Impact of anatomical changes on radiation absorbed dose of prostate and bladder in a potential scenario of magnetic resonance imaging (MRI)-guided carbon-ion radiotherapy (MRgCT) of prostate cancer
Introduction
Technological delivery, indications, and effectiveness of carbon-ion (C-ion) radiotherapy (CIRT) have enjoyed a rapid advance so far [1], [2]. Due to specific characteristics of C-ions compared to photon beams, CIRT is more sensitive to anatomical changes of the patient [3]. Many attempts have been made to reduce uncertainties during treatment delivery, one of which is the image-guided particle therapy (IGPT). Due to its intrinsically higher geometric sensitivity, ion therapy can benefit from IGPT even more than photon therapy [4].
A magnetic resonance imaging (MRI) system is an exceptional candidate for IGRT [5]. Researchers in institutions have studied the clinical applicability of coupling MRI and linear accelerator (LINAC)-based radiotherapy units [6], [7]. Although the combination of MRI and LINAC as an integrated system (MR-LINAC) is currently being applied in the clinic, there are many challenges to the integration of MRI and hadron therapy systems.
Magnetic field influence in charged particle beam therapy scenario guided by MRI is more considerable than on photon beam therapy-based MRI guidance, because, in the case of charged particles, both the primary ions and generated secondary charged particles, are influenced [8], which leads to more dose disturbance in the patient's body. To compensate or correct these perturbations, they should be assessed as accurately as possible.
On the other hand, Monte Carlo (MC) techniques have been applied for evaluating dose perturbations induced by magnetic fields in MR-guided X-ray therapy (MRgXT) [9], [10], as well as MR-guided proton therapy (MRgPT) [8], [11], [12].
Furthermore, the impact of variation in magnetic field intensity and C-ion beam energies on C-ion range variation and dose changes under magnetic fields in homogeneous and heterogeneous phantoms have been assessed in the literature so far [11], [13], [14], [15].
In a cancer patient-specific scenario, intra- and inter-fractional changes may greatly affect the robustness of particle therapy [16], [17], [18], [19]. Therefore, the investigation on the dosimetric impact of anatomical changes in the presence of a magnetic field should be of concern.
In existing studies, only very limited information is available on the dosimetric variation of C-ion beams occurring in materials under a magnetic field, and the dosimetric impacts of anatomical changes, under a perpendicular magnetic field, have not yet been investigated in the literature. For this reason, the main novelty point that has been a concern in this study is the assessment of the radiation absorbed dose variation due to anatomical changes in the case of Gaussian C-ion beams transportation in a media affected by a 1.5 T transverse magnetic field with the motivation of application to potential future development of magnetic resonance imaging-guided (MRI-guided) C-ion radiotherapy (MRgCT).
According to the International Agency for Research on Cancer (IARC) report, prostate cancer is the second most frequent malignancy in men worldwide [20]. The outcomes of radical radiotherapy for localized prostate cancer are recommended to be equal to or even better than prostatectomy [21]. CIRT is one of the radical non-surgical radiotherapy method for achieving high local control rates in localized prostate cancer [22].
One of the challenges in prostate cancer treatment with C-ions is the anatomical changes in the pelvic area. These anatomical changes lead to changes in the dose distribution of the prostate (as a target volume) and surrounding critical organs, such as the bladder. These changes have been evaluated in the absence of a magnetic field in the literature [23], but no data has been reported in the presence of a magnetic field in the pelvis to treat prostate cancer.
Therefore, in this study, the impact of some anatomical changes scenarios on radiation absorbed dose of the prostate and bladder was assessed for potential application of MRgCT for prostate cancer.
Section snippets
MC simulation
In this study, MC simulation was performed using the FLUKA code (version 2011.2c.5). FLUKA is a general-purpose code for transporting different nuclear and elementary particles in various materials [24]. It has been demonstrated that the FLUKA MC code offers a favourable compromise with the empirical data for calculating dosimetric parameters of charged hadrons [25], [26]. This code also has a fast algorithm for traversing charged particles in a magnetic field [24], [27].
General settings of the FLUKA code cards
The FLUKA MC simulation
The simulated C-ion beam validation in the absence of the magnetic field
Fig. 3 compares the relative depth-ionization curve of a C-ion beam with an energy of 250 MeV/n in the absence of a magnetic field with the experimental data reported in the literature [28]. The discrepancy between the calculated and the experimental values [28] regarding the location of the Bragg peak was 0.4 mm, which is in the range of allowable uncertainties suggested in the references [29] for the Bragg depth of C-ion beams. The reason that the experimental data [28] was shown after a
Conclusion
One of the major challenges in ion therapy is to evaluate the dosimetric impact of anatomical changes of the organs [23], [37], [38]. These changes in the presence of a 1.5 T magnetic field that can occur in an MRgCT scenario might be of greater concern, but they have not been addressed in the literature so far. One of the important areas where the effects of anatomical changes on the tumour dose and its surrounding sensitive organs should be of concern is the pelvic area in men.
In this study,
CRediT authorship contribution statement
M. Akbari: Conceptualization, Methodology, Validation, Formal analysis, Writing - original draft, Writing - review & editing, Visualization, Supervision, Project administration, Funding acquisition. A. Karimian: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Resources, Data curation, Writing - original draft, Writing - review & editing, Visualization.
Declaration of Competing Interest
The authors declared that there is no conflict of interest.
Acknowledgment
This study was supported by the support program of the Ph.D. thesis at the University of Isfahan, Isfahan, Iran. The authors are extremely thankful to Dr. Andrea Mairani from Heidelberg Ion-Beam Therapy Center (HIT) for providing some helpful beam data relevant to this work.
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