modulator for a complex target in scanned proton therapy

The purpose of this work was to develop and manufacture a 3D range-modulator (3D RM) for a complex target contour for scanned proton therapy. The 3D RM is considered to be a viable technique for the very fast dose application in patient-specific tumors with only one fixed energy. The RM was developed based on a tumor from a patient CT and manufactured with high-quality 3D printing techniques with both polymer resin and aluminum. Monte Carlo simulations were utilized to investigate its modulating properties and the resulting dose distribution. Additionally, the simulation results were validated with measurements at the Marburg Ion-Beam Therapy Centre. For this purpose, a previously developed water phantom was used to conduct fast, automated high-resolution dose measurements. The results show a very good agreement between simulations and measurements and indicate that highly homogeneous dose distributions are possible. The delivered dose is conformed to the distal as well as to the proximal edge of the target. The 3D range-modulator concept combines a high degree of dose homogeneity and conformity, comparable to standard IMPT with very short irradiation times, promising clinically applicable dose distributions for lung and/or FLASH treatment, comparable and competitive to those from conventional irradiation techniques.

Applications of various range shifters for proton pencil beam scanning radiotherapy

Background

A range pull-back device, such as a machine-related range shifter (MRS) or a universal patient-related range shifter (UPRS), is needed in pencil beam scanning technique to treat shallow tumors.

Methods

Three UPRS made by QFix (Avondale, PA, USA) allow treating targets across the body: U-shaped bolus (UB), anterior lateral bolus (ALB), and couch top bolus. Head-and-neck (HN) patients who used the UPRS were tested. The in-air spot sizes were measured and compared in this study at air gaps: 6 cm, 16 cm, and 26 cm. Measurements were performed in a solid water phantom using a single-field optimization pencil beam scanning field with the ALB placed at 0, 10, and 20 cm air gaps. The two-dimensional dose maps at the middle of the spread-out Bragg peak were measured using ion chamber array MatriXX PT (IBA-Dosimetry, Schwarzenbruck, Germany) located at isocenter and compared with the treatment planning system.

Results

A UPRS can be consistently placed close to the patient and maintains a relatively small spot size resulting in improved dose distributions. However, when a UPRS is non-removable (e.g. thick couch top), the quality of volumetric imaging is degraded due to their high Z material construction, hindering the value of Image-Guided Radiation Therapy (IGRT). Limitations of using UPRS with small air gaps include reduced couch weight limit, potential collision with patient or immobilization devices, and challenges using non-coplanar fields with certain UPRS. Our experience showed the combination of a U-shaped bolus exclusively for an HN target and an MRS as the complimentary device for head-and-neck targets as well as for all other treatment sites may be ideal to preserve the dosimetric advantages of pencil beam scanning proton treatments across the body.

Conclusion

We have described how to implement UPRS and MRS for various clinical indications using the PBS technique, and comprehensively reviewed the advantage and disadvantages of UPRS and MRS. We recommend the removable UB only to be employed for the brain and HN treatments while an automated MRS is used for all proton beams that require RS but not convenient or feasible to use UB.

Monte Carlo simulations and dose measurements of 2D range-modulators for scanned particle therapy

This paper introduces the concept of a 2D range-modulator as a static device for generating spread-out Bragg peaks at very small distances to the target. The 2D range-modulator has some distinct advantages that can be highly useful for different research projects in particle therapy facilities. Most importantly, it creates an instantaneous, quasi-static irradiation field with only one energy, thus decreasing irradiation time tremendously. In addition, it can be manufactured fast and cost efficiently and its SOBP width and shape can be adjusted easily for the specific purpose/experiment. As the modulator is a static element, there is no need for rotation (e.g. like in a modulation wheel) or lateral oscillation and due to the small base structure period it can be positioned close to the target. Two different rapid prototyping manufacturing techniques were utilized. The modulation properties of one polymer and one steel modulator were investigated with both simulations and measurements. For this purpose, a sophisticated water phantom system (WERNER), that can perform fast, completely automated and high resolution dose measurements, was developed. Using WERNER, the dose distribution of a modulator can be verified quickly and reliably, both during experiments, as well as in a time constrained clinical environment. The maximum deviation between the Monte Carlo simulations and dose measurements in the spread-out Bragg peak region was 1.4% and 4% for the polymer and steel modulator respectively. They were able to create spread-out Bragg peaks with a high degree of dose homogeneity, thus validating the whole process chain, from the mathematical optimization and modulator development, to manufacturing, MC simulations and dose measurements. Combining the convenience, flexibility and cost-effectiveness of rapid prototyping with the advantages of highly customizable modulators, that can be adapted for different experiments, the 2D range-modulator is considered a very useful tool for a variety of research objectives. Moreover, we have successfully shown that the manufacturing of 2D modulators with high quality and high degree of homogeneity is possible, paving the way for the further development of the more complex 3D range-modulators, which are considered a viable option for the very fast treatment of moving targets and/or FLASH irradiation.

