Out-of-field Dosimetry Of A 1.5 T MR-Linac

The goal of radiation therapy is to spare the surrounding normal tissues as much as possible and reduce the risk of radiation complications.With the arrival of the new era of Image-guided radiation therapy(IGRT),magnetic resonance imaging-linac(MR-Linac)has been recognized for clinical application because of the benefits of superior soft-tissue contrast,nonradiative exposure,and real-time anatomical image tracking.However,in clinical practice,it has been found that the dose distribution of the MR-Linac is affected by the presence of the static 1.5 T transverse magnetic field.The actual trajectory of secondary electrons of the MR-Linac is curved by the Lorentz force in the magnetic field,resulting in electron streaming effect(ESE).Furthermore,the electron return effect(ERE)is observed at interfaces between two materials with large density differences such as between air and tissue.These contaminated electrons have the same order of magnitude as the scattered or leaked photons for the larger irradiation fields and may cause extra dose deposition outside the target volume.In addition,considering the contaminated photons originating from patient scatter,collimator scatter and head leakage,the additional radiation outside the treatment field may increase the risk of deleterious effects and even the secondary malignancy,the importance of which cannot be ignored.Due to the complexity of dose distribution outside the treatment field and many influencing factors involved,the simulated model of out-of-field dose distribution established in the treatment planning system has some limitations and the accuracy of out-of-field dose calculation appears to be undesirable.In recent years,out-of-field dose has been quantified primarily through Monte Carlo(MC)simulation algorithms and phantom experiments.Therefore,in view of the promising application of MR-Linac in the future,relevant dosimetry data are needed to provide the reference for clinical planning and decision-making to evaluate the necessity of radiation protection outside the treatment field.Currently,there is no literature on the out-of-field dosimetry of MR-Linac.This paper focuses on the phantom experiments of the MR-Linac with the optically stimulated luminescence dosimeters.Experimental contents and contributions are summarized as follows:(1)A cubic solid water phantom,with optically stimulated luminescence dosimeters(OSLDs)on the surface,was vertically irradiated by MR-Linac square fields with different sizes.In addition,OSLDs were arranged out of the beam edges in four directions.The discrete measured point doses using OSLDs were connected to a curve.Then the out-of-field dose distribution in the magnetic field was quantified,and the influencing factors of the out-of-field dose distribution were analyzed.(2)Using optically stimulated luminescence dosimeters,25 checkpoints were measured on surfaces(20 checkpoints)and internal organs(5 checkpoints)of the anthropomorphic phantom for square radiation fields on the MR-Linac,with similar prescribed doses for the head,thoracic,and pelvic sites.The same plan was delivered to the conventional linac for anthropomorphic phantom irradiation,and the measured data were read out.The surface and internal dose differences between MR-Lianc and conventional linac were compared,and the reasons of which were analyzed.(3)The out-of-field dose of surface and internal checkpoints were calculated by the Monaco treatment planning system in comparison with the OSLD-measured data.The inverse of the function that describes the out-of-field dose with respect to the isocentric distance from the radiation field was fitted.In addition,the relationship between the dose evaluation error of the treatment planning system and the variation in the isocentric distance from the radiation field is described,and the accuracy of the out-of-field dose calculation was evaluated.(4)The differences of surface dose dosimetry of anthropomorphic phantom in different age groups were compared to provide the clinical data for reference and evaluate the necessity of individual radiation protection in the out-of-field exposure.The following results were drawn:(1)At different field sizes(5×5 to 20×20 cm~2)and distances(1 to 10 cm)to beam edge,the out-of-field surface dose measured on MR-Linac varied from 0.16%(10 cm to 5×5 cm~2 edge)to 7.02%(1 cm to 20×20cm~2 edge)of the maximum dose laterally and from 0.14%(10 cm to 5×5 cm~2 edge)to 8.56%(1 cm to 20×20 cm~2 edge)of the maximum dose longitudinally;(2)Compared to the conventional linac,MR-Linac delivered higher average values of out-of-field dose on surface check points(20%,19%,21%)and internal simulated OARs(42%,37%,9%)of the anthropomorphic phantom at head,thoracic and pelvic irradiations,respectively;(3)Compared to the OSLDs measured data,the Monaco TPS presented an overestimate of the out-of-field dose of OARs at 0–2%isodose area on both surface and internal check points,and the overestimation gets greater as the distance increases.The underestimation was found to be 0–35%at 2%–5%isodose area on both surface and internal check points;(4)Irradiated with the same target volume,the out-of field dose distribution of two paediatric anthropomorphic phantoms were similar to that of adult anthropomorphic phantom.To sum up,compared to the conventional linac,MR-Linac has the same out-of-field dose distribution.However,considering the absolute dose values,MR-Linac delivered relatively higher out-of-field doses on both surface and internal OARs.Additional radiation shielding to patients undergoing MR-Linac may provide protection from out-of-field exposure.