Monte Carlo simulations of patient dose perturbations in rotational‐type radiotherapy due to a transverse magnetic field: A tomotherapy investigation

Abstract

Purpose: Several groups are exploring the integration of magnetic resonance (MR) image guidance with radiotherapy to reduce tumor position uncertainty during photon radiotherapy. The therapeutic gain from reducing tumor position uncertainty using intrafraction MR imaging during radiotherapy could be partially offset if the negative effects of magnetic field-induced dose perturbations are not appreciated or accounted for. The authors hypothesize that a more rotationally symmetric modality such as helical tomotherapy will permit a systematic mediation of these dose perturbations. This investigation offers a unique look at the dose perturbations due to homogeneous transverse magnetic field during the delivery of Tomotherapy ® Treatment System plans under varying degrees of rotational beamlet symmetry. Methods: The authors accurately reproduced treatment plan beamlet and patient configurations using the Monte Carlo code 4. This code has a thoroughly benchmarked electromagnetic particle transport physics package well-suited for the radiotherapy energy regime. The three approved clinical treatment plans for this study were for a prostate, head and neck, and lung treatment. The dose heterogeneity index metric was used to quantify the effect of the dose perturbations to the target volumes.Results: The authors demonstrate the ability to reproduce the clinical dose-volume histograms (DVH) to within 4% dose agreement at each DVH point for the target volumes and most planning structures, and therefore, are able to confidently examine the effects of transverse magnetic fields on the plans. The authors investigated field strengths of 0.35, 0.7, 1, 1.5, and 3 T. Changes to the dose heterogeneity index of 0.1% were seen in the prostate and head and neck case, reflecting negligible dose perturbations to the target volumes, a change from 5.5% to 20.1% was observed with the lung case. Conclusions: This study demonstrated that the effect of external magnetic fields can be mitigated by exploiting a more rotationally symmetric treatment modality. C 2015 American Association of Physicists in Medicine. [http://dx

Technical Note: Dose effects of 1.5 T transverse magnetic field on tissue interfaces in MRI-guided radiotherapy

“…These results are generally consistent with data from previous studies, such as that by Raaijmakers et al 8 However, there was some concern about the large increases in DHI of the PTV that resulted from the high-field magnetic fields in lung cancer cases. 12 In order to address this concern, we obtained the DHI values for all four sites studied. Similar to the result reported by Yang et al, 13 we observed increases (though smaller) of DHI for the reconstructed VMAT plans in the presence of 1.5 T TMF for the two lung cancer cases, case Lung2 resulting in a similar but slightly higher DHI, and case Lung1 with an even higher DHI.…”

“…6 Consequently, the ERE can change the dose at tissue interfaces, according to magnetic field strength and surface orientation, and can cause an under or overdose at air cavity walls, along with lung-tissue interfaces in the patient's body and on the skin. [5][6][7][8] The effects of transverse magnetic field (TMF) on intensitymodulated radiation therapy (IMRT) plans [9][10][11] and helical beam therapy plans 12 have received considerable attention. Raaijmakers et al 9 first investigated whether IMRT could be used to compensate for ERE on prostate, larynx, and oropharyngeal cases.…”

“…van Heijst et al 10 investigated and compared the ERE on skin dose for whole breast irradiation (WBI) and accelerated partial breast irradiation (APBI) during the MR-guided breast RT. Recently, Yang et al 12 computed the dosimetric impact of TMFs on helical tomotherapy plans using Monte Carlo methods and reported that the plan quality (e.g., dose uniformity in target) could be reduced dramatically for high TMFs (e.g., ≥1.5 T). This observation is not surprising, however, since the authors failed to incorporate TMF during plan optimization.…”

Concurrent Monte Carlo transport and fluence optimization with fluence adjusting scalable transport Monte Carlo

“…The in-house forward transport user code has been benchmarked to show good agreement with EGSnrc (Ref. 18) and clinical TPS for patient geometries, 19 and served as a gold-standard benchmark for a GPU-based Monte Carlo dose calculation engine. 7 An "optimization manager" class was implemented, repurposing the forward transport user code to allow onthe-fly fluence optimization without user intervention.…”

New concept on an integrated interior magnetic resonance imaging and medical linear accelerator system for radiation therapy

“…Hence, treatment planning with the magnetic field considered is necessary, which has also been suggested previously by other studies. [28][29][30][31][32][33] The most distinct feature differentiating our conceptual system from the existing MRI-LINAC systems is its smaller FoV. As opposed to integrating a full body MRI scanner with a LINAC, we propose using a new MRI design that has a small (∼15 cm diameter) FoV.…”