ABSTRACT
Background and purpose:Now 3-dimensional conformal radiotherapy(3DCRT)and intensitymodulated radiotherapy(IMRT)are widely used in the treatment of head and neck tumor.For the parotid,this target area is located on the side of the head and the tumor has a concave shape.What kind of radiation method can be used more eriectively to achieve dose uniformity and protection organs at risk is the topic of much discussion.The postoperative irradiation of parotid tumor is varied in the techniques used.In this study,3-dimensional conformal radiotherapy (3DCRT),intensity-modulated radiotherapy(IMRT)and simplified forward planned multi-segment radiotherapy (MSRT)were compared to conventional planning techniques in order to investigate the potential advantages of these new treatments.Methods:The conventional planning included the large opposed lateral fields with 2 or 3 weight ratio at the target lateral(2F-2D)and the unilateral field with mixture of 6 MV photon and electron beams(X+E).The 3D techniques included 3DCRT,MSRT and IMRT.Their dose distributions were calculated and compared for 8 patients treated in our center.Different beam arrangements were used for 3D techniques.In each case.the dose of PTV was prescribed to 60 Gy.All plans were compared using dose-volume histogram data.The conformity index(CI)and heterogeneity index(HI)of dose were used to evaluate the dose coverage of the target volume.Dose sparing of brain stem.spinal cord and the contra lateral parotid was also compared.To compare IMRT and MS RT,the timing ofplanning and radiation delivery was recorded.Results:Compared to conventional planning,the 3DCRT,MSRT and IMRT plans produced adequate target coverage,and the CI showed 3DCRT plans(0.78)produced poorer target coverage than MSRT(0.81)and IMRT(0.85).MRST and IMRT plans showed a significant reduction in maximum dose to the spinal cord,brainstem and the contra lateral parotid,compared to the conventional plans,while the 3DCRT plan did not show significant sparing of these structures.MSRT and IMRT plans produced better dose coverage among all the techniques.The efficacy of beam delivery comparing between two modulated planning showed MSRT was better.Conclusion:For postoperative irradiation of parotid cancer,3D planning techniques generated better target dose-coverage,without compromising the dose-sparing advantages of important structures.A satisfactory dose distribution can be obtained using MSRT and such a simple technique may be suitable for replacing IMRT.