Determining Out-of-Field Doses and Second Cancer Risk From Proton Therapy in Young Patients—An Overview

Study on induced radioactivity and individual dose evaluation in Gantry room for Varian ProBeam Proton Therapy System

Proton therapy has emerged as an advantageous modality for tumor radiotherapy due to its favorable physical and biological properties. However, this therapy generates induced radioactivity through nuclear reactions between the primary beam, secondary particles, and surrounding materials. This study focuses on systematically investigating the induced radioactivity in the gantry room during pencil beam scanning, utilizing both experimental measurements and Monte Carlo simulations. Results indicate that patients are the primary source of induced radioactivity, predominantly producing radionuclides such as 11C, 13N, and 15O. Long-term irradiation primarily generates radionuclides like 22Na, 24Na, and 54Mn etc. Additionally, this study estimates the individual doses received by medical workers in the gantry room, the irradiation dose for patient escorts, and the additional dose to patients from residual radiation. Finally, the study offers recommendations to minimize unnecessary irradiation doses to medical workers, patient escorts, and patients.

Secondary neutron dosimetry for conformal FLASH proton therapy

BACKGROUND

Cyclotron-based proton therapy systems utilize the highest proton energies to achieve an ultra-high dose rate (UHDR) for FLASH radiotherapy. The deep-penetrating range associated with this high energy can be modulated by inserting a uniform plate of proton-stopping material, known as a range shifter, in the beam path at the nozzle to bring the Bragg peak within the target while ensuring high proton transport efficiency for UHDR. Aluminum has been recently proposed as a range shifter material mainly due to its high compactness and its mechanical properties. A possible drawback lies in the fact that aluminum has a larger cross-section of producing secondary neutrons compared to conventional plastic range shifters. Accordingly, an increase in secondary neutron contamination was expected during the delivery of range-modulated FLASH proton therapy, potentially heightening neutron-induced carcinogenic risks to the patient.

PURPOSE

We conducted neutron dosimetry using simulations and measurements to evaluate excess dose due to neutron exposure during UHDR proton irradiation with aluminum range shifters compared to plastic range shifters.

METHODS

Monte Carlo simulations in TOPAS were performed to investigate the secondary neutron production characteristics with aluminum range shifter during 225 MeV single-spot proton irradiation. The computational results were validated against measurements with a pair of ionization chambers in an out-of-field region ( ≤ $\le$ 30 cm) and with a Proton Recoil Scintillator-Los Alamos rem meter in a far-out-of-field region (0.5-2.5 m). The assessments were repeated with solid water slabs as a surrogate for the conventional range shifter material to evaluate the impact of aluminum on neutron yield. The results were compared with the International Electrotechnical Commission (IEC) standards to evaluate the clinical acceptance of the secondary neutron yield.

RESULTS

For a range modulation up to 26 cm in water, the maximum simulated and measured values of out-of-field secondary neutron dose equivalent per therapeutic dose with aluminum range shifter were found to be( 0.57 ± 0.02 ) mSv/Gy $(0.57\pm 0.02)\ \text{mSv/Gy}$ and( 0.46 ± 0.04 ) mSv/Gy $(0.46\pm 0.04)\ \text{mSv/Gy}$ , respectively, overall higher than the solid water cases (simulation:( 0.332 ± 0.003 ) mSv/Gy $(0.332\pm 0.003)\ \text{mSv/Gy}$ ; measurement:( 0.33 ± 0.03 ) mSv/Gy $(0.33\pm 0.03)\ \text{mSv/Gy}$ ). The maximum far out-of-field secondary neutron dose equivalent was found to be (8.8 ± 0.5 $8.8 \pm 0.5$ ) μ Sv / Gy $\umu {\rm Sv/Gy}$ and (1.62 ± 0.02 $1.62 \pm 0.02$ ) μ Sv / Gy $\umu {\rm Sv/Gy}$ for the simulations and rem meter measurements, respectively, also higher than the solid water counterparts (simulation: (3.3 ± 0.3 $3.3 \pm 0.3$ ) μ Sv / Gy $\umu {\rm Sv/Gy}$ ; measurement: (0.63 ± 0.03 $0.63 \pm 0.03$ ) μ Sv / Gy $\umu {\rm Sv/Gy}$ ).

