Occupational radiation exposure of therapists in clinical use of spot scanning proton-carbon ion synchrotron systems

Abstract

This study addresses the escalating radiation safety concerns among medical professionals due to the rising application of proton and carbon ion radiotherapy. It evaluates the occupational radiation exposure faced by therapists when utilizing the Siemens IONTRIS Proton-Carbon Ion System. Through random sampling of 80 patients treated between January and June 2024, we recorded particle types and counts and measured dose rates using a photon/neutron radiation dose meter. Notably, 1 min post-treatment, the dose rate peaked at 16.00 μSv/h near the tumor's skin surface, showing a significant correlation with particle count. The therapist's standing position and the surfaces of the range shifter and ripple filter registered average dose rates of 1.25 μSv/h and 3.63 μSv/h, 0.33 μSv/h, respectively. Other points averaged 0.08 μSv/h, with no neutron detection. The study concludes that the annual average occupational exposure for therapists, at ~300 μSv, is significantly below the International Commission on Radiological Protection's recommended dose equivalent limit, confirming the safety of the Siemens IONTRIS device in clinical settings.

INTRODUCTION

Background

Since the Lawrence Berkeley National Laboratory first applied proton and carbon ion radiotherapy in clinical settings in 1954, its superior physical and biological properties have been widely recognized in the field of radiation oncology [1, 2]. With the development and popularization of technology, medical institutions worldwide have adopted this advanced treatment method, and the clinical application of proton and carbon ion therapy has shown a significant growth trend [3]. This trend not only promotes the increase in the number of professionals such as radiation oncologists, physicists, therapists, engineers and nurses but also places higher demands on their professional skills and safety protection.

Rationale and knowledge gap

Although proton and carbon ion therapy has shown significant efficacy in treating tumors, the characteristics of high-energy particle interactions with matter also pose certain occupational health risks [4]. During the treatment process, therapists must enter the treatment room ~1 min after treatment to remove the patient's restraint and assist the patient. This procedure exposes the therapist to induced radioactive nuclide radiation. Although existing studies have shown that the dose rate at a distance of 30 cm from the patient's tumor after irradiation with proton and carbon ion beams is 0.96 μSv/h and 0.30 μSv/h [5], respectively, and the exposure dose received by medical personnel and patient companions is well below the limit recommended by the International Commission on Radiological Protection [6, 7], these studies mainly focus on the measurement of a few cases and lack comprehensive and systematic research to assess the occupational exposure received by therapists in clinical applications.

Objective

The purpose of this study is to conduct a more comprehensive and systematic investigation of the occupational exposure received by therapists when using Siemens IONTRIS proton and carbon ion systems for clinical treatment [5]. By summarizing the factors influencing occupational exposure and evaluating the average annual dose received by therapists, this study will provide more scientific guidance for occupational safety and health practitioners, ensuring that they can protect their health and safety while providing high-quality medical services.

MATERIALS AND METHODS

Equipment and instruments

The treatment device is the Siemens IONTRIS proton and carbon ion facility with spot scanning technology, a synchrotron capable of accelerating carbon ions to 85–430 MeV/u and protons to 50–250 MeV. The measuring instrument is the Neutron RAE II photon/neutron radiation dose meter from Radiation Monitoring Devices, Inc., of the United States, which is regularly calibrated by the National Institute of Metrology of China. Its photon detector is a 1 cm³ CsI scintillator, and the neutron detector is a 1 cm³ LiI scintillator; the photon energy response range is 0.06–3.0 MeV, the neutron energy response range is thermal neutrons to 14 MeV; the photon sensitivity is >30 cps/Sv/h, and the neutron sensitivity is 1–2 cps/2.5 neutrons/s/cm².

Detailed explanation of uncertainty sources

We have provided a comprehensive description of the sources of uncertainty, including:

Statistical Uncertainty

This arises from the natural variability of measurement data. We have quantified this by calculating the standard deviation and confidence intervals of our measurements.

Systematic Uncertainty

This includes instrument calibration errors, detector response stability and the influence of environmental conditions (e.g. temperature and humidity) on measurement results. We have implemented regular calibration of our measurement devices, monitored environmental conditions and employed quality control procedures to assess and control systematic uncertainties.

Model Uncertainty

This pertains to assumptions made in dose calculation models. We have carefully reviewed these assumptions and their potential impact on the results.

