Dosimetric comparison of photon and particle radiotherapy devices for prostate cancer: a rapid systematic review with frequentist and Bayesian network meta-analysis

Every year, millions of men around the world face a diagnosis of prostate cancer, a disease that, when caught early, can often be cured with radiation. For decades, doctors have relied on beams of light—technically known as photons—to zap the tumor while trying to spare the healthy organs nearby, such as the bladder and the rectum. Over time, the tools used to deliver these beams have become incredibly sophisticated. Some machines spin around the patient, painting the tumor with radiation from every angle; others use robotic arms to aim from dozens of different directions. More recently, a different kind of weapon has entered the scene: particle therapy. Instead of light, these machines fire heavy particles, like protons or carbon ions, which behave differently inside the body. They can stop more abruptly, potentially delivering their energy right where it is needed and leaving less behind. But with so many different machines and techniques available, it has been difficult to know which one truly offers the best balance of hitting the target and protecting the patient.

A team of researchers set out to solve this puzzle by bringing together scattered pieces of evidence from around the world. They did not treat new patients themselves; instead, they gathered data from nine existing studies that had already compared these different radiation machines on paper. These studies used the same computer scans of patients to create virtual treatment plans, allowing the researchers to see how each machine would perform if it were used on the same person. The team looked at six distinct technologies: standard intensity-modulated radiation therapy, a spinning arc technique, a robotic system, a helical machine that spirals around the patient, and two types of particle therapy using protons and carbon ions. By combining the results of these nine studies into one large analysis, they could rank the machines against each other to see which one produced the most precise and safest radiation plans.

The results of this massive comparison revealed a clear leader in the race for precision, though with an important caveat. The carbon ion machine, which uses heavy particles, consistently produced the most perfectly shaped radiation clouds around the tumor. It was the best at making sure the entire tumor received the dose it needed while keeping the dose inside the tumor itself very even. Furthermore, this machine was the most effective at keeping the average amount of radiation hitting the bladder and the rectum as low as possible. In simple terms, it spared the healthy tissue the most on average. However, the study also found a specific weakness in this otherwise superior machine. While the average dose to the rectum was low, the highest single point of radiation hitting the front wall of the rectum was actually higher with the carbon ion machine than with the others. This suggests that while the machine is excellent at overall protection, it can deliver a very intense, concentrated burst of energy to a tiny spot right next to the tumor.

Among the machines that use light, or photons, the picture was more mixed, with different tools excelling at different tasks. The robotic system, which can aim from many non-standard angles, was the best at ensuring the very edges of the tumor received enough radiation and at keeping the highest point of radiation hitting the rectum low. The spinning arc machine was particularly good at protecting the bladder and managing the volume of the rectum that received a high dose. The standard, non-rotating light machine performed best at sparing the rectum from receiving moderate amounts of radiation. The helical machine, which spirals around the patient, was the top performer for ensuring the tumor received a very high dose at its most intense point. The study could not include the proton machine in the main ranking for the most aggressive treatment schedules because the available data for protons came from different types of treatment plans, but the limited data that did exist suggested protons also offer strong protection for healthy organs.

The researchers were careful to note that their findings come with significant uncertainty. The analysis relied on a relatively small number of studies, and the data for the advanced particle machines was especially thin, with the carbon ion results resting heavily on just one study. There were also large differences in how the original studies were conducted, which made the results somewhat variable. Despite these limitations, the study provides the first comprehensive ranking of these six technologies. It suggests that while carbon ion therapy offers the best overall precision and lowest average doses to healthy organs, doctors must be cautious about the specific high-dose spots it creates near the rectum. The study concludes that no single machine is perfect for every situation; instead, the best choice depends on which specific part of the treatment plan needs the most attention, whether it is hitting the tumor perfectly, keeping the average dose low, or avoiding a single high-intensity point.