CONCLUSIONS

We conducted simulations and measurements of secondary neutron production under proton irradiation at FLASH energy with range shifters. We found that the secondary neutron yield increased when using aluminum range shifters compared to conventional materials while remaining well below the non-primary radiation limit constrained by the IEC regulations.

A Monte Carlo study on the secondary neutron generation by oxygen ion beams for radiotherapy and its comparison to lighter ions

Objective.To study the secondary neutrons generated by primary oxygen beams for cancer treatment and compare the results to those from primary protons, helium, and carbon ions. This information can provide useful insight into the positioning of neutron detectors in phantom for future experimental dose assessments.Approach.Mono-energetic oxygen beams and spread-out Bragg peaks were simulated using the Monte Carlo particle transport codesFLUktuierende KAskade, tool for particle simulation, and Monte Carlo N-Particle, with energies within the therapeutic range. The energy and angular distribution of the secondary neutrons were quantified.Main results.The secondary neutron spectra generated by primary oxygen beams present the same qualitative trend as for other primary ions. The energy distributions resemble continuous spectra with one peak in the thermal/epithermal region, and one other peak in the fast/relativistic region, with the most probable energy ranging from 94 up to 277 MeV and maximum energies exceeding 500 MeV. The angular distribution of the secondary neutrons is mainly downstream-directed for the fast/relativistic energies, whereas the thermal/epithermal neutrons present a more isotropic propagation. When comparing the four different primary ions, there is a significant increase in the most probable energy as well as the number of secondary neutrons per primary particle when increasing the mass of the primaries.Significance.Most previous studies have only presented results of secondary neutrons generated by primary proton beams. In this work, secondary neutrons generated by primary oxygen beams are presented, and the obtained energy and angular spectra are added as supplementary material. Furthermore, a comparison of the secondary neutron generation by the different primary ions is given, which can be used as the starting point for future studies on treatment plan comparison and secondary neutron dose optimisation. The distal penumbra after the maximum dose deposition appears to be a suitable location for in-phantom dose assessments.

Range shifter contribution to neutron exposure of patients undergoing proton pencil beam scanning

BACKGROUND

Superficial targets require the use of the lowest energies within the available energy range in proton pencil-beam scanning (PBS) technique. However, the lower efficiency of the energy selection system at these energies and the requirement of a greater number of layers may represent disadvantages for this approach. The alternative is to use a range shifter (RS) at nozzle exit. However, one of the concerns of using this beamline element is that it becomes an additional source of neutrons that could irradiate organs situated far from the target.

PURPOSE

The purpose of this study is to assess the increase in neutron dose due to the RS in proton PBS technique. Additionally, an analytical model for the neutron production is tested.

METHODS

Two clinical plans, designed to achieve identical target coverage, were created for an anthropomorphic phantom. These plans consisted of a lateral field delivering an absorbed dose of 60 Gy (RBE) to the target. One of the plans employed the RS. The MCNP code was used to simulate the plans, evaluating the distribution of neutron dose equivalent (Hn ) and the equivalent dose in organ. In the plan with the RS plan, neutron production from both the patient and the RS were assessed separately. Hn values were also fitted versus the distance to field edge using a Gaussian function.

RESULTS

Hn per prescription dose, in the plan using the RS, ranged between 1.4 and 3.7 mSv/Gy at the field edge, whereas doses at 40 cm from the edge ranged from 9.9 to 32 μSv/Gy. These values are 1.2 to 10 times higher compared to those obtained without the RS. Both this factor and the contribution of neutrons originating from the RS increases with the distance from field edge. A triple-Gaussian function was able to reproduce the equivalent dose in organs within a factor of 2, although underestimating the values.

CONCLUSIONS

The dose deposited in the patient by the neutrons originating from the RS predominantly affects areas away from the target (beyond approximately 25 cm from field edge), resulting in a neutron dose equivalent of the order of mSv. This indicates an overall low neutron contribution from the use of RS in PBS.

Modelling the influence of radiosensitivity on development of second primary cancer in out-of-field organs following proton therapy for paediatric cranial cancer

OBJECTIVE

Radiobiological modelling the risks of second primary cancer (SPC) after proton therapy (PT) for childhood cranial cancer remains largely unknown. Organ-specific dose-response risk factors such as radiosensitivity require exploration. This study compared the influence of radiosensitivity data (slope of βEAR) on children's lifetime attributable risks (LAR) of SPC development in out-of-field organs following cranial scattering and scanning PT.

