Feasibility of a 3D-printed anthropomorphic patient-specific head phantom for patient-specific quality assurance of intensity-modulated radiotherapy

A quick guide on implementing and quality assuring 3D printing in radiation oncology

As three-dimensional (3D) printing becomes increasingly common in radiation oncology, proper implementation, usage, and ongoing quality assurance (QA) are essential. While there have been many reports on various clinical investigations and several review articles, there is a lack of literature on the general considerations of implementing 3D printing in radiation oncology departments, including comprehensive process establishment and proper ongoing QA. This review aims to guide radiation oncology departments in effectively using 3D printing technology for routine clinical applications and future developments. We attempt to provide recommendations on 3D printing equipment, software, workflow, and QA, based on existing literature and our experience. Specifically, we focus on three main applications: patient-specific bolus, high-dose-rate (HDR) surface brachytherapy applicators, and phantoms. Additionally, cost considerations are briefly discussed. This review focuses on point-of-care (POC) printing in house, and briefly touches on outsourcing printing via mail-order services.

Fabrication of 3D printed head phantom using plaster mixed with polylactic acid powder for patient-specific QA in intensity-modulated radiotherapy

This study aimed to fabricate a heterogeneous phantom replicating the commercial Rando phantom by mixing plaster powder and polylactic acid (PLA) powder. Producing a heterogeneous phantom using Plaster and PLA is cheaper because it can be easily obtained in the commercial market. Additionally, patient-specific Quality Assurance can be easily performed because the phantom can be produced based on the patient's CT image. PLA has been well studied in the field of radiation therapy and was found to be safe and effective. To match the mean Hounsfield unit (HU) values of the Rando phantom, the bone tissue was changed using plaster and 0-35% PLA powder until an appropriate HU value was obtained, and soft tissue was changed using the PLA infill value until an appropriate HU value was obtained. Bone tissue (200 HU or higher), soft issue (- 500 to 200 HU), and air cavity (less than - 500 HU) were modeled based on the HU values on the computed tomography (CT) image. The bone tissue was modeled as a cavity, and after three-dimensional (3D) printing, a solution containing a mixture of plaster and PLA powder was poured. To evaluate the bone implementation of the phantom obtained by the mixture of plaster and PLA powder, the HU profile of the CT images of the 3D-printed phantom using only PLA and the Rando phantom printed using only PLA was evaluated. The mean HU value for soft tissue in the Rando phantom (- 22.5 HU) showed the greatest similarity to the result obtained with an infill value of 82% (- 20 HU). The mean HU value for bone tissue (669 HU) showed the greatest similarity to the value obtained with 15% PLA powder (680 HU). Thus, for the phantom composed of plaster mixed with PLA powder, soft tissue was fabricated using a 3D printer with an infill value of 82%, and bone tissue was fabricated with a mixture containing 15% PLA powder. In the HU profile, this phantom showed a mean difference of 61 HU for soft tissue and 109 HU for bone tissue in comparison with the Rando phantom. The ratio of PLA powder and plaster can be adjusted to achieve an HU value similar to bone tissue. A simple combination of PLA powder and plaster enabled the creation of a custom phantom that showed similarities to the Rando phantom in both soft tissue and bone tissue.

End to end comparison of surface-guided imaging versus stereoscopic X-rays for the SRS treatment of multiple metastases with a single isocenter using 3D anthropomorphic gel phantoms

INTRODUCTION

Two end-to-end tests evaluate the accuracy of a surface-guided radiation therapy (SGRT) system (CRAD Catalyst HD) for position verification in comparison to a stereoscopic x-ray imaging system (Brainlab Exactrac ) for single-isocenter, multiple metastases stereotactic radiosurgery (SRS) using 3D polymer gel inserts.

MATERIALS AND METHODS

A 3D-printed phantom (Prime phantom, RTsafe PC, Athens, Greece) with two separate cylindrical polymer gel inserts were immobilized in open-face masks and treated with a single isocentric, multitarget SRS plan. Planning was done in Brainlab (Elements) to treat five metastatic lesions in one fraction, and initial setup was done using cone beam computed tomography. Positional verification was done using orthogonal X-ray imaging (Brainlab Exactrac) and/or a surface imaging system (CRAD Catalyst HD, Uppsala, Sweden), and shift discrepancies were recorded for each couch angle. Forty-two hours after irradiation, the gel phantom was scanned in a 1.5 Tesla MRI, and images were fused with the patient computed tomography data/structure set for further analysis of spatial dose distribution.