Reference to Calibration Certificates

We have included the calibration certificates for our dosimeters, which detail the instrument's inherent errors, statistical fluctuations, dose response and alarm threshold deviations. Based on these calibration data, we have evaluated the uncertainty of the dosimeters to ensure the accuracy and reliability of our measurements.

Clinical data

A total of 80 patients treated from January 2024 to June 2024 were selected by simple random sampling, with 40 cases each receiving proton and carbon ion therapy. The average number of particles for carbon ion cases was 20.11 × 10⁸ and for proton cases was 1537.86 × 10⁸. Beam irradiation time ranged from 5 to 30 min, and the number of irradiation fields ranged from 2 to 5. Irradiation sites included the head and neck, thorax and abdomen, pelvic cavity and lower extremities.

Measurement method

The Siemens IONTRIS proton and carbon ion system uses a synchrotron pencil beam electromagnetic scanning technique. The energy of the protons or carbon ions is deposited almost entirely at the patient's tumor site, so that the tumor with significant induced radioactivity is considered the primary radiation source for occupational exposure. Within the treatment room, the ripple filter (RF) that serves to broaden the Bragg peak and the range shifter (RS) that increases the superficial dose may appear on the beam path, depending on the patient's treatment plan and are considered secondary sources of occupational exposure. The type of particle and the total number of particles used were recorded for each patient.

After each patient began treatment, the radiation dose rate reading from the dose meter in the control room was monitored and a stable reading was recorded; 1 min after the end of treatment, when the treatment bed was moved to the lowest safe position, the measuring instrument was placed on the skin closest to the tumor for 5 s, and once the reading stabilized, the dose rate reading was recorded; then, a relatively stable dose rate reading was recorded at a position ~30 cm from the tumor (where the therapist stands); finally, dose rate values were recorded on the surfaces of the patient's immobilization device, bed, robotic arm, RF and RS.

Statistical analysis

Data were processed using SPSS 27 software. Scatter plots were generated separately for the total number of proton and carbon ion particles vs the dose rate at the skin closest to the tumor; the correlation between the total number of particles and the dose rate at the skin closest to the tumor was analyzed using the Pearson's method.

RESULTS

Comparison of particle counts and dose rates for carbon ions and protons

Statistical summary results for the 80 patients are shown in Table 1. The average total number of protons is 76.47 times that of carbon ions, and the average dose rate at the skin closest to the tumor after proton therapy is 13.69 times that of carbon ions. This indicates that the number of protons is greater than the number of carbon ions in a single treatment session, resulting in a higher level of induced radiation at the skin closest to the tumor compared to carbon ions.

Table 1.

Particle type, total particle count for 80 patients, shown with dose rate at each measurement point

