Optimising Stereotactic Body Radiotherapy (SBRT)with A Phantom-A Planning Study

Part One:A Dosimetry Studies with a Uniformity Phantom Optimising Stereotactic Body Radiotherapy (SBRT) for Different Prescription Dose Levels and Radiation Treatment Planning MethodsOBJECTIVE:To study uniform phantom to simulate different prescription dose Llevels surrounding the PTV, coplanar and non-coplanar plannings,3DCRT, IMRT, VMAT plannings, to compare the dose volume histogram (DVH) of the above different radiotherapy methods with the same prescription dose and risk at organs (OAR) restrictions under the same conditions, looking for the most essential and primitive distinction among various radiotherapy planning methods, to choose the best solutions for the clinical application of radiotherapy planning SBRT treatment technology and provide a reference.METHODS:Building a virtual30cm high30cm diameter cylindrical tissue with similar density to human with Eclipse treatment planning system for studying, CT value is OHu; virtually construct a3cm diameter spherica tumor (Hu50) at its center, to simulate a T1early stage non-small cell lung cancer patient. PTV (planning target volume) is formed by the outer GTV expansion1cm in all directions. Virtual outline around five organs at risk, representing the serial organs, parallel organs at different distances, three adjacent organs, respectively. For the PTV, five-field coplanar and non-coplanar plans and a seven-field coplanar plan were produced and optimised. Eclipse treatment planning system was used to generate five, seven and nine field coplanar and non-coplanar plans and one or two volumetric intensity-modulated arc therapy (VMAT) plans. Application1or2Arc, made radiotherapy plannings with60%,70%,80%,90%isodose level (Isocenter100%) surrounding the PTV,respectively. The resultin of various target and OAR dose distribution, including PTV90, PTV95, GTV90, GTV95, CI, HI, OAR2:V20, V10, V5, the average radiation dose,OAR1, OAR3,OAR4maximum and average radiation dose, respectively, though dose-volume histogram statistics.RESULTS:Analysis of variance showed non-coplanar plans to have lower VI0, V5, and higher PTV90and PTV95than coplanar plans. VMAT showed equivalent V20and target coverage when compared with the best non-coplanar plans, but with a faster delivery time (2min8s versus12min40s). All plans have shown that when using the higher prescription dose level, target uniformity, conformal index is good, organ at risk with lower radiation dose. To improve normal tissue sparing the dose should be prescribed at an isodose higher in SBRT.CONCLUSION:Whether CRT, IMRT or VMAT, different prescription dose levels (90%,80%,70%,60%) surrounding the PTV, rule is consistent.with the use of a percentage lower dose level, PTV uniformity index is getting worse, higher dose to surrounding tissues. The advantage of use of low prescription dose levels is that large doses of the target area gradient may bring benefits for tumor control.Same prescription dose levels surrounding the PTV, CRT, IMRT, VMAT three techniques, no significant difference to the regular shape of PTV. VMAT on OAR and applied according to the amount of time by slightly advantage, disadvantage is the low-dose region than the other two kind of technology increases. IMRT and VMAT technology have more advantages than the CRT to the irregular target.We suggest meeting the requirements of target conformal index premise, trying using a higher isodose level as the prescription dose. To the regular shape of PTV, there is no different among three techniques. CRT planning facilitates, fast, and, IMRT, VMAT plans, complex, time-consuming. VMAT plans have advantage in normal tissue sparing and delivery time, but need pay attention to the low-dose region increases. Although VMAT has been demonstrated to exhibit advantages in the treatment of other kinds of malignancies, the dosimetric advantage of VMAT in this study was not always evident when compared with IMRT. In addition, it is unclear whether IMRT should be replaced by VMAT. Considering the lower MUs, shorter delivery times and reduced low-dose exposure of OARs, the use of VMAT in postoperative radiotherapy remains suitable for gastric carcinoma; however, the clinical implications and outcome require further study.Part two:Determining the optimal prescription dose level on the planning target volume in stereotactic body radiotherapy for lung tumorsObjectives:The aim of this study was to determine a method of dose prescription that minimizes normal tissue irradiation outside of the planning target volume (PTV) for stereotactic body radiotherapy (SBRT) of lung tumors.Materials/Methods:One typical T1lung tumor with peripheral lesion in the lung was chosen for analysis. A PTV and several organs at risk (OARs) were constructed for dose calculated. Six treatment plans employing intensity modulated radiotherapy (IMRT) technique were made in which the dose was prescribed to encompass the PTV with the prescription isodose level (PIL) set at50%,60%,70%,%0%,90%,and95%of the isocenter dose, respectively. Four OARs around the PTV were constructed to assess the dose in the tissues adjacent to the PTV.Results:The MDPD increases with decreasing PIL from1.07to2.37, and the CI increases with decreasing PIL from1.05to1.22except the95%PIL with CI1.08. The90%PIL plans are nearly identical for CI and OARs sparing, except the volume of lower dose of lung. In summary, the plans tend to improve with higher PILs except the volume of lower dose of lung. The PTV coverage is similar among with six PIL plan, and the MDPD increases with decreasing PIL. The maximal dose of spinal cord, mean dose of ipsilateral lung-PTV, mean dose of trachea and chest wall, it is seen that the dose increases with decreasing PIL.Conclusions:The use of higher PILs for SBRT results in OARs sparing better except the volume of lower dose of lung. The use of lower PILs will create significant target dose inhomogeneity which may be beneficial for tumor control with worst conformity indices.