RESULTS

Discrepancies between the CRAD Catalyst HD system and Brainlab Exactrac were <1 mm in the translational direction and <0.5° in the angular direction at noncoplanar couch angles. Dose parameters (DMean% , D95% ) and 3D gamma index passing rates were evaluated for both setup modalities for each planned target volume (PTV) at a variety of thresholds: 3%/2 mm (Exactrac≥93.1% and CRAD ≥87.2%), 5%/2 mm (Exactrac≥95.6% and CRAD ≥94.6%), and 5%/1 mm (Exactrac≥81.8% and CRAD ≥83.7%).

CONCLUSION

Dose metrics for a setup with surface imaging was found to be consistent with setup using x-ray imaging, demonstrating high accuracy and reproducibility for treatment delivery. Results indicate the feasibility of using surface imaging for position verification at noncoplanar couch angles for single-isocenter, multiple-target SRS using end-to-end quality assurance (QA) testing with 3D polymer gel dosimetry.

Clinical performance of FractionLab in patient-specific quality assurance for intensity-modulated radiotherapy

Background

This study was aimed at comparing and analyzing the results of FractionLab (Varian/Mobius Medical System) with those of portal dosimetry that uses an electronic portal imaging device. Portal dosimetry is extensively used for patient-specific quality assurance (QA) in intensity-modulated radiotherapy (IMRT).

Methods

The study includes 29 patients who underwent IMRT on a Novalis-Tx linear accelerator (Varian Medical System and BrainLAB) between June 2019 and March 2021. We analyzed the multileaf collimator DynaLog files generated after portal dosimetry to evaluate the same condition using FractionLab. The results of the recently launched FractionLab at various gamma indices (0.1%/0.1 mm–1%/1 mm) are analyzed and compared with those of portal dosimetry (3%/3 mm).

Results

The average gamma passing rates of portal dosimetry (3%/3 mm) and FractionLab are 98.1% (95.5%–100%) and 97.5% (92.3%–99.7%) at 0.6%/0.6 mm, respectively. The results of portal dosimetry (3%/3 mm) are statistically comparable with the QA results of FractionLab (0.6%/0.6 mm–0.9%/0.9 mm).

Conclusion

This paper presents the clinical performance of FractionLab by the comparison of the QA results of FractionLab using portal dosimetry with various gamma indexes when performing patient-specific QA in IMRT treatment. Further, the appropriate gamma index when performing patient-specific QA with FractionLab is provided.

Use of Three-Dimensional Printing in Modelling an Anatomical Structure with a High Computed Tomography Attenuation Value: A Feasibility Study

Introduction: Three-dimensional (3D) printing provides an opportunity to develop anthropomorphic computed tomography (CT) phantoms with anatomical and radiological features mimicking a range of patients’ conditions, thus allowing development of individualised, low dose scanning protocols. However, previous studies of 3D printing in CT phantom development could only create anatomical structures using potassium iodide with attenuation values up to 1200 HU which is insufficient to mimic the radiological features of some high attenuation structures such as cortical bone. This study aimed at investigating the feasibility of using 3D printing in modelling cortical bone with a non-iodinated material. Methods: This study had 2 stages. Stage 1 involved a vat photopolymerisation 3D printer to directly print cube phantoms with different percentage compositions of calcium phosphate (CP) and resin (approach 1), and approach 2 using a material extrusion 3D printer to develop a cube mould for infilling of the CP with hardener as the phantom. The approach able to create the cube phantom with the CT attenuation value close to that of a tibial mid-diaphysis cortex of a real patient, 1475±205 HU was employed to develop a tibial mid-diaphysis phantom. The mean CT numbers of the cube and tibia phantoms were measured and compared with that of the original CT dataset through unpaired t-test. Results: All phantoms were scanned by CT using a lower extremity scanning protocol. The moulding approach was selected to develop the tibia middiaphysis phantom with CT attenuation value, 1434±184 HU which was not statistically significantly different from the one of the original dataset (p = 0.721). Conclusion: This study demonstrates the feasibility to use the material extrusion 3D printer to create a tibial mid-diaphysis mould for infilling of the CP as an anthropomorphic CT phantom and the attenuation value of its cortex matches the real patient’s one.

