Development of a whole-body dose assessment system by JAEA and QST through study on heavy ion radiation therapy and secondary cancer incidence

The research group of Takuya Furuta of the Research Group for Radiation Transport Analysis, Nuclear Science and Engineering Center, Japan Atomic Energy Agency (JAEA) and Yusuke Koba of the Department of Radiation Regulatory Science Research, National Institute of Radiological Sciences, Quantum Life and Medical Science Directorate, the National Institutes for Quantum Science and Technology (QST), announced on August 25 that they had developed "RT-PHITS for CIRT," a system that can evaluate the radiation dose to a patient's entire body from heavy-ion radiation therapy in order to clarify the mechanism of secondary cancer development after such therapies. Based on past treatment planning data, a computer reproduces the complex irradiation systems that differ from patient to patient, and then the simulations are used to accurately evaluate exposure dosage over the patient's entire body. The results are expected to lead to lower-risk radiotherapy, including lower-risk heavy particle radiotherapy. The results were posted in the international science journal Physics in Medicine and Biology.

Radiation therapy treats tumors and cancers, which are the treatment targets, by irradiating them (delivering doses of radiation energy) to kill the tumor cells. Although treatment plans are designed to concentrate the radiation on the tumor as much as possible, radiotherapy potentially carries the risk of the patient developing secondary cancer because radiation to other parts of the body cannot be completely blocked. Secondary cancers are all cancers that develop after radiotherapy and that have a source or origin other than metastasis or recurrence and are reported to occur at significantly higher frequency with X-ray therapy than with non-radiation treatments such as surgery.

Heavy-ion radiation therapy, one type of radiotherapy, can concentrate radiation doses more intensively than X-ray therapy, and has been reported to have a lower incidence of secondary cancers than X-ray therapy, particularly for prostate cancer. On the other hand, there is no system that can measure the patient's whole-body dose away from the irradiation site where secondary cancers can occur, and quantitative evaluations have not been conducted. As such, dose distribution evaluations for heavy-ion radiation therapy have been limited to the areas around the irradiation site for the purpose of treatment efficacy and to avoid acute damage to the surrounding vital organs.

The probability of secondary cancer occurring is extremely low, and, to clarify the causes of such cancers, it is necessary to investigate the degree of radiation exposure and the incidence of secondary cancer for each organ in many patients based on the detailed radiation dose distribution in those patients' bodies.

Therefore, the research group aimed to develop a system that enables whole-body dosimetry in order to bring to light the mechanism of secondary cancers caused by heavy particle radiotherapy and to clarify the causes of these low frequency of occurrences.

In heavy-ion therapy, patients are treated with a complex irradiation system that forms the beam into the shape of each person's tumor (by placing multiple shields on the heavy ion beam, which is composed of carbon ions), and then focusing the radiation dosage on the tumor. This information is then recorded in the patient's treatment plan. Because carbon ions are composed of composite particles, secondary particles are generated when they collide with examination devices, the shields, or the patient's body during irradiation, and the radiation doses away from the irradiated area due to these secondary particles. In order to evaluate a patient's whole-body radiation dose, it was necessary to reproduce the complex irradiation system of heavy particle beams on a computer using an anthropomorphic phantom and to have the simulation reflect the generation of secondary particles.

The system that the research group developed uses a virtual space in a computer to reproduce the irradiation system used during a patient's treatment, based on the heavy-ion radiotherapy planning data. By accurately simulating the behavior of the heavy ion beams and the secondary particles in the irradiation system by using the "PHITS" Monte Carlo radiation behavior analysis code (which was primarily developed at JAEA), the research group was able to realize a detailed radiation dosage distribution for the patient's entire body. By automating these processes, the research group has established a system that can process vast amounts of patient data. In addition to estimating dosage distributions at treatment sites, the research group also confirmed that it is possible to evaluate dosage distributions at a distance from the treatment site, and even at low dose distributions of 1% or less of the radiation dose.

In addition, the software can also convert radiation dosage distributions to DICOM data and integrate the images based on it, which makes it possible to overlay patient CT images onto the dosage distributions.

Furuta commented on these developments, "In the future, we plan to use this system to reevaluate cases of heavy ion radiotherapy that are held by QST, which has the largest number of cases in the world. By conducting dosimetry and follow-up studies of each individual patient's treatment in these cases, we hope to clarify under what conditions secondary cancers occur and why they are less likely to occur with heavy particle radiotherapy by comparing whether or not patients have developed secondary cancers and whether or not they have experienced any side effects. To begin with, we will analyze about 200 cases of prostate cancer that have been reported with X-ray therapy. We believe that clarifying these mechanisms will lead to safer radiotherapy in the future."

Koba added that, "In the future, we would like to re-analyze the treatment data for treatments that QST conducted, and, in collaboration, with other research institutes, we would like to compare the re-analyzed treatment data with dose evaluations for X-ray therapy and clarify the factors that cause secondary cancers."

Journal Information
Publication: Physics in Medicine & Biology
Title: Development of the DICOM-based Monte Carlo dose reconstruction system for a retrospective study on the secondary cancer risk in carbon ion radiotherapy
DOI: 10.1088/1361-6560/ac7998