Breast phantoms for 2D digital mammography with realistic anatomical structures and attenuation characteristics based on clinical images using 3D printing

The aim of this work was to develop a production process for breast phantoms for 2D digital mammography (DM) with realistic anatomical structures and attenuation characteristics based on clinical images using 3D printing. The presented production process is based on PolyJet 3D printing technology using a polypropylene like printing material. First, an attenuation calibration function for this material and the achievable lateral resolution of the printing process of about 200 µm was determined. Subsequently, to generate the digital 3D model of the breast phantom, the pixel intensities of the unprocessed clinical image that are related to the attenuation along the z-axis of the breast, were converted to corresponding phantom heights using the calibration function. To validate the process, an image of the 3D printed breast phantom was acquired on the full field digital mammography (FFDM) system used for calibration and compared with the clinical image in terms of anatomical structures and associated attenuation characteristics. The exposure parameters and image impression of the phantom were evaluated using five other FFDM systems of different manufacturers and types. Results demonstrated that the anatomical structures in the images and the attenuation characteristics of a female breast and the derived phantom agreed on the FFDM system used for calibration. The automatic exposure control segmentation, the automatically selected exposure parameters and the image postprocessing of the clinical and phantom image indicated a high level of conformity. As shown, the phantom is also suitable for other FFDM systems. In conclusion, an approach to produce anthropomorphic breast phantoms for DM offering realistic anatomical structures and attenuation characteristics based on clinical images was successfully developed. As shown, the phantom realistically simulated the original female breast. Therefore, it is expected that such phantoms are promising to support bridging the gap between physical-technical and diagnostic image quality assessment. In addition, they enable a variety of practical and scientific applications for which present technical phantoms are not suitable.

Preclinical dose verification using a 3D printed mouse phantom for radiobiology experiments

PURPOSE

Dose verification in preclinical radiotherapy is often challenged by a lack of standardization in the techniques and technologies commonly employed along with the inherent difficulty of dosimetry associated with small-field kilovoltage sources. As a consequence, the accuracy of dosimetry in radiobiological research has been called into question. Fortunately, the development and characterization of realistic small-animal phantoms has emerged as an effective and accessible means of improving dosimetric accuracy and precision in this context. The application of three-dimensional (3D) printing, in particular, has enabled substantial improvements in the conformity of representative phantoms with respect to the small animals they are modeled after. In this study, our goal was to evaluate a fully 3D printed mouse phantom for use in preclinical treatment verification of sophisticated therapies for various anatomical targets of therapeutic interest.

METHODS

An anatomically realistic mouse phantom was 3D printed based on segmented microCT data of a tumor-bearing mouse. The phantom was modified to accommodate both laser-cut EBT3 radiochromic film within the mouse thorax and a plastic scintillator dosimeter (PSD), which may be placed within the brain, abdomen, or 1-cm flank subcutaneous tumor. Various treatments were delivered on an image-guided small-animal irradiator in order to determine the doses to isocenter using a PSD and validate lateral- and depth-dose distributions using film dosimeters. On-board cone-beam CT imaging was used to localize isocenter to the film plane or PSD active element prior to irradiation. The PSD irradiations comprised a 3 × 3 mm² brain arc, 5 × 5 mm² parallel-opposed pair (POP), and 5-beam 10 × 10 mm² abdominal coplanar arrangement while two-dimensional (2D) film dose distributions were acquired using a 3 × 3 mm² arc and both 5 × 5 and 10 × 10 mm² 3-beam coplanar plans. A validated Monte Carlo (MC) model of the source was used as to verify the accuracy of the film and PSD dose measurements. computer-aided design (CAD) geometries for the mouse phantom and dosimeters were imported directly into the MC code to allow for highly accurate reproduction of the physical experiment conditions. Experimental and MC-derived film data were co-registered and film dose profiles were compared for points above 90% of the dose maximum. Point dose measurements obtained with the PSD were similarly compared for each of the candidate (brain, abdomen, and tumor) treatment sites.

