Material selection for a 3D-printed tissue-equivalent cost-effective phantom for radiotherapy quality assurance

Radiation therapy is a powerful tool for treating cancer, using high-energy beams to destroy tumors while sparing the healthy tissue around them. Because these beams are invisible and their effects are measured in tiny fractions of energy, doctors rely on rigorous checks before treating a patient. They use models, called phantoms, to simulate how a specific patient's body will absorb the radiation. Traditionally, these models are made from standard blocks of plastic that look like a generic human body. While reliable, these standard blocks cannot capture the unique shape of an individual patient or the complex mix of tissues inside them. To fix this, scientists have turned to 3D printing, a technology that builds objects layer by layer, to create custom models that match a patient's exact anatomy. However, a major hurdle remains: the plastic used in 3D printers does not naturally behave like human flesh when hit by radiation. If the plastic absorbs too much or too little energy compared to real tissue, the safety checks will be wrong, potentially leading to incorrect treatment plans.

A team of researchers set out to solve this problem by finding the right materials for these custom 3D-printed models. They needed to identify plastics that mimic human soft tissue not just in appearance, but in how they interact with radiation at two very different energy levels. The first level is the low energy used for medical imaging, like a standard CT scan, which creates the map of the patient's body. The second is the high energy used to actually treat the cancer, which penetrates deep into the body. The researchers wanted to know if they could print a model that looked right on a scan and delivered the correct dose of radiation during treatment, all while keeping the cost low enough for hospitals to use regularly.

To find the answer, the team created over sixty small cubes of plastic, representing about thirty different types of materials commonly used in 3D printing. They used three different printing machines: two that melt and lay down thin threads of plastic, and one that cures liquid resin with light. They printed some cubes completely solid and others with varying amounts of empty space inside, a feature known as infill density, to see if they could tune the material's properties. After printing, they scanned every cube with a clinical CT scanner to measure how the material appeared on the image, recorded by a value called a Hounsfield Unit. They also weighed the cubes to determine their mass and calculated how many electrons were packed into each one, a critical factor for how radiation travels through matter. Finally, they tested how the materials behaved under the high-energy beams used in actual cancer treatment by placing a tiny sensor inside a hole in the center of larger cubes and measuring the energy deposited.

The results revealed that several common plastics could indeed mimic human soft tissue very closely. When printed at 100% density, the materials showed a wide range of properties. Some plastics appeared very dark on the CT scan, similar to fat, while others appeared bright, resembling denser organs. The mass density of these solid cubes ranged from slightly lighter than water to about twenty percent heavier. Crucially, when the researchers tested these materials under the high-energy treatment beams, the amount of radiation they absorbed matched the computer predictions almost perfectly. The difference between what the machines predicted and what the sensors measured was tiny, never exceeding about one and a third percent. This level of agreement suggests that these printed materials behave almost exactly like water, which is the standard reference for human tissue in radiation therapy.

The study also discovered that changing the amount of empty space inside the printed objects allowed the researchers to fine-tune the materials further. By adjusting the infill density from ten percent to ninety percent, they could shift the properties of a single plastic type to cover a vast spectrum. For example, a plastic that was too dense to mimic fat when solid could be printed with large gaps to mimic the low density of air-filled lungs. This flexibility meant that a single material could be used to represent different parts of the body simply by changing the printing settings. The researchers found that specific combinations of plastic type and infill density could closely match the radiological properties of specific human tissues, such as the liver, heart, or breast tissue.

While the findings are promising, the researchers noted some limitations. The high-end machine that used liquid resin produced excellent results but was significantly more expensive than the machines that used plastic threads. Additionally, the liquid resin samples showed some signs of deformation after being stored for several months, whereas the plastic thread samples remained stable. The study focused only on soft tissues, leaving the challenge of finding materials that mimic bone for future work. Despite these caveats, the research demonstrates that it is possible to create cost-effective, patient-specific models for radiation safety checks. By selecting the right plastic and adjusting the internal structure, hospitals can produce phantoms that are both affordable and accurate, ensuring that the radiation doses delivered to patients are safe and precise.