3D Printing for Radiotherapy

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3D printing (also known as additive manufacturing) is a digital process. Three-dimensional objects are fabricated layer-by-layer by depositing layers of melted plastics from a heated nozzle (shown in Figure 1) or by the curing of resins using lasers. This manufacturing process requires a virtual or digital model of the object to be produced that is used to create a series of instructions for the printer. These models can be created from scratch or based on 3D scans (e.g. constructed from photographs) or medical imaging data (such as CT or MRI images) of patients, volunteers or inanimate objects.

Advances in 3D print technologies and the development of easy-to-use computer-aided design solutions during the past two decades has provided a positive impact on the manufacturing of parts and components in the healthcare industry. The new 3D printing technology enables health professionals to make cost-efficient, patient-matched medical devices at the point-of-care. Some examples are the making of:

• splints,
• prostheses,
• stents,
• meshes and matrices that promote wound healing,
• models that facilitate surgical planning, and
• educational tools.

Examples of 3D devices or accessories tailor-made for radiotherapy patient treatments are:

• immobilisation set-up devices,
• tissue-equivalent bolus,
• shielding for low energy X-ray beam shaping, and
• special brachytherapy applicators.

Handy physics quality assurance jigs and phantoms can also be made by 3D printing. The improved design of these 3D physics tools enables more accurate radiation beam measurements and the patient’s treatment – a precision that must be regularly monitored by the physicists.

This article summarises some of the 3D print work at Royal Brisbane and Women’s Hospital made in conjunction with the Herston Biofabrication Institute.

Bolus for superficial treatments

The most common 3D print job at the Royal Brisbane and Women’s Hospital is the making of personalised bolus used for patients receiving X-ray or electron treatments on the skin surface or similar locations. Figure 2 shows the moulded bolus placed on the patient’s hand. The bolus is used to increase the dose to superficial treatment skin area and reduces the dose at depth in the hand. If there are any air gaps between the bolus and the patient’s hand due to a poor fitting, the treatment can be compromised.

The 3D printed bolus (Figure 2) is modelled using either medical imaging or 3D scan data. The 3D bolus provides a better fit with fewer air gaps and can be more reliably positioned in-situ for each treatment.

Low kilovoltage X-ray beam shielding

Previously, thin lead shielding was used to shape the low kilovoltage X-ray beam (also referred to as superficial X-rays). A large majority of these cases involved the treatment of patients who had skin cancers on their head and neck area. The mould room technician made a plaster cast negative of the patient’s head and neck area and then a positive plaster cast from the negative (Figure 3).

The skin cancer treatment area is mostly relatively small and irregular in shape and occurs in difficult regions such as the eyes, nose, mouth or ear. The treatment field shape is cut out of approximately 2mm thick lead and then moulded to fit on the patient’s positive plaster cast, as shown in figure 3.

Apart from the labour intense procedure for making the lead shield, there are occupational precautions necessary to avoid staff accidently absorbing very small quantities of toxic lead. Patients also found the making of the plaster cast distressing (especially those who suffered claustrophobia to any extent) and some had painful skin lesions. Figure 4. shows the patient set up for the lead shield arrangement.

Metal-plastic composites, sufficient to attenuate the superficial X-ray beam, can be tailor-made for each patient by the 3D print method. These composites contain materials like copper or tungsten, avoiding the need for precautions taken with lead. A 2 mm 3D printed copper-PLA shield is shown in Figure 5.

Physics Quality assurance tools

The goal of physics QA is to ensure that all equipment related to patient treatment operate safely and are accurately calibrated. This is to ensure that the cancer is accurately targeted and receives the very high dose required to eradicate it while other surrounding critical structures receive as little dose as possible. Radiation safety of patients and staff alike is also an important aspect. QA is an ongoing regular requirement and having the 3D physics tools that help to streamline and make more efficient procedures have become valuable assets for this work.

Before any new radiotherapy equipment or method of treatment by calculation is introduced into clinical use for patient treatment, check measurements on a tissue-equivalent phantom must be completed and confirmed as being within acceptable clinical accuracy. This is normally achieved by using anthropomorphic phantoms to replicate physical characteristics of the patient’s anatomy. Figure 6 shows part of a virtual phantom as a series of 3D printed slabs, reproducing soft tissues, lung and bone. A composite of stone powder and plastic is used in the phantom to reproduce the CT image characteristics of normal bone. By placing radiosensitive film between the slices,  the radiotherapy delivered dose can be measured and checked against the computer calculated treatment.

Conclusion

3D printing provides a cost-effective production of bespoke medical devices and quality assurance tools for radiotherapy purposes. While departments considering point-of-care manufacturing will need to comply with applicable standards (e.g. International Standards Organisation 13485 for quality management of medical devices) and regulations (e.g. TGA requirements for patient-matched devices), the barriers to entry continue to reduce as better solutions evolve in the healthcare industry. 

Acknowledgements

Thanks to Emily Simpson-Page, Tanya Kairn, Sanphat Sangudsup, Lovissa Jessen and staff at the Royal Brisbane and Women’s Hospital and Herston Biofabrication Institute for their contributions included here.

Scott Crowe PhD 8 July 2021

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