[Sterilisation of patient specific surgical guide for dental implantology made in a hospital: Validation of a sterility test and structural deformation study]

Medical device made to measure by 3D printing are now emerging in hospital. In order to improve the precision of surgery and facilitate the treatment of complicated cases, patient specific surgical guides for dental implantology are made by stereolithography in our facial surgical unit. This new activity requires to ensure the safety of patients and health personnel by validating the various step of the manufacturing circuit. In this context, the goal of this work was to study the quality of autoclave sterilisation of the patient specific surgical guide made to measure in our hospital. A protocol of sterility test was designed and validated. Sterility of implantology guides 0, 7, 14 and 28 days after sterilisation was checked. The impact of the autoclave sterilisation on the medical device structure was evaluated by visual check and during surgeries. The sterility of the implantology guides up to 28 days after sterilisation was also validated. The protocol of sterility test executed can be extended to other hospitals interested in validating a sterility test. No deformation was observed by surgeons during the dental implant process. Future studies may be necessary to check the accurate impact of sterilisation on surgical guide structure.

Development of a novel and low-cost anthropomorphic pelvis phantom for 3D dosimetry in radiotherapy

Purpose

The aim of this study was to construct a low-cost, anthropomorphic, and 3D-printed pelvis phantom and evaluate the feasibility of its use to perform 3D dosimetry with commercially available bead thermoluminescent dosimeters (TLDs).

Material and methods

A novel anthropomorphic female phantom was developed with all relevant pelvic organs to position the bead TLDs. Organs were 3D-printed using acrylonitrile butadiene styrene. Phantom components were confirmed to have mass density and computed tomography (CT) numbers similar to relevant tissues. To find out clinically required spatial resolution of beads to cause no perturbation effect, TLDs were positioned with 2.5, 5, and 7.5 mm spacing on the surface of syringe. After taking a CT scan and creating a 4-field conformal radiotherapy plan, 3 dose planes were extracted from the treatment planning system (TPS) at different depths. By using a 2D-gamma analysis, the TPS reports were compared with and without the presence of beads. Moreover, the bead TLDs were placed on the organs’ surfaces of the pelvis phantom and exposed to high-dose-rate (HDR) ⁶⁰Co source. TLDs’ readouts were compared with the TPS calculated doses, and dose surface histograms (DSHs) of organs were plotted.

Results

3D-printed phantom organs agreed well with body tissues regarding both their design and radiation properties. Furthermore, the 2D-gamma analysis on the syringe showed more than 99% points passed 3%- and 3-mm criteria at different depths. By calculating the integral dose of DSHs, the percentage differences were –1.5%, 2%, 5%, and 10% for uterus, rectum, bladder, and sigmoid, respectively. Also, combined standard uncertainty was estimated as 3.5% (k = 1).

Conclusions

A customized pelvis phantom was successfully built and assessed to confirm properties similar to body tissues. Additionally, no significant perturbation effect with different bead resolutions was presented by the external TPS, with 0.1 mm dose grid resolution.

Creation of an anthropomorphic CT head phantom for verification of image segmentation

Purpose

Many methods are available to segment structural magnetic resonance (MR) images of the brain into different tissue types. These have generally been developed for research purposes but there is some clinical use in the diagnosis of neurodegenerative diseases such as dementia. The potential exists for computed tomography (CT) segmentation to be used in place of MRI segmentation, but this will require a method to verify the accuracy of CT processing, particularly if algorithms developed for MR are used, as MR has notably greater tissue contrast.

Methods

To investigate these issues we have created a three‐dimensional (3D) printed brain with realistic Hounsfield unit (HU) values based on tissue maps segmented directly from an individual T1 MRI scan of a normal subject. Several T1 MRI scans of normal subjects from the ADNI database were segmented using SPM12 and used to create stereolithography files of different tissues for 3D printing. The attenuation properties of several material blends were investigated, and three suitable formulations were used to print an object expected to have realistic geometry and attenuation properties. A skull was simulated by coating the object with plaster of Paris impregnated bandages. Using two CT scanners, the realism of the phantom was assessed by the measurement of HU values, SPM12 segmentation and comparison with the source data used to create the phantom.

Results

Realistic relative HU values were measured although a subtraction of 60 was required to obtain equivalence with the expected values (gray matter 32.9–35.8 phantom, 29.9–34.2 literature). Segmentation of images acquired at different kVps/mAs showed excellent agreement with the source data (Dice Similarity Coefficient 0.79 for gray matter). The performance of two scanners with two segmentation methods was compared, with the scanners found to have similar performance and with one segmentation method clearly superior to the other.

Conclusion

The ability to use 3D printing to create a realistic (in terms of geometry and attenuation properties) head phantom has been demonstrated and used in an initial assessment of CT segmentation accuracy using freely available software developed for MRI.

