Influence of beam incidence and irradiation parameters on stray neutron doses to healthy organs of pediatric patients treated for an intracranial tumor with passive scattering proton therapy

doi:10.1016/j.ejmp.2016.03.009

Physica Medica

Volume 32, Issue 4, April 2016, Pages 590-599

https://doi.org/10.1016/j.ejmp.2016.03.009 Get rights and content

Highlights

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MC calculations were used to assess neutron doses to healthy organs.
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Lateral incidences induce more neutron doses than the anterior–superior incidence.
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Impact of some irradiation parameters on neutron doses changes with beam incidence.
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Investigations on internal neutron fluence and doses were carried out.
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Beam incidence impacts the fluence and energy of neutrons reaching the patient.

Abstract

Purpose

In scattering proton therapy, the beam incidence, i.e. the patient’s orientation with respect to the beam axis, can significantly influence stray neutron doses although it is almost not documented in the literature.

Methods

MCNPX calculations were carried out to estimate stray neutron doses to 25 healthy organs of a 10-year-old female phantom treated for an intracranial tumor. Two beam incidences were considered in this article, namely a superior (SUP) field and a right lateral (RLAT) field. For both fields, a parametric study was performed varying proton beam energy, modulation width, collimator aperture and thickness, compensator thickness and air gap size.

Results

Using a standard beam line configuration for a craniopharyngioma treatment, neutron absorbed doses per therapeutic dose of 63 μGy Gy⁻¹ and 149 μGy Gy⁻¹ were found at the heart for the SUP and the RLAT fields, respectively. This dose discrepancy was explained by the different patient’s orientations leading to changes in the distance between organs and the final collimator where external neutrons are mainly produced. Moreover, investigations on neutron spectral fluence at the heart showed that the number of neutrons was 2.5 times higher for the RLAT field compared against the SUP field. Finally, the influence of some irradiation parameters on neutron doses was found to be different according to the beam incidence.

Conclusion

Beam incidence was thus found to induce large variations in stray neutron doses, proving that this parameter could be optimized to enhance the radiation protection of the patient.

Introduction

Proton therapy is one of the external radiation therapy techniques which enables a highly conformal dose delivery to the target volume while sparing organs at risk (OAR) nearby [1]. However, secondary neutrons generated by proton nuclear interactions with beam line elements (called external neutrons) and within the patient himself (called internal neutrons) raise a true concern, especially for pediatric patients, as they increase the lifetime risk of developing a secondary malignancy [2], [3], [4]. Thus, experimental measurements [5], [6], [7] and Monte Carlo (MC) calculations [8], [9], [10] have been extensively used to assess the exposure of healthy organs to stray neutrons. These studies showed that neutron dose equivalents largely tend to vary between proton therapy facilities, from a few mSv Gy⁻¹ to 80 mSv Gy⁻¹. This large variation is mainly due to the dependence of stray neutrons on tumor size and location, the design and composition of beam line elements as well as patient’s size and morphology.

Hence, various studies have investigated the variation of neutron doses with irradiation parameters such as proton beam energy, collimator aperture, modulation width and air gap size [11], [12], [13], [14], [15], [16]. Authors found that neutron doses to healthy organs increased with proton beam energy, collimator aperture and modulation width and decreased with air gap size. Only [15] considered an intracranial tumor treatment but the proton beam energy, collimator aperture and modulation width were simultaneously changed in the calculations. Therefore, it is necessary to individually estimate the influence of each irradiation parameter on neutron doses while performing intracranial treatments. In addition, the influence of beam incidence, i.e. patient’s orientation with respect to the beam axis, is an equally important parameter but less documented in the literature [8], [17]. Indeed, varying beam incidence is expected to have a strong impact on neutron doses considering changes in the distance between healthy organs and the final collimator which is the main source of external neutrons. [8] used two cranial fields, left and right posterior oblique, which were part of a craniospinal irradiation. These two fields, being symmetrical with respect to the patient, provided little information on the effect of patient’s orientation on out-of-field neutron doses. Meanwhile, [17] considered five different beam incidences typically used for intracranial tumor treatments and showed that lateral fields (the patient is oriented 90° with respect to the proton beam direction) induce up to 3.2 times higher stray neutron doses compared against a superior (SUP) field (the patient is oriented along the proton beam direction). The result is somewhat counter-intuitive and requires further investigations since the SUP field is expected to be the most unfavorable incidence for the patient who is oriented along the beam axis and thus receives the most energetic neutrons (namely intra-nuclear cascade neutrons) emitted in the forward direction. In addition, the variation of neutron doses with irradiation parameters could be different for SUP and RLAT fields. Indeed, changing the beam incidence is expected to modify the fluence and energy distribution of neutrons reaching the patient.