Particle type	Total particle count (108)	Dose rate at skin nearest to tumor (μSv/h)	Dose rate at therapist's standing position (μSv/h)	Dose rate on RS surface (μSv/h)	Dose rate at RF (μSv/h)	Dose rate at immobilization device, bed, robotic arm (μSv/h)
Proton	1990	24.2	0.11	28.28	0.11	0.08
	3320	29.58	0.36	34.47	1.37	0.08
	2610	28.68	0.41	34.71	0.25	0.08
	678	45.17	0.26	8.77	0.1	0.08
	884	19.52	0.25	18.03	1.92	0.08
	687	10.83	0.15	0.26	0.93	0.08
	735	24.26	1.33	0.13	0.13	0.08
	2018	17.83	1.33	3.66	0.4	0.08
	2120	23.4	1.23	15.06	0.14	0.08
	526	18.91	0.6	0.11	0.11	0.08
	1651	29.43	1.63	0.08	1.37	0.08
	877	10.37	0.26	2.6	0.25	0.08
	409.1	7.1	0.16	0.85	0.1	0.08
	919	9.19	0.35	3.9	0.08	0.08
	2116	33.66	0.37	5.47	0.4	0.08
	263	9.62	0.15	1.6	0.4	0.08
	1579.3	10.67	0.5	6.7	0.12	0.08
	3082	28.7	2.3	6.3	0.18	0.08
	2440	28.71	1.6	11.52	0.5	0.08
	2316	11.95	1.2	28.71	0.43	0.08
	644	12.68	2.35	15.06	0.11	0.08
	1333	23.63	2.3	0.11	1.37	0.08
	2460	54.35	6.2	0.08	0.25	0.08
	2976	58.18	11.4	2.6	0.1	0.08
	1990	35.89	12.6	0.85	1.92	0.08
	2560	51.48	8.02	0.21	0.93	0.08
	908	16.44	1.68	0.13	0.13	0.08
	905	28.92	1.55	3.66	0.4	0.08
	821	59.02	1.63	15.06	0.14	0.08
	497	24.01	2.15	0.11	0.25	0.08
	3144	26.74	2.24	0.08	0.1	0.08
	1219	40.11	2.77	2.6	0.08	0.08
	649	17.27	2.1	0.85	0.4	0.08
	1220	43.12	1.8	3.9	0.4	0.08
	1150	64.25	2.16	0.26	0.12	0.08
	2997	70.78	4.5	0.13	0.18	0.08
	1749	51.01	1.65	3.66	0.5	0.08
	1107	19.04	1.25	15.06	0.43	0.08
	472	33.55	1.08	0.11	0.18	0.08
	1493	41.32	2.15	0.08	0.5	0.08
Average	1537.86	29.84	2.15	6.90	0.44	0.08
carbon	17.42	1.27	0.08	0.95	0.08	0.08
	13.09	1.03	0.1	0.95	0.08	0.08
	5.47	0.65	0.11	0.95	0.08	0.08
	16.73	0.61	0.08	0.95	0.08	0.08
	20.63	4.1	0.16	0.95	0.08	0.08
	13.04	0.83	0.08	0.95	0.08	0.08
	33.6	1.66	0.12	0.95	0.22	0.08
	16.18	2.31	0.16	0.95	0.08	0.08
	29.98	1.84	0.08	1.22	0.08	0.08
	35.68	1.2	0.08	0.2	0.66	0.08
	19.52	0.46	0.08	0.22	0.47	0.08
	22.1	3.82	0.18	0.13	0.13	0.08
	28.34	2.16	0.08	0.1	0.58	0.08
	13.21	1.13	0.08	0.08	0.56	0.08
	14.43	1.83	0.08	0.08	0.55	0.08
	29.8	3.46	0.23	0.08	1.16	0.08
	22.8	0.15	2.35	0.11	0.11	0.08
	16.58	1.68	0.26	0.15	0.16	0.08
	13.23	0.38	0.08	0.08	0.08	0.08
	13.97	0.77	0.08	0.08	0.08	0.08
	4.39	0.97	0.14	0.08	0.08	0.08
	14.8	0.5	0.1	0.08	0.08	0.08
	14.9	3.26	0.48	0.08	0.08	0.08
	1.58	3.11	0.3	0.08	0.08	0.08
	12.66	0.6	0.14	0.08	0.22	0.08
	26.5	5.8	0.86	0.08	0.08	0.08
	8.08	1.04	0.16	0.08	0.08	0.08
	13.83	3	0.5	0.08	0.66	0.08
	9.34	3	0.18	0.08	0.47	0.08
	23	5.7	1.02	0.95	0.13	0.08
	4.62	0.61	0.16	0.95	0.58	0.08
	76.6	4.55	0.43	1.22	0.08	0.08
	52.7	3.11	0.77	0.2	0.08	0.08
	6.93	0.79	0.18	0.22	0.08	0.08
	28.83	1.74	0.26	0.13	0.08	0.08
	50.9	10	2.36	0.1	0.08	0.08
	15.86	3.44	0.66	0.08	0.08	0.08
	11.87	0.92	0.15	0.08	0.08	0.08
	27.9	2.77	0.64	0.08	0.08	0.08
	3.34	0.78	0.16	0.08	0.08	0.08
Average	20.11	2.18	0.36	0.37	0.22	0.08

Comparison of dose rates at the skin closest to the tumor and at a distance of ~30 cm from the tumor

The average dose rate at the skin closest to the tumor and the average dose rate at the therapist's standing position are 16.01 μSv/h and 1.25 μSv/h, respectively. The former is 12.81 times higher than the latter. This indicates that the radiation intensity generated after irradiating the patient's tumor with protons or carbon ions decreases with increasing distance from the radiation source. Therefore, therapists should be aware that adopting appropriate distance protection can reduce occupational exposure.

