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
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
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