This study was thus carried out to extend the previous work [17] by quantifying the influence of beam incidence on neutron absorbed doses to healthy organs while individually assessing the impact of the proton beam energy at the nozzle entrance, modulation width, collimator aperture and thickness, compensator thickness and air gap size. Calculations reproduced typical craniopharyngioma treatments while neutron doses to healthy organs were calculated to a 10-year-old female phantom. The two treatment fields considered here were an SUP field and a right lateral (RLAT) field. Additionally, to fully investigate discrepancies between these two fields, calculations of neutron spectral fluences to healthy organs were also performed. Finally, the work focuses on calculating the contribution of internal neutrons to the total neutron dose and analyzing internal neutron spectral fluence to healthy organs.

Section snippets

Typical treatment plan for a craniopharyngioma

A craniopharyngioma is a deep-seated brain tumor located close to the pituitary gland and mainly developed during the childhood from 5 to 14 years old and by people aged from 65 to 74 years old [18]. Although it is a benign tumor, it can substantially grow and affect the hypothalamus, the pituitary gland and optic nerves as well as nearby parts of the brain leading to serious hormonal and optical disorders. In addition to surgery, proton radiotherapy enables to efficiently treat the tumor while

Impact of beam incidence on neutron absorbed doses

Table 2 presents neutron absorbed doses per proton-Gray delivered to the PTV calculated with the standard beam line configuration (cf. Table 1) for 25 healthy organs of the 10-year-old phantom and for the two treatment fields considered in this study, namely the SUP and the RLAT fields. First, the table shows a strong decrease of neutron absorbed doses with increasing distance between healthy organs and both the PTV and the collimator. Using the SUP field, the calculated neutron dose to the

Discussion and study limits

First, beam incidence was found to strongly influence neutron absorbed doses, especially owing to the different patient’s orientations with respect to the beam axis; the number and energy of neutrons reaching healthy organs are therefore impacted when changing from one particular treatment field to another. Moreover, the influence of some irradiation parameters on neutrons doses also appeared to change as a function of beam incidence, namely for collimator aperture and thickness as well as

Conclusion

The MC model of the CPO’s gantry beam line and treatment facility was used to calculate neutron absorbed doses as well as neutron spectral fluences to the healthy organs of a 10-year-old female phantom treated for a craniopharyngioma.

Beam incidence was found to strongly influence neutron absorbed doses since the SUP field involves lower neutron absorbed doses for all healthy organs compared against the RLAT field. It was shown that it is due to the proximity of patient’s organs with the final

References (38)