RESULTS

For each treatment configuration and anatomical target, the MC-calculated and measured doses met the proposed 5% agreement goal for dose accuracy in radiobiology experiments. The 2D film and MC dose distributions were successfully registered and mean doses for lateral profiles were found to agree to within 2.3% in all cases. Isocentric point-dose measurements taken with the PSD were similarly consistent, with a maximum percentage deviation of 3.2%.

CONCLUSIONS

Our study confirms the utility of 3D printed phantom design in providing accurate dose estimates for a variety of preclinical treatment paradigms. As a tool for pretreatment dose verification, the phantom may be of particular interest to researchers for its ability to facilitate precise dosimetry while fostering a reduction in cost for radiobiology experiments.

Validation of new 2D ripple filters in proton treatments of spherical geometries and non-small cell lung carcinoma cases

A ripple filter (RiFi) is a passive energy modulator used in scanned particle therapy to broaden the Bragg peak, thus lowering the number of accelerator energies required for homogeneous target coverage, which significantly reduces the irradiation time. As we have previously shown, a new 6 mm thick RiFi with 2D groove shapes produced with 3D printing can be used in carbon ion treatments with a similar target coverage and only a marginally worse planning conformity compared to treatments with in-use 3 mm thick RiFis of an older 1D design. Where RiFis are normally not used with protons due to larger scattering and straggling effects, this new design would be beneficial in proton therapy too. Measurements of proton Bragg curves and lateral beam profiles were carried out for different RiFi designs and thicknesses as well as for no RiFi at the Heidelberg Ionenstrahl-Therapiezentrum. Base data for proton treatment planning were generated with the Monte Carlo code SHIELD-HIT12A with and without the 2D 6 mm RiFi. Plans on spherical targets in water were calculated with TRiP98 for a systematic RiFi performance analysis and for comparisons with carbon ion plans for the same respective energy depth step sizes. Plans for 9 stage I static non small cell lung cancer patients were calculated with Eclipse 13.7.15. Dose-volume-histograms, spatial dose distributions and dosimetric indexes were used for plan evaluation. Measurements confirm the functionality of the new 2D RiFi design, which reduces the beam spot size compared to 1D RiFis of the same thickness. Planning studies show that a 6 mm thick 2D RiFi could be used in proton therapy to lower the irradiation time. Although slightly worse planning conformity and dose homogeneity were found for plans with the RiFi compared to plans without, satisfactory results within the planning objective were obtained for all cases.

Development and dosimetric assessment of a patient-specific elastic skin applicator for high-dose-rate brachytherapy

PURPOSE

The purpose of this study was to develop a patient-specific elastic skin applicator and to evaluate its dosimetric characteristics for high-dose-rate (HDR) brachytherapy.

METHODS AND MATERIALS

We simulated the treatment of a nonmelanoma skin cancer on the nose. An elastic skin applicator was manufactured by pouring the Dragon Skin (Smooth-On Inc., Easton, PA) with a shore hardness of 10A into an applicator mold. The rigid skin applicator was printed using high-impact polystyrene with a shore hardness of 73D. HDR plans were generated using a Freiburg Flap (FF) applicator and patient-specific rigid and elastic applicators. For dosimetric assessment, dose-volumetric parameters for target volume and normal organs were evaluated. Global gamma evaluations were performed, comparing film measurements and treatment planning system calculations with various gamma criteria. The 10% low-dose threshold was applied.

RESULTS

The V_120% values of the target volume were 56.9%, 70.3%, and 70.2% for HDR plans using FF, rigid, and elastic applicators, respectively. The maximum doses of the right eyeball were 21.7 Gy, 20.5 Gy, and 20.5 Gy for the HDR plans using FF, rigid, and elastic applicators, respectively. The average gamma passing rates were 82.5% ± 1.5%, 91.6% ± 0.8%, and 94.8% ± 0.2% for FF, rigid, and elastic applicators, respectively, with 3%/3 mm criterion.

CONCLUSIONS

Patient-specific elastic skin applicator showed better adhesion to irregular or curved body surfaces, resulting in better agreement between planned and delivered dose distributions. The applicator suggested in this study can be effectively implemented clinically.

Technical Note: Manufacturing of a realistic mouse phantom for dosimetry of radiobiology experiments

PURPOSE

The goal of this work was to design a realistic mouse phantom as a useful tool for accurate dosimetry in radiobiology experiments.