Relationship between induced radioactivity level after tumor irradiation and total particle count

There is a significant positive correlation between the dose rate at the skin closest to the tumor 1 min after the end of tumor treatment and the total number of particles used (r = 0.75, P < 0.01, Figs 1 and 2). To reduce the random error in measuring the induced radiation level after patient treatment, this study emphasizes the measurement of the dose rate at the skin closest to the tumor, using it to represent the induced radioactivity level after patient tumor treatment to explore its relationship with the number of particles used in the treatment. The more particles, the more nuclear reactions occur and the more radioactive isotopes and positrons are produced by nuclear fission, resulting in greater induced radioactivity and, consequently, a higher dose rate at the skin closest to the tumor. However, due to the varying depth of tumors in patients, the varying duration of treatment and the randomness of measurement methods and tools, some points deviate significantly from the fitted line.

Fig. 1.

The dose rate to the skin closest to the tumor changes with the total number of protons (y = 0.02×).

Fig. 2.

The dose rate to the skin closest to the tumor varies with the total number of carbons (y = 0.125×).

Dose rate at other positions and neutron measurement situation

The average dose rates at the surfaces of the RS and RF are 3.63 μSv/h and 0.33 μSv/h, respectively. The average dose rate at the patient's immobilization device, bed and robotic arm is 0.0–0.08 μSv/h, with no neutrons detected at any location. For superficial tumors, such as breast and neck tumors, an RS is required in the ion path to increase the superficial dose, resulting in a higher level of induced radioactivity on the surface of the RS after treatment for these patients. Larger volume tumors require an RF to broaden the Bragg peak, and these patients will also have induced radioactivity on the RF surface after treatment. Therapists should be careful to maintain a distance from the just-used RS and RF unless necessary.

Annual occupational dose for therapists

Approximately 1 min after the end of treatment, therapists enter the treatment room to release the immobilization devices and accompany the patient for ~3 min. To estimate a larger dose equivalent, the effect of radioactive decay is neglected. Assuming that each therapist works at maximum capacity, with contact with 20 patients per day and ~250 working days per year, the average annual dose is calculated to be ~300 μSv.

DISCUSSION

Key findings

This study found a significant correlation between induced radiation levels after treatment and the number of particles used. The equivalent dose of carbon ions is 2 to 3 times that of protons, meaning that fewer carbon ions are needed to achieve the same therapeutic effect. As a result, patients treated with protons have higher levels of induced radiation than those treated with carbon ions. In addition, the occupational exposure of therapists in this study, which included an equal number of proton and carbon ion cases, is likely higher than the actual exposure due to the current prevalence of fewer proton treatments.

During proton and carbon ion therapy, high-energy particle beams interact with various components of the accelerator, producing secondary neutrons and gamma rays. These secondary neutrons further induce multiple activation reactions with the accelerator components, the concrete shielding walls of the treatment room, water within the accelerator and air in the treatment room. The most common interactions are through (n, γ) reactions. When the energy of the incident neutrons is sufficiently high to induce particle emission, other types of activation reactions, such as (n, p), (n, α) and (n, 2n), can occur. These reactions produce a series of radionuclides with varying half-lives, including ²⁴Na, ¹⁴O, ¹⁵O, ¹³N, ¹¹C, ⁷Be, ³H and ⁴¹Ar. The presence of these radionuclides may contribute additional dose exposure.

Comparison with existing guidelines

We will provide a detailed comparison of our study results with the recommendations in American Association of Physicists in Medicine (AAPM) TG-136 and National Council on Radiation Protection and Measurements (NCRP) reports, as follows:

A‌APM TG-136 Guidelines

 

Description. TG-136 provides detailed technical recommendations for dose measurement and quality assurance in proton and heavy-ion therapy.

 

Comparison. We will compare the dose measurement methods used in our study with the standard methods recommended by TG-136. This comparison will focus on differences in measurement accuracy, instrument calibration and uncertainty assessment.

NCRP Reports

 

Description. NCRP reports offer comprehensive recommendations for occupational exposure protection, including dose limits, protective measures and monitoring requirements.

 

Comparison. We will compare our study's assessment of occupational exposure with the recommended standards in NCRP reports. This will involve analyzing whether our findings meet these standards in practical clinical applications and exploring potential reasons for any discrepancies.

