| PURPOSE:The heart may be exposed to the radiation field and induced to the damage in the progress of thoracic radiotherapy,leading to the radiation-induced heart disease(RIHD).And the survival time of cancer patients has increased sifnificantly with the development of radiotherapy and other antitumor techniques.Therefore,the impact of delayed RIHD on survival has gradually attracted people’s attention.The purpose of this study was to quantify the kinematic changes and the extension range of the heart and its substructures in patients with thoracic malignant tumor who had undergone radiotherapy by breath-hold and electrocardiogram gated 4-dimensional magnetic resonance imaging(4D-MRI).The other aim was to evaluate the dosimetric variations affected by the periodic movement on heart and its substructures,through comparing the dose on the planning CT with the actual delivery dose.METHODS:The planning CT and the 4D-MRI images were uploaded to a commercial software MIM Maestro 6.7.6 workstation,and the boundaries of the heart and its substructures were delineated at 20 phases.The heart and its substructures included:the whole heart,the heart,the atrial septum,the ventricular septum,the left ventricular muscle,the anterolateral papillary muscle and the posteromedial papillary muscle.The heart and its substructure boundaries in 20 phases were fused and the fusion volume was generated on the base of the "fusion" function of MIM workstation.And the planning CT and the 4D-MRI images were registrated based on the frame assistant.The compensatory expansion range was calculated by expending the bondary of the heart and its substrucrures on the palnning CT to that of the fusion volume and covering at least 95%of the fusion volume(A∩B/A ≥95%).The radiation field of the planning CT was then matched to the 20 phases images of the 4D-MRI on the base of the "dose conversion" function of MIM workstation,and therefore corresponding dose-volume diagrams(DVH)of the heart and its substructures were generated.RESULTS:The boundary displacements of the heart and its substructures ranged from 4 mm to 20 mm,and the average compensatory extension ranged from 0.9 cm to 6.6 cm.The compensatory extension range of the anterolateral papillary muscle(mean value=4.42cm)and the posteromedial papillary muscle(mean value=4.57cm)were larger than that of the whole heart(mean value=2.52cm),the heart(mean value=2.10cm)and the left ventricular muscle(mean value=2.63cm).The Dmean of the whole heart and the heart(about 3%)were significantly lower than that of the other structures.The Dmean of the left ventricular papillary muscle was as high as 18.37%and 21.92%,and about 15.28%of the left ventricular muscle,which were almost five times larger than that of the whole heart and the heart.The Vx of the heart and its substructures also showed a similar trend.The maximum variation of the ventricular muscle was 81.18%,20 times higher than that of the whole heart and the heart.CONCLUSION:The displacements of the heart and its substructures were obvious due to the cardiac periodic activity,and the motion amplitude of different substructures was different.And it was not precise enough to evaluate the real delivery radiation dose of the heart and its substructures only assessed by static planning CT because of the displacements in the process of thoracic radiotherapy.The differences between the planned dose and the real delivery dose were acceptable of the whole heart and the heart,but extra radiation dose could not be avoided considering the left ventricular muscle and atrioventricular septum.At this time,it may not meet the purpose of protecting these important substructures of the heart in thoracic radiotherapy if only outline the whole heart as the organ at risk(OAR). |
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