D.R. Olsen et al.
Proton therapy – a systematic review of clinical effectiveness
Radiother Oncol
(2007)
C. Zacharatou Jarlskog et al.
Risk of developing second cancer from neutron dose in proton therapy as function of field characteristics, organ, and patient age
Int J Radiat Oncol Biol Phys
(2008)
B.S. Athar et al.
Comparison of second cancer risk due to out-of-field doses from 6-MV IMRT and proton therapy based on 6 pediatric patient treatment plans
Radiother Oncol
(2011)
X. Yan et al.
Measurement of neutron dose equivalent to proton therapy patients outside of the proton radiation field
Nucl Instrum Methods Phys Res
(2002)
S.C. Roy et al.
Scattered neutron dose equivalent to a fetus from proton therapy of the mother
Radiat Phys Chem
(2004)
D. Shin et al.
Secondary neutron doses for several beam configurations for proton therapy
Int J Radiat Oncol Biol Phys
(2009)
N.S. Boehling et al.
Dosimetric comparison of three-dimensional conformal proton radiotherapy, intensity-modulated proton therapy, and intensity-modulated radiotherapy for treatment of pediatric craniopharyngiomas
Int J Radiat Oncol Biol Phys
(2012)
H. Paganetti et al.
Relative biological effectiveness (RBE) values for proton beam therapy
Int J Radiat Oncol Biol Phys
(2002)
M.B. Chadwick et al.
Nuclear data for accelerator-driven systems
Prog Nucl Energy
(2001)
A.T. Meadows et al.
Late effects of early childhood cancer therapy
Br J Cancer
(1992)

P.J. Binns et al.

Secondary dose exposures during 200 MeV proton therapy

Radiat Prot Dosimetry

(1997)

P.J. Taddei et al.

Stray radiation dose and second cancer risk for a pediatric patient receiving craniospinal irradiation with proton beams

Phys Med Biol

(2009)

B.S. Athar et al.

Neutron equivalent doses and associated lifetime cancer incidence risks for head and neck and spinal proton therapy

Phys Med Biol

(2009)

W. Newhauser et al.

The risk of developing a second cancer after receiving craniospinal proton irradiation

Phys Med Biol

(2009)

J.C. Polf et al.

Calculations of neutron dose equivalent exposures from range-modulated proton therapy beams

Phys Med Biol

(2005)

G. Mesoloras et al.

Neutron scattered dose equivalent to a fetus from proton radiotherapy of the mother

Med Phys

(2006)

Y. Zheng et al.

Monte Carlo study of neutron dose equivalent during passive scattering proton therapy

Phys Med Biol

(2007)

J. Fontenot et al.

Equivalent dose and effective dose from stray radiation during passively scattered proton radiotherapy for prostate cancer

Phys Med Biol

(2008)

C. Zacharatou Jarlskog et al.

Assessment of organ specific neutron equivalent doses in proton therapy using computational whole-body age-dependent voxel phantoms

Phys Med Biol

(2008)

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Original PaperInfluence of beam incidence and irradiation parameters on stray neutron doses to healthy organs of pediatric patients treated for an intracranial tumor with passive scattering proton therapy

Highlights

Abstract

Purpose

Methods

Results

Conclusion

Introduction

Section snippets

Typical treatment plan for a craniopharyngioma

Impact of beam incidence on neutron absorbed doses

Discussion and study limits

Conclusion

Radiother Oncol

Int J Radiat Oncol Biol Phys

Radiother Oncol

Nucl Instrum Methods Phys Res

Radiat Phys Chem

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Prog Nucl Energy

Late effects of early childhood cancer therapy

Br J Cancer

Secondary dose exposures during 200 MeV proton therapy

Radiat Prot Dosimetry

Stray radiation dose and second cancer risk for a pediatric patient receiving craniospinal irradiation with proton beams

Phys Med Biol

Neutron equivalent doses and associated lifetime cancer incidence risks for head and neck and spinal proton therapy

Phys Med Biol

The risk of developing a second cancer after receiving craniospinal proton irradiation

Phys Med Biol

Calculations of neutron dose equivalent exposures from range-modulated proton therapy beams

Phys Med Biol

Neutron scattered dose equivalent to a fetus from proton radiotherapy of the mother

Med Phys

Monte Carlo study of neutron dose equivalent during passive scattering proton therapy

Phys Med Biol

Equivalent dose and effective dose from stray radiation during passively scattered proton radiotherapy for prostate cancer

Phys Med Biol

Assessment of organ specific neutron equivalent doses in proton therapy using computational whole-body age-dependent voxel phantoms

Phys Med Biol

Original Paper
Influence of beam incidence and irradiation parameters on stray neutron doses to healthy organs of pediatric patients treated for an intracranial tumor with passive scattering proton therapy