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Lu, et al.
including light absorbers, radical inhibitors, and diluents. printing and direct printing. The former is to first print a
The light absorbers can restrain the curing depth, the shell of the bolus and then fill it with other polymers, and
radical inhibitors can prevent premature solidification the latter is to print a bolus directly with a 3D printing
of liquids, and the diluents can decrease the liquid technique.
viscosity . Compared to the large polymer family,
[74]
the variety of photopolymers is relatively small. Novel 4.1. Indirect printing bolus (casting)
photopolymers can be developed by grafting specific As the name suggests, the indirect printing means that it
photosensitive groups with normal polymers. Up to now, takes at least two steps to produce a customized bolus.
various photopolymers have been used for photocuring The typical workflow to make bolus by the indirect
printing, including photocuring resins, hydrogels, and printing is shown in Figure 6A. In this method, the
silicone elastomers . bolus shells are first printed by 3D printing technology,
[75]
4. Customization of bolus through 3D then the materials were cast into the chamber of shells.
printing After demolding, a customized bolus is obtained. To
be precise, this method should be called casting, which
With the improvement of 3D printing technology, it is suitable for processing most of hydrogels and some
has been widely used in various fields, especially the elastomers with low melting point. Up to now, various
biomedical field requiring customization of medical polymers have been widely processed as customized
devices. 3D printing is currently a promising approach bolus using this method. For example, Kong et al.
to achieve the customization of bolus. Compared to the printed a shell of bolus using the PLA and then filled it
commonly used flat bolus, the 3D-printed bolus allows with silicone rubber and hydrogels for non-melanoma
for a more match to a patient’s skin surface and shows skin cancer radiotherapy [10] . Park et al. casted a urethane
desirable curative effect of radiotherapy (Figure 5). liquid rubber and liquid silicon compound into the
However, 3D printing bolus still stays in the early stages mold to make the customized boluses [81] . Resins are
of development due to shortcomings of printable and commonly used to print mold shells due to their high
appropriate materials. Reviewing the research progress stiffness, low swellability, and low flexibility. Compared
of 3D printing bolus in recent years, we divide the current to the commonly used sheet bolus, the bolus cast by
3D printing bolus into two main categories: indirect this method can contact closer with the body, greatly

A B C D

E F G

Figure 5. Some 3D-printed boluses reported in the literature. (A) A nose bolus printed with Tangoplus (Reproduced from Ref licensed
[76]
under Creative Commons Attribution 4.0 license). (B) A bolus printed with ABS on the head phantom surface (Reproduced from Ref
[77]
licensed under Creative Commons Attribution 4.0 license). (C) 3D-printed bolus of the 4 and 5 knuckle (Reproduced from Ref licensed
th
[8]
th
under Creative Commons Attribution 4.0 license). (D) 3D-printed bolus fitting the ear of a volunteer (Reproduced from Ref licensed under
[8]
Creative Commons Attribution 4.0 license). (E) 3D-printed Ninjaflex bolus covering the right-hand side of the head phantom (Reproduced
from Ref licensed under Creative Commons Attribution 4.0 license). (F) 3D-printed breast bolus (Reproduced from Ref licensed
[79]
[78]
under Creative Commons Attribution 4.0 license). (G) Bolus printed with PLA on the Alderson RANDO phantom (Reproduced from Ref
[80]
licensed under Creative Commons Attribution 4.0 license).
International Journal of Bioprinting (2021)–Volume 7, Issue 4 35