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International Journal of Bioprinting 3D bioprinting of boluses for radiotherapy

added to each well. The cells were incubated in the dark A virtual bolus with a thickness of 5 mm was generated
for 4 h. Subsequently, the supernatant was aspirated, and in the Monaco TPS to achieve the desired dose coverage
the resulting crystals were dissolved in 1 mL of dimethyl of the PTV. During plan optimization, the PTV served as
sulfoxide (DMSO). The absorbance of the solution was the objective structure, while rings surrounding it were
measured at 492 nm using a microplate reader (Infinite utilized to restrict doses outside its boundaries. The target
F50, TECAN). volume, achieving 95% coverage, was normalized to the
prescribed dose following optimization. Subsequently, the
2.5. Stability test virtual bolus was converted into a standard tessellation
The cured gel was subjected to an environment with a language (STL) file using MIM. The STL file was
temperature range of 22–26°C and a humidity level between subsequently subjected to smoothing using 3D-matic
40% and 60%. Computed tomography (CT) imaging software in order to eliminate CT slicing artifacts, and the
was performed using Philips CT scanner to evaluate the resultant smoothed file was printed utilizing a DLP printer.
homogeneity of the gels. The CT scanner was configured with Subsequently, CT images of the phantom were acquired
acquisition parameters: energy at 120 kV, current at 350 mA, using both the printed and commercial boluses. IMRT/
and a slice thickness of 3 mm. The images were subsequently VMAT plans were then generated for each type of bolus,
imported into Monaco treatment planning system (TPS), including virtual ones, while adhering to identical dose
where Hounsfield units (HU) and relative density (RD) constraints. A total prescription of 60 Gy was administered
values were obtained from five 1 × 1 cm regions of interest to the PTV for 30 consecutive days, with dose distribution
2
located in the central axial CT slice of the gels. Changes in calculated using the Monte Carlo algorithm for planning
shape were observed at various time points (0 h, 15 h, and purposes. Subsequently, maximum gaps between skin and
1 week) through CT imaging. The selected time points for commercial/3D-printed boluses were investigated.
observation were based on the practical application scenario
of personalized boluses, which typically involve a maximum 2.7. Statistical analysis
usage of 5 h per week over a duration of 3 weeks. Statistical analyses were conducted using SPSS (version
19.0), with a significance threshold of P <0.05.
2.6. Radiological evaluation
A DLP projector (405 nm, Nova Bene 4, China) was utilized 3. Results and discussion
for printing our composite bioink, which comprises of 3.1. System design
ALG (0.5 wt%), AM (20 wt%), PEGDA (0.2 mol% of AM), The gel designed for bolus application is illustrated in
LAP (0.1 wt%), GLY (30 wt%), and Tar (0.0035 wt%) based Figure 1A. The DLP-based 3D bioprinting technique
on mechanical and adhesive testing. The printer’s G-code employs a blue light (405 nm) to activate the photoinitiator
was generated through the use of commercial software, LAP, leading to the generation of free radicals that
with printing parameters set to a thickness of 100 µm, initiate the breaking of C=C double bonds in AM and
five base layers, and a curing time of 9 s. After printing, PEGDA [25,26] . This process results in the formation of a
the constructs were briefly rinsed with water to eliminate crosslinking network and solidification of the bioink into
unreacted solution. Subsequently, the models underwent a desired pattern. Figure 1B illustrates the formation of
post-curing in a specialized box to enhance their internal DN hybrid gels, which results from the interpenetration
structural stability and reinforce their printed form. of a chemically crosslinked network and a physically
To assess the accuracy of the DLP system in utilizing crosslinked network. The former is synthesized through
bioink, gel cuboid arrays were printed orthogonally to the acrylamide polymerization with PEGDA and entanglement
build platform. Two cuboid boluses (5 cm × 10 cm × 0.5 with ALG, where PEGDA serves as a crosslinker to facilitate
cm) were designed using CAD software, and the printing covalent network formation. The second crosslinking
accuracy was assessed by calculating the deviation between network is built with the hydrogen bonds provided by PA,
the printed and intended dimensions. The gel can be used ALG, PAM, and GLY. The hydrogen bonds endowed the
to prepare bolus for head radiotherapy. gel with enhanced tensile and adhesive properties. These
intermolecular forces endow the gel with superior tensile
The moldability of the gel was confirmed through CT strength and adhesive properties.
imaging of a Rando phantom, wherein an artificial gross
tumor volume (GTV) was initially defined below the 3.2. Structure of the gels
skin surface in no-bolus CT images with a reconstruction SEM images of the freeze-dried gels are presented in Figure
thickness of 1 mm. A planning target volume (PTV) 2A, revealing interconnected and porous structures. The
was generated by expanding a 3 mm margin around the gel without ALG displayed a loosely arranged network
GTV, while ensuring it remained on the patient’s surface. structure, whereas the inclusion of ALG led to a more

Volume 10 Issue 2 (2024) 253 doi: 10.36922/ijb.1589