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3D-printed Bolus in Radiotherapy
reducing the air gap and improving the efficiency of customized bolus directly processed using 3D printing
radiotherapy. has also attracted wide attention of doctors and scholars
However, the indirect printing method is in the field of radiotherapy.
complicated and time-consuming, and the accuracy of Up to now, various polymers have been used to
this method is relatively low. The process of this method fabricate bolus using different 3D printing technologies,
mainly includes molding and casting, which determines mainly including FDM, inkjet and SLA [79,80] . FDM is
the preparing efficiency of customized bolus. Chiu et al. one of the earliest 3D printing technologies used to print
have constructed a silicone bolus using this method bolus. For example, Kim et al. have used the FDM method
for head-and-neck radiotherapy. The time taken for to print a nose bolus using ABS resins as the printing
construction including mold printing and fillers casting is material . Using ABSplus thermoplastic as printing
[22]
about 2 days depending on bolus surface area, complexity materials, Park et al. have fabricated a customized ear
and volume . In this method, two pieces of mold shells bolus by applying the FDM method . A customized
[26]
[82]
including a positive and a negative are first printed and breast bolus composed of PLA was also printed by means
then put together to form a chamber for casting fillers. of this method . Through this method, a semi-flexible
[24]
The accuracy of the bolus is seriously influenced by the TPU was processed into a customized leg bolus used
thickness of mold shells. This method is suitable for the for treating the primary cutaneous lymphoma . In the
[83]
preparation of bolus with a large size used for breast or current study, the thermoplastic polymers were mainly
head radiotherapy, but not for small bolus with complex used to process customized bolus based on the printing
structures used for ear or nose. In a word, although the principle of FDM technology. Compared with both the
indirect printing method enables the customization of commercial sheet bolus and the indirectly printed bolus,
bolus and improves the efficiency of radiotherapy to some the customized bolus processed by FDM has a better fit
extent, the complicated and time-consuming process will to the irregular body skin. However, these thermoplastic
greatly limit the commercialization of this technology in polymers used for FDM are stiffer than the soft tissues.
the field of radiotherapy. It inevitably results in the patient experiencing pain and
air gaps, influencing the efficiency of radiotherapy. In
4.2. Direct printing bolus addition, the time required for preparing a customized
Direct printing means that 3D objects can be directly bolus using this method was relatively long due to the
realized from CAD models by means of computer time-consuming process of FDM. Consequently, a more
numerical control printers. CAD models are translated malleable and printable polymer should be used to
into computer-readable formats (usually standard process customized bolus.
tessellation language file), and then sliced into control Recently, some other 3D printing technologies
codes which can control printers to solidify material in suitable for processing soft polymers have begun to be
a layer-by-layer manner (Figure 6B). With the rapid used to produce customized bolus. Park et al. used an
[76]
development of 3D printing technology, increasing inkjet printing technology to print a nose bolus composed
researchers from different backgrounds have started to of a malleable rubber-like material . Using the same
[76]
manufacture devices or structures with high complexity method, Baltz et al. also made a customized bolus cap
using this powerful technology [8,77,78] . In recent years, the composed of a rubber-like photopolymer resin used for

A

B

Figure 6. (A) Indirect printing workflow of bolus. (B) Direct printing workflow of bolus.

36 International Journal of Bioprinting (2021)–Volume 7, Issue 4