Development and validation of a 3D‐printed bolus cap for total scalp irradiation

Abstract Purpose The goal of total scalp irradiation (TSI) is to deliver a uniform dose to the scalp, which requires the use of a bolus cap. Most current methods for fabricating bolus caps are laborious, yet still result in nonconformity and low reproducibility, which can lead to nonuniform irradiation of the scalp. We developed and validated patient‐specific bolus caps for TSI using three‐dimensional (3D) printing. Methods and materials 3D‐printing materials were radiologically analyzed to identify a material with properties suitable for use as a bolus cap. A Python script was developed within a commercial treatment planning system to automate the creation of a ready‐to‐print, patient‐specific 3D bolus cap model. A bolus cap was printed for an anthropomorphic head phantom using a commercial vendor and a computed tomography simulation of the anthropomorphic head phantom and bolus cap was used to create a volumetric‐modulated arc therapy TSI treatment plan. The planned treatment was delivered to the head phantom and dosimetric validation was performed using thermoluminescent dosimeters (TLD). The developed procedure was used to create a bolus cap for a clinical TSI patient, and in vivo TLD measurements were acquired for several fractions. Results Agilus‐60 was validated as a new 3D‐printing material suitable for use as bolus. A 3D‐printed Agilus‐60 bolus cap had excellent conformality to the phantom scalp, with a maximum air gap of 4 mm. TLD measurements showed that the bolus cap generated a uniform dose to the scalp within a 2.7% standard deviation, and the delivered doses agreed with calculated doses to within 2.4% on average. The patient bolus was conformal and the average difference between TLD measured and planned doses was 5.3%. Conclusions We have developed a workflow to 3D‐print highly conformal bolus caps for TSI and demonstrated these caps can reproducibly generate a uniform dose to the scalp.


| INTRODUCTION
Total scalp irradiation (TSI) is a specialized treatment technique that aims to deliver a uniform dose to the entire scalp. In the past, TSI has been faced with two major obstacles. First, dose homogeneity, which is substantially limited by the complex field matching required with electron or electron-photon-based techniques. 1 This obstacle has been substantially addressed through transition to using intensity modulation radiation therapy and volumetric modulated arc therapy (VMAT) techniques to eliminate field matching. [2][3][4][5] The second major obstacle, which still remains, is the need for a scalp bolus in order to ensure adequate dose to the skin.
Making a bolus that is conformal to the scalp is difficult owing to the convex shape of the scalp. Our current standard-of-care technique uses sheets of soft 0.5 cm thick commercial bolus material that are cut and taped together and placed on the patient's head to create a bolus cap that is held in place under a swim cap. This method is laborious, time-consuming, and ultimately produces a bolus cap that is difficult to reproduce for daily treatments and prone to deforming under the swim cap, causing random air gaps.
Other bolus fabrication methods for photon-based TSI presented in the literature suffer from similar limitations. Bedford et al. used an immobilization shell with 1 cm of wax built up on the interior surface. This method suffered from large air gaps between the wax bolus and scalp surface, which led to errors as large as 12% between the planned and delivered dose to the scalp. 2 Lin et al. used a thermoplastic mesh mask formed to the posterior of the patient's head and then glued 0.5 cm bolus slabs to the surface of the mask. This method had good daily setup reproducibility but still had air gaps as large as 1.5 cm and required the construction of a custom head rest and immobilization device. 5 Most other TSI studies described in the literature have used 0.5-to 1.0-cm-thick solid sheets of thermoplastic material that are heated and formed to the patient's medial scalp, with sheets of soft bolus material taped on to cover the lateral portions of the scalp. 4,6,7 Although these methods have demonstrated good conformality, they still require manual fabrication and are prone to patient discomfort and reproducibility issues.
The use of three-dimensional (3D) printing to create specialized radiation therapy devices has been a growing area of research. 3D printing offers a minimally labor intensive method to create custom patient-specific devices using 3D models of patient anatomy that can be derived from CT DICOM data. This has been demonstrated through the use of 3D-printers in the fabrication of bolus, compensators, and patient-specific phantoms. [8][9][10][11][12] Thus far, most 3D-printed boluses have been used for small and/or relatively flat treatment sites, such as the nose, 11 ear, 13 eye canthi, 14 and foot surface. 10 Additionally, most 3D-printed bolus has used standard, rigid thermoplastic materials, including polylactic acid and acrylonitrile butadiene styrene. While these traditional 3D-printing materials have been shown to be suitable for use as bolus, these materials are unsuitable for a TSI bolus due to the unique challenges associated with TSI.
Because the scalp is a relatively extensive treatment area, a bolus made from these materials would be extremely rigid and not practical to fit onto the patient's head in one piece. Additionally, a rigid bolus would be uncomfortable to fit onto a patient and for the patient to wear while in the immobilization setup as TSI patients' scalps are very sensitive due to radiation-induced acute skin toxicity. 15 These issues represent the unique challenges that were considered when developing a 3D printed scalp bolus for TSI. The purpose of this study was to design a 3D-printed bolus to be used in TSI that improves upon the current problems of nonconformality and limited reproducibility of the bolus cap and that can be readily fabricated as part of a clinical workflow.