A Systematic Review on 3D-Printed Imaging and Dosimetry Phantoms in Radiation Therapy

INTRODUCTION

Additive manufacturing or 3-dimensional printing has become a widespread technology with many applications in medicine. We have conducted a systematic review of its application in radiation oncology with a particular emphasis on the creation of phantoms for image quality assessment and radiation dosimetry. Traditionally used phantoms for quality assurance in radiotherapy are often constraint by simplified geometry and homogenous nature to perform imaging analysis or pretreatment dosimetric verification. Such phantoms are limited due to their ability in only representing the average human body, not only in proportion and radiation properties but also do not accommodate pathological features. These limiting factors restrict the patient-specific quality assurance process to verify image-guided positioning accuracy and/or dose accuracy in "water-like" condition.

METHODS AND RESULTS

English speaking manuscripts published since 2008 were searched in 5 databases (Google Scholar, Scopus, PubMed, IEEE Xplore, and Web of Science). A significant increase in publications over the 10 years was observed with imaging and dosimetry phantoms about the same total number (52 vs 50). Key features of additive manufacturing are the customization with creation of realistic pathology as well as the ability to vary density and as such contrast. Commonly used printing materials, such as polylactic acid, acrylonitrile butadiene styrene, high-impact polystyrene and many more, are utilized to achieve a wide range of achievable X-ray attenuation values from -1000 HU to 500 HU and higher. Not surprisingly, multimaterial printing using the polymer jetting technology is emerging as an important printing process with its ability to create heterogeneous phantoms for dosimetry in radiotherapy.

CONCLUSION

Given the flexibility and increasing availability and low cost of additive manufacturing, it can be expected that its applications for radiation medicine will continue to increase.

Evaluation of a 3D-printed heterogeneous anthropomorphic head and neck phantom for patient-specific quality assurance in intensity-modulated radiation therapy

We evaluated an anthropomorphic head and neck phantom with tissue heterogeneity, produced using a personal 3D printer, with quality assurance (QA), specific to patients undergoing intensity-modulated radiation therapy (IMRT). Using semi-automatic segmentation, 3D models of bone, soft tissue, and an air-filled cavity were created based on computed tomography (CT) images from patients with head and neck cancer treated with IMRT. For the 3D printer settings, polylactide was used for soft tissue with 100% infill. Bone was reproduced by pouring plaster into the cavity created by the 3D printer. The average CT values for soft tissue and bone were 13.0 ± 144.3 HU and 439.5 ± 137.0 HU, respectively, for the phantom and 12.1 ± 124.5 HU and 771.5 ± 405.3 HU, respectively, for the patient. The gamma passing rate (3%/3 mm) was 96.1% for a nine-field IMRT plan. Thus, this phantom may be used instead of a standard shape phantom for patient-specific QA in IMRT.

Characterization of a novel 3D printed patient specific phantom for quality assurance in cranial stereotactic radiosurgery applications

In single-isocenter stereotactic radiosurgery/radiotherapy (SRS/SRT) intracranial applications, multiple targets are being treated concurrently, often involving non-coplanar arcs, small photon beams and steep dose gradients. In search for more rigorous quality assurance protocols, this work presents and evaluates a novel methodology for patient-specific pre-treatment plan verification, utilizing 3D printing technology. In a patient's planning CT scan, the external contour and bone structures were segmented and 3D-printed using high-density bone-mimicking material. The resulting head phantom was filled with water while a film dosimetry insert was incorporated. Patient and phantom CT image series were fused and inspected for anatomical coherence. HUs and corresponding densities were compared in several anatomical regions within the head. Furthermore, the level of patient-to-phantom dosimetric equivalence was evaluated both computationally and experimentally. A single-isocenter multi-focal SRS treatment plan was prepared, while dose distributions were calculated on both CT image series, using identical calculation parameters. Phantom- and patient-derived dose distributions were compared in terms of isolines, DVHs, dose-volume metrics and 3D gamma index (GI) analysis. The phantom was treated as if the real patient and film measurements were compared against the patient-derived calculated dose distribution. Visual inspection of the fused CT images suggests excellent geometric similarity between phantom and patient, also confirmed using similarity indices. HUs and densities agreed within one standard deviation except for the skin (modeled as 'bone') and sinuses (water-filled). GI comparison between the calculated distributions resulted in passing rates better than 97% (1%/1 mm). DVHs and dose-volume metrics were also in satisfying agreement. In addition to serving as a feasibility proof-of-concept, experimental absolute film dosimetry verified the computational study results. GI passing rates were above 90%. Results of this work suggest that employing the presented methodology, patient-equivalent phantoms (except for the skin and sinuses areas) can be produced, enabling literally patient-specific pre-treatment plan verification in intracranial applications.