METHODS

A subcutaneous tumor-bearing mouse was scanned in a microCT scanner, its organs manually segmented and contoured. The resulting geometries were converted into a stereolithographic file format (STL) and sent to a multimaterial 3D printer. The phantom was split into two parts to allow for lung excavation and 3D-printed with an acrylic-like material and consisted of the main body (mass density ρ=1.18 g/cm³ ) and bone (ρ=1.20 g/cm³ ). The excavated lungs were filled with polystyrene (ρ=0.32 g/cm³ ). Three cavities were excavated to allow the placement of a 1-mm diameter plastic scintillator dosimeter (PSD) in the brain, the center of the body and a subcutaneous tumor. Additionally, a laser-cut Gafchromic film can be placed in between the two phantom parts for 2D dosimetric evaluation. The expected differences in dose deposition between mouse tissues and the mouse phantom for a 220-kVp beam delivered by the small animal radiation research platform (SARRP) were calculated by Monte Carlo (MC).

RESULTS

MicroCT scans of the phantom showed excellent material uniformity and confirmed the material densities given by the manufacturer. MC dose calculations revealed that the dose measured by tissue-equivalent dosimeters inserted into the phantom in the brain, abdomen, and subcutaneous tumor would be underestimated by 3-5%, which is deemed to be an acceptable error assuming the proposed 5% accuracy of radiobiological experiments.

CONCLUSIONS

The low-cost mouse phantom can be easily manufactured and, after a careful dosimetric characterization, may serve as a useful tool for dose verification in a range of radiobiology experiments.

Holey-Cavity-Based Compressive Sensing for Ultrasound Imaging

The use of solid cavities around electromagnetic sources has been recently reported as a mechanism to provide enhanced images at microwave frequencies. These cavities are used as measurement randomizers; and they compress the wave fields at the physical layer. As a result of this compression, the amount of information collected by the sensing array through the different excited modes inside the resonant cavity is increased when compared to that obtained by no-cavity approaches. In this work, a two-dimensional cavity, having multiple openings, is used to perform such a compression for ultrasound imaging. Moreover, compressive sensing techniques are used for sparse signal retrieval with a limited number of operating transceivers. As a proof-of-concept of this theoretical investigation, two point-like targets located in a uniform background medium are imaged in the presence and the absence of the cavity. In addition, an analysis of the sensing capacity and the shape of the point spread function is also carried out for the aforementioned cases. The cavity is designed to have the maximum sensing capacity given different materials and opening sizes. It is demonstrated that the use of a cavity, whether it is made of plastic or metal, can significantly enhance the sensing capacity and the point spread function of a focused beam. The imaging performance is also improved in terms cross-range resolution when compared to the no-cavity case.

Intrapatient study comparing 3D printed bolus versus standard vinyl gel sheet bolus for postmastectomy chest wall radiation therapy

PURPOSE

This patient study evaluated the use of 3-dimensional (3D) printed bolus for chest wall radiation therapy compared with standard sheet bolus with regard to accuracy of fit, surface dose measured in vivo, and efficiency of patient setup. By alternating bolus type over the course of therapy, each patient served as her own control.

METHODS AND MATERIALS

For 16 patients undergoing chest wall radiation therapy, a custom 5.0 mm thick bolus was designed based on the treatment planning computed tomography scan and 3D printed using polylactic acid. Cone beam computed tomography scanning was used to image and quantify the accuracy of fit of the 2 bolus types with regard to air gaps between the bolus and skin. As a quality assurance measure for the 3D printed bolus, optically stimulated luminescent dosimetry provided in vivo comparison of surface dose at 7 points on the chest wall. Durations of patient setup and image guidance were recorded and compared.

RESULTS

In 13 of 16 patients, the bolus was printed without user intervention, and the median print time was 12.6 hours. The accuracy of fit of the bolus to the chest wall was improved significantly relative to standard sheet bolus, with the frequency of air gaps 5 mm or greater reduced from 30% to 13% (P < .001) and maximum air gap dimension diminished from 0.5 ± 0.3 to 0.3 ± 0.3 mm on average. Surface dose was within 3% for both standard sheet and 3D printed bolus. On average, the use of 3D printed bolus reduced the setup time from 104 to 76 seconds.

CONCLUSIONS

This study demonstrates 3D printed bolus in postmastectomy radiation therapy improves fit of the bolus and reduces patient setup time marginally compared with standard vinyl gel sheet bolus. The time savings on patient setup must be weighed against the considerable time needed for the 3D printing process.