2.A | Material analysis
Due to the unique challenges presented in developing a 3D-printed bolus for TSI, we consulted with a local 3D-printing company (3D Print Bureau of Texas, Houston, TX, USA) on potential 3D-printing materials that could be suitable as bolus and meet the requirements of patient comfort for TSI. This company has commercial-grade Poly-Jet 3D printers capable of printing materials with many different properties. One such material is Agilus (Stratasys, Eden Prairie, MN, USA), which is a soft-curing rubber-like photopolymer resin that can be blended in discrete concentrations with a hard-curing photopolymer resin during printing to produce objects with varying elasticity ranging from very soft rubber material to a solid block. The ability of 3D-printed Agilus to produce materials with differing elasticity meant we could select a mixture where the final material closely mimics the flexibility and softness of traditional bolus material. However, the radiological properties of Agilus have not previously been evaluated.
Thus, to determine which Agilus mixture would be most appropriate for use as a scalp bolus, we conducted radiological analysis on a spectrum of printed samples with varying mixtures. Agilus mixtures are characterized by their Shore durometer value.
For example, Agilus-27 has a Shore value of 27 and is the softest material that can be printed, and Agilus-100 is the firmest material.
We first conducted a CT analysis to determine how well the material's physical density was predicted by our standard CT calibration curve. We obtained 25 mm × 200 mm × 5 mm strips printed in the following Shore values: Agilus-27, Agilus-40, Agilus-50, Agilus-60, and Agilus-70. The average CT number of each strip was measured using the DICOM imaging software OsiriX (Pixmeo, Bernex, Switzerland) and the clinical CT calibration curve was used to predict the density of each strip. The predicted density was then compared with the true density, which was calculated on the basis of weight (measured with a high-accuracy scale) and dimensions (measured with calipers).
Of the materials evaluated, Agilus-60 was identified as the most suitable for a bolus cap (see Section 3) and was therefore further evaluated with percent depth dose (PDD) measurements in 3D printed Agilus-60 blocks using a method described by Craft and Howell 8 and briefly summarized here. The external vendor printed blocks of varying sizes with holes for an Exradin A1SL small-volume ionization chamber. A Varian Truebeam linear accelerator (Varian Medical Systems, Palo Alto, CA, USA) was used to acquire PDD measurements for a 6-MV beam. A CT scan of the blocks was imported into RayStation, where we modeled the PDD measurement setup in two different ways: one with the density of the blocks overridden with the true measured density of 1.14 g/cm 3 and one with the density of the blocks derived from the CT calibration curve.

2.B | Phantom study
A CIRS ATOM anthropomorphic head phantom (CIRS, Norfolk, VA, USA) was used to develop a fabrication workflow and dosimetrically validate the 3D-printed bolus cap. The head phantom was scanned using a Philips Brilliance Big Bore CT scanner (Philips Healthcare, Andover, MA, USA) using our institution's standard head and neck protocol for CT simulation (3 mm slice thickness, 120 kVp, 400 mAs). The scan was then imported into our commercial treatment planning system (TPS) RayStation 6.99 (RaySearch Laboratories, Stockholm, Sweden).
In the interest of reducing fabrication time to develop an optimal clinical workflow, a Python script (provided in Appendix S1) was developed for RayStation to automatically generate a 5 mm thick patient-specific bolus cap contour. Before running the script, a rough outline of the desired extent of the bolus cap on the scalp is required. Upon execution, the script generates an external contour of the phantom, and automatically performs the necessary expansions, contractions, and Boolean operations to create the patient-specific bolus cap contour. Finally, the script uses a built-in RayStation function to export the bolus cap contour as an .stl file that is compatible with 3D modeling/printing software.

2.B.1 | Printing the bolus cap
The Agilus-60 compound is soft and flexible, like conventional sheets of commercial bolus material, yet is rigid enough to maintain its shape. These properties meant that the bolus cap could be printed in one piece and easily fit onto a patient's head. Because of this, no further modification to the one-piece bolus .stl file was necessary.
The bolus cap was printed in Agilus-60 by the 3D printing company using a Stratasys PolyJet J750 3D printer.

