Explore The Influence Of Spiral Tomotherapy Plan Parameters On The Radiotherapy Plan

As an important means of cancer treatment,radiation therapy has experienced the development from ordinary external irradiation to three-dimensional conformal radiotherapy(3DCRT)and now to intensity-modulated radiation therapy(IMRT).As a new technology of intensity-modulated radiotherapy(IMRT),tomotherapy(TOMO)has the advantages of wide range of treatment,strong intensity-modulation ability,high dose suitability in target area and low dose of normal tissue compared with traditional Linac.It can protect the normal target tissue while performing high precision treatment on the tumor target area.Tomotherapy is based on the principle of CT tomography to achieve intensity-modulated radiotherapy.The design of radiotherapy plan is different from traditional medical linear accelerator.The planning parameters mainly include field Width,pitch and modulation factor(MF).For different diseases,it is necessary to properly set these planning parameters to achieve a relative balance between the radiation dose and the planned execution efficiency.This study focused on the influence of planning parameters of tomotherapy on head and neck tumors and lung tumors on the distribution of therapeutic doses,and proposed the external factors affecting the dose distribution,which will provide reference for the design of future tomo plans.The results show that the field width mainly affects the dose distribution of the patient's head and foot direction and the time of the radiotherapy;the pitch mainly affected the dose distribution in left and right directions;the modulation factor affected the conformity and uniformity of dose,and also had a great influence on the planned time of radiotherapy.For nasopharyngeal carcinoma patients,the parameters of helical tomotherapy plan are recommended to set the field width to 2.5 cm and the pitch to 0.287.The modulation factor is set between 1.6and 2.8 depending on the stage of the tumor and the degree of invasion.In general,the modulation factor of the early nasopharyngeal cancer plan is about 2.2,and 2.6 or more in the middle and late stage patients with the radiotherapy plan close to the brainstem and spinal cord.Different from nasopharyngeal cancer radiotherapy plan,lung cancer due to the existence of respiratory movement,in order to ensure accurate projection of the dose during the execution of the radiotherapy plan,the pitch should be adjusted so that the rotation period of the gantry is as long as possible.The lung cancer radiotherapy plan sets the lead gate width to be 5.0 cm(except for stereotactic radiotherapy),and the modulation factor is set between 1.6 and 2.6.The control of low dose area of lung can be achieved by setting double lung block.Since TOMO has no uniform system compared to conventional accelerators,the higher of the dose rate closer the isocenter.In addition to the field width,the modulation factor affects the treatment time,the size of the tumor from the center of the machine will also significantly affect the treatment time for peripheral lung cancer(centralized tumor).Therefore,under the premise of ensuring that patients can safely enter the TOMO aperture treatment,the placement center should be located near the tumor center as far as possible.