Design and fabrication of a personalized anthropomorphic phantom using 3D printing and tissue equivalent materials

To fabricate an individualized anthropomorphic lung phantom with tissue-equivalent radiation attenuation properties using a cost-effective three-dimensional (3D) printing technique. Based on anonymized human chest CT images, the phantom contained a 3D-printed skin shell, filled with tissue equivalent materials with similar radiation attenuation characteristics. The filling materials were a mixture of CaCO₃, MgO, agarose, NaCl, pearl powder and silica gel. The dose calculation accuracy of different treatment planning system (TPS) algorithms was validated and compared with the ion chamber measurements in the phantom, including tumor and surrounding normal tissues. The chest phantom was shown to represent a human's chest in terms of radiation attenuation property and human anatomy. The Hounsfield unit ranges were -60 to -100, 20 to 60, and 120 to 300 for fat, muscle, and bone, respectively. The actual measured values of the ionization chamber were 213.7 cGy for the tumor, 53.85 cGy for normal lung tissue, and 4.1 cGy for the spinal cord, compared to 214.1, 55.2, and 4.5 cGy, respectively, with use of the Monte Carlo algorithm in TPS. The application of 3D printing in anthropomorphic phantoms can improve personalized medical need and efficiency with reduce costs thus, can be used for radiation dose verification.

Gel dosimetry for three dimensional proton range measurements in anthropomorphic geometries

Proton beams used for radiotherapy have potential for superior sparing of normal tissue, although range uncertainties are among the main limiting factors in the accuracy of dose delivery. The aim of this study was to benchmark an N-vinylpyrrolidone based polymer gel to perform three-dimensional measurement of geometric proton beam characteristics and especially to test its suitability as a range probe in combination with an anthropomorphic phantom. For single proton pencil beams as well as for 3×3cm² mono-energy layers depth dose profiles, lateral dose distribution at different depths and proton range were evaluated in simple cubic gel phantoms at different energies from 75 to 115MeV and different dose levels. In addition, a 90MeV mono-energetic beam was delivered to an anthropomorphic 3D printed head phantom, which was filled with gel. Subsequently, all phantoms underwent magnetic resonance imaging using an axial pixel size of 0.68-0.98mm and with slice thicknesses of 2 or 3mm to derive a 3-dimensional distribution of the T₂ relaxation time, which correlates with radiation dose. Indices describing lateral dose distribution and proton range were compared against predictions from a treatment planning system (TPS, for cubic and head phantoms) and Monte Carlo simulations (MC, for the head phantom) after manual rigid co-registration with the T₂ relaxation time datasets. For all pencil beams, the FWHM agreement with TPS was better than 1mm or 7%. For the mono-energetic layer, the agreement with TPS in this respect was even better than 0.3mm in each case. With respect to range, results from gel measurements differed no more than 0.9mm (1.6%) from values predicted by TPS. In case of the anthropomorphic phantom, deviations with respect to a nominal range of about 61mm as well as in FWHM were slightly higher, namely within 1.0mm and 1.1mm respectively. Average deviations between gel and TPS/MC were similar (-0.3mm±0.4mm/-0.2±0.5mm). In conclusion, polymer gel dosimetry was found to be a valuable tool to determine geometric proton beam properties three-dimensionally and with high spatial resolution in simple cubic as well as in a more complex anthropomorphic phantom. Post registration range errors of the order of 1mm could be achieved. The additional registration uncertainty (95%) was 1mm.

Inter-comparison of quantitative imaging of lutetium-177 (177Lu) in European hospitals

BACKGROUND

This inter-comparison exercise was performed to demonstrate the variability of quantitative SPECT/CT imaging for lutetium-177 (177Lu) in current clinical practice. Our aim was to assess the feasibility of using international inter-comparison exercises as a means to ensure consistency between clinical sites whilst enabling the sites to use their own choice of quantitative imaging protocols, specific to their systems. Dual-compartment concentric spherical sources of accurately known activity concentrations were prepared and sent to seven European clinical sites. The site staff were not aware of the true volumes or activity within the sources-they performed SPECT/CT imaging of the source, positioned within a water-filled phantom, using their own choice of parameters and reported their estimate of the activities within the source.

RESULTS

The volumes reported by the participants for the inner section of the source were all within 29% of the true value and within 60% of the true value for the outer section. The activities reported by the participants for the inner section of the source were all within 20% of the true value, whilst those reported for the outer section were up to 83% different to the true value.

CONCLUSIONS

A variety of calibration and segmentation methods were used by the participants for this exercise which demonstrated the variability of quantitative imaging across clinical sites. This paper presents a method to assess consistency between sites using different calibration and segmentation methods.