2.B.2 | CT TSI simulation and treatment planning
The 3D-printed bolus cap was fitted onto the anthropomorphic head phantom and CT scanned using our standard immobilization setup.
The head phantom was rested on an Orfit (Orfit Industries, Wijnegem, Belgium) head support and fitted with a three-point thermoplastic immobilization mask.
The CT scan was imported into RayStation and a VMAT plan was generated using our standard-of-care treatment technique: single isocenter, two arcs, 6-MV photons, and prescription of 60 Gy to 99% of the scalp clinical target volume (CTV) delivered in 30 fractions. The plans were reviewed and approved by a radiation oncologist (A.S.G.) specializing in head and neck treatments.

2.B.3 | Dosimetric validation
To verify that the 3D-printed bolus cap achieved the necessary surface buildup for a TSI treatment, we performed dosimetric validation using thermoluminescent dosimeters (TLDs). The anthropomorphic head phantom was marked with radio-opaque markers

3.A | Material analysis
The results of the CT analysis and physical measurements of the Agilus compounds are presented in Fig. 3. The Agilus-27 strip's density was accurately predicted by the CT calibration curve, with a 0.85% error.

However, this compound was too deformable, and a test print of an
Agilus-27 bolus cap demonstrated that it could not hold its own shape and would be prone to reproducibility errors. Agilus-60 was the next best modeled by the CT calibration curve, with a 1.39% error. This material better held its shape, while remaining soft and semi-flexible, which is why it was chosen for the bolus cap.
The results of the PDD measurements are presented in Fig. 4.
The water commissioning PDD curve is provided as reference to show the measured PDD behaved similar to water, but deviated at depth due to the higher density of the material. The TPS data agreed very well with the measured data except for points shallower than D max , in which the TPS calculated a higher than measured PDD.
Overriding the block density in the TPS did not significantly affect the modeled PDD, suggesting that the TPS accurately modeled the heterogeneity correction using our standard CT calibration curve.
From the PDD blocks, we found Agilus-60 to have a density of 1.14 g/cm 3 and a mean (±SD) HU of 84 ± 33. While Fig. 4 shows the Agilus-60 strip had a measured density of 1.09 g/cm 3 , this is within expected density variation of 3D-printed materials. 17 The CT and PDD measurements demonstrated Agilus-60 to be a tissue equivalent material suitable for use as bolus.

3.B.2 | CT simulation and treatment planning
Photographs of the CT simulation setup and CT images of the bolus cap on the head phantom are presented in Fig. 6. The CT images showed that the bolus cap was conformal to the phantom's scalp, with 4 mm being the maximum air gap observed.
Isodose distributions for the VMAT treatment plan generated in RayStation are presented in Fig. 7. The plan achieved the prescription of 60 Gy to 99% of the CTV, with clinically acceptable doses to the brain and brain stem. The white spots seen inside the CTV in the isodose distributions represent 105% hot spots (63 Gy). Hot spots were minimized as much as possible during planning while still maintaining prescription coverage to 99% of the CTV, which is a F I G . 4. Plot comparing measured PDD in Agilus-60 (square) with the water commissioning data (line) and TPS modeled PDD with no density override (star) and density overridden to 1.14 g/cm 3 (triangle).
F I G . 5. Pictures of the one-piece Agilus-60 3D-printed bolus cap printed by the external company. Note the bolus cap can be printed in any color desired.
planning priority and the presence of hot spots to achieve the prescription coverage is an acceptable trade off.

3.C.1 | Patient study
The Agilus patient-specific bolus cap was printed in similar time to that of the phantom bolus cap and cost $1,700. The patient's CT simulation scan showed the bolus to have overall good conformality to the patient's scalp, shown in Fig. 8. The maximum air gap measured was 7 mm, which was larger than observed in the phantom study. The gap was observed on the patient's right side, and may be due to pulling caused by the additional bolus that was added to treat the ear and temple. For future patients with added bolus, conformality could be improved by instructing radiation therapists to better form the thermoplastic mask around the bolus during CT simulation to support the bolus against the scalp.   Future research will include using 3D-printed Agilus bolus for other complex anatomy requiring bolus such as soft-tissue sarcomas and generalizing the python script to be compatible with creating bolus for these applications.
In conclusion, we developed a semi-automated workflow for the fabrication of highly conformal, patient-specific 3D-printed bolus caps for use in TSI. Material analysis identified Agilus-60 as a new 3D-printing material with suitable physical and radiological properties for use as a bolus in radiation therapy. An end-to-end phantom study demonstrated that the fabrication method developed created a conformal bolus and subsequent dosimetric validation measurements demonstrated that the 3D-printed bolus cap generated a uniform dose to the scalp that could be accurately calculated by the TPS, and therefore met the clinical requirements for TSI. A patient study showed the technique worked well with a patient and the bolus cap reproducibly delivered full dose to the scalp. Additionally, the technique offered significant advantages to our clinical workflow for TSI.