Specific Comparison Content

We will conduct a detailed analysis of the dose rate distribution, radiation contributions from activation products and the effectiveness of protective measures measured in our study. These findings will be compared with the recommendations in AAPM and NCRP reports to validate the rationality and reliability of our results.

Strengths and limitations

A strength of this study is its focus on the occupational exposure of therapists, which provides valuable insight into the radiation levels encountered during proton and carbon ion therapy. However, the study has limitations, including the exclusion of exposures of physicists and engineers involved in treatment planning, equipment quality assurance and maintenance. The use of portable measurement devices, while convenient, may also present challenges in accuracy and sensitivity compared to other instruments. In addition, the limited number of cases and potentially incomplete measurement points suggest the need for a more comprehensive study in the future [8, 9].

The presence of these radionuclides in the treatment room significantly increases the risk of occupational exposure. For instance, ²⁴Na and ¹⁵O, with short half-lives, emit high-energy gamma rays during decay, contributing significantly to dose exposure shortly after treatment. Radionuclides with longer half-lives, such as ¹³N and ¹¹C, emit beta particles and gamma rays that persist in the treatment room, increasing cumulative dose exposure. Long-lived radionuclides such as ⁷Be and ³H can spread through the air, posing long-term exposure risks.

Comparison with similar researches

The results of this study can be compared to similar research in the field of radiation therapy, particularly with regard to occupational radiation exposure among medical professionals [10, 11]. While this study provides specific data on therapists, a broader comparison with research that includes physicists and engineers would provide a more comprehensive view of occupational radiation exposure in the medical field.

Explanations of findings

The significant induced radiation levels observed with proton therapy are due to the higher number of particles used compared to carbon ion therapy. The unique Bragg peak distribution of proton and carbon ion beams, coupled with the advanced scanning and modulation capabilities of the Siemens IONTRIS system, minimizes activation of air and normal tissues, thereby reducing therapists' occupational exposure.

The radionuclides produced by activation reactions are primarily concentrated in accelerator components and the treatment bed surface, which release high-dose rates shortly after treatment, posing a direct threat to staff. Activation can also increase background radiation levels in the treatment room, further contributing to staff's cumulative dose exposure.

Implications and actions needed

The results suggest that while the Siemens IONTRIS system with its advanced features reduces the risk of radiation exposure to therapists, caution is still required, especially with regard to the close proximity to RS and RF surfaces after treatment. Future actions include expanding the study to include physicists and engineers, using more precise measurement devices and increasing the number of cases and measurement points for a more representative analysis.

CONCLUSION

The study concludes that occupational radiation exposure in proton and carbon ion therapy is influenced by the number of particles used and the type of ionizing radiation. While the Siemens IONTRIS equipment enhances safety through its advanced technology, the potential for induced radiation remains, particularly with equipment components such as RS and RF. The limitations of the study highlight the need for a broader and more comprehensive approach to understanding and mitigating occupational radiation exposure in medical radiation therapy settings. Future research should aim to include a wider range of professionals, use more accurate measurement tools and analyze a larger and more diverse set of cases to provide a comprehensive assessment of radiation risks and inform necessary safety measures.

CONFLICT OF INTEREST

The authors declare that there are no conflicts of interest regarding the publication of this paper. All authors have no financial or personal relationships with other people or organizations that could inappropriately influence or bias the content of this paper.

AUTHOR CONTRIBUTIONS

Zhulei Liu and Xiaowa Wang (Conception and design), Dan You and Dan Zhou (Administrative support), Zhulei Liu and Ruirui Bu (Provision of study materials or patients), Zhulei Liu and Yao Li (Collection and assembly of data), Zhulei Liu and Xiaowa Wang (Data analysis and interpretation), Zhulei Liu and Xiaowa Wang (Manuscript writing) and All authors (Final approval of manuscript)

FUNDING

This work was supported by the National Natural Science Foundation of China, grant No. 12075062.

ETHICS APPROVAL INFORMATION

This study has been approved by the Shanghai Proton and Heavy Ion Center Institutional Review Board with the ethics number 241112EXP-01. The review process was conducted in compliance with the principles of the Declaration of Helsinki and the ethical guidelines provided by the International Committee of Medical Journal Editors (ICMJE). The study was granted approval on 6 December 2024, with a research period from December 2024 to December 2025. The IRB ensured that all participants were fully informed about the study procedures and potential risks and that informed consent was obtained from all participants involved in the study.