METHODS

Out-of-field radiosensitivity parameter estimates for organs (α/β and βEAR) were sourced from literature. Physical distances for 13 out-of-field organs were measured and input into Schneider's SPC model. Sensitivity analyses were performed as a function of radiosensitivity (α/β of 1-10 Gy) and initial slope (βEAR) from Japanese/UK data to estimate the influence on the risk of radiation-induced SPC following scattering and scanning PT.

RESULTS

Models showed similar LAR of SPC estimates for age and sex-matched paediatric phantoms, however, for breast there was a significant increase using Japanese βEAR data. For most organs, scattering PT demonstrated a larger risk of LAR for SPC which increased with α/β.

CONCLUSION

Breast tissue exhibited the highest susceptibility in calculated LAR risk, demonstrating the importance for accurate data input when estimating LAR of SPC.

ADVANCES IN KNOWLEDGE

The findings of this study demonstrated younger female patients undergoing cranial proton therapy have a higher risk of developing second primary cancer of the breast tissue. Long-term multicenter registries are important to improve predictive radiobiological modelling studies of side effects.

Comparison of Acute Hematological Adverse Reactions Induced by Craniospinal Irradiation With Intensity-Modulated Radiotherapy and Conventional Radiotherapy

Background

With the update of equipment, the hospital base of this study began to adopt craniospinal irradiation (CSI) intensity-modulated radiotherapy (IMRT) in May 2018 to replace the traditional CSI conventional radiotherapy (CRT) technology. The purpose of this study was designed to compare the differences in acute hematological adverse reactions induced by CSI-IMRT and CSI-CRT.

Methods

The clinical data and hematological data of 102 patients with central nervous system malignant tumors who underwent CSI treatment at the 900th Hospital of Joint Logistics Support Force of PLA from January 2008 to August 2022 were analyzed retrospectively. The patients included 63 men and 39 women, aged 3 to 56 years old. On the basis of the radiotherapy technique used, the patients were divided into the CSI-IMRT group (38 cases) and CSI-CRT group (64 cases). Acute hematological adverse reactions during radiotherapy were compared between the two groups according to the Common Terminology Criteria for Adverse Events version 4.0. The Mann-Whitney U test was used to compare the measurement data, and the χ² test was used to compare the count data.

Results

No significant difference was found between the CSI-IMRT group and the CSI-CRT group in terms of sex, histopathological type, tumor location, spinal cord invasion, surgery, and the Eastern Cooperative Oncology Group score (χ² = 0.004 to 6.213; all P > .05). No significant difference was found in onset time of myelosuppression (11 days (interquartile range [IQR]: 7 to 14; minimum [min] to maximum [max]: 0 to 26) vs 8 days (IQR: 7 to 15; min to max: 3 to 29)) and nadir time of myelosuppression (21 days (IQR: 18 to 25; min to max: 12 to 35) vs 22 days (IQR: 15 to 25; min to max: 12 to 36)) between the CSI-IMRT group and the CSI-CRT group (Z = -0.856, -0.248; all P > .05). There were no significant differences in the incidence of decreased white blood cell counts (WBC), platelet counts, and hemoglobin concentration between the CSI-IMRT group and the CSI-CRT group, 86.8% (33/38) vs 78.1% (50/64), 57.9% (22/38) vs 42.2% (27/64), 57.9% (22/38) vs 53.1% (34/64); χ² = 1.195, 2.357, 0.219; all P > .05. There were no significant differences in the incidence of decreased WBC, platelet counts, and hemoglobin concentration (severe myelosuppression) in grades III and IV, 23.7% (9/38) vs 21.9% (14/64), 7.9% (3/38) vs 3.1% (2/64), 5.3% (2/38) vs 9.4% (6/64); χ² = 0.045, 1.164, 0.558; all P > .05.

Conclusions

There was no significant difference in the incidence of myelosuppression and severe myelosuppression (grade III or above) induced by CSI-IMRT and CSI-CRT. CSI-IMRT is worthy of further clinical application.