3D range-modulator for scanned particle therapy: development, Monte Carlo simulations and experimental evaluation

The purpose of this work was to design and manufacture a 3D range-modulator for scanned particle therapy. The modulator is intended to create a highly conformal dose distribution with only one fixed energy, simultaneously reducing considerably the treatment time. As a proof of concept, a 3D range-modulator was developed for a spherical target volume with a diameter of 5 cm, placed at a depth of 25 cm in a water phantom. It consists of a large number of thin pins with a well-defined shape and different lengths to modulate the necessary shift of the Bragg peak. The 3D range-modulator was manufactured with a rapid prototyping technique. The FLUKA Monte Carlo package was used to simulate the modulating effect of the 3D range-modulator and the resulting dose distribution. For that purpose, a special user routine was implemented to handle its complex geometrical contour. Additionally, FLUKA was extended with the capability of intensity modulated scanning. To validate the simulation results, dose measurements were carried out at the Heidelberg Ion Beam Therapy Center with a 400.41 MeV/u ¹²C beam. The high resolution dosimetric measurements show a good agreement between simulated and measured dose distributions. Irradiation of the monoenergetic raster plan took 3 s, which is approximately 20 times shorter than a comparable plan with 16 different energies. The combination of only one energy and a 3D range-modulator leads to a tremendous decrease in irradiation time. 'Interplay effects', typical for moving targets and pencil beam scanning, can be immensely reduced or disappear completely, making the delivery of a homogeneous dose to moving targets more reliable. Combining high dose conformity, very good homogeneity and extremely short irradiation times, the 3D range-modulator is considered to become a clinically applicable method for very fast treatment of lung tumours.

Towards 3D printed multifunctional immobilization for proton therapy: Initial materials characterization

PURPOSE

3D printing technology is investigated for the purpose of patient immobilization during proton therapy. It potentially enables a merge of patient immobilization, bolus range shifting, and other functions into one single patient-specific structure. In this first step, a set of 3D printed materials is characterized in detail, in terms of structural and radiological properties, elemental composition, directional dependence, and structural changes induced by radiation damage. These data will serve as inputs for the design of 3D printed immobilization structure prototypes.

METHODS

Using four different 3D printing techniques, in total eight materials were subjected to testing. Samples with a nominal dimension of 20 × 20 × 80 mm³ were 3D printed. The geometrical printing accuracy of each test sample was measured with a dial gage. To assess the mechanical response of the samples, standardized compression tests were performed to determine the Young's modulus. To investigate the effect of radiation on the mechanical response, the mechanical tests were performed both prior and after the administration of clinically relevant dose levels (70 Gy), multiplied with a safety factor of 1.4. Dual energy computed tomography (DECT) methods were used to calculate the relative electron density to water ρ_e, the effective atomic number Z_eff, and the proton stopping power ratio (SPR) to water SPR. In order to validate the DECT based calculation of radiological properties, beam measurements were performed on the 3D printed samples as well. Photon irradiations were performed to measure the photon linear attenuation coefficients, while proton irradiations were performed to measure the proton range shift of the samples. The directional dependence of these properties was investigated by performing the irradiations for different orientations of the samples.

RESULTS

The printed test objects showed reduced geometric printing accuracy for 2 materials (deviation > 0.25 mm). Compression tests yielded Young's moduli ranging from 0.6 to 2940 MPa. No deterioration in the mechanical response was observed after exposure of the samples to 100 Gy in a therapeutic MV photon beam. The DECT-based characterization yielded Z_eff ranging from 5.91 to 10.43. The SPR and ρ_e both ranged from 0.6 to 1.22. The measured photon attenuation coefficients at clinical energies scaled linearly with ρ_e. Good agreement was seen between the DECT estimated SPR and the measured range shift, except for the higher Z_eff. As opposed to the photon attenuation, the proton range shifting appeared to be printing orientation dependent for certain materials.

CONCLUSIONS

In this study, the first step toward 3D printed, multifunctional immobilization was performed, by going through a candidate clinical workflow for the first time: from the material printing to DECT characterization with a verification through beam measurements. Besides a proof of concept for beam modification, the mechanical response of printed materials was also investigated to assess their capabilities for positioning functionality. For the studied set of printing techniques and materials, a wide variety of mechanical and radiological properties can be selected from for the intended purpose. Moreover the elaborated hybrid DECT methods aid in performing in-house quality assurance of 3D printed components, as these methods enable the estimation of the radiological properties relevant for use in radiation therapy.