Dosimetric Study in Tomotherapy Based on AAPM TG ۱۱۹ Structures: A Longitudinal Moving Phantom Case

Introduction: Tomotherapy beam delivery is in the helical form. Therefore, the dose distribution will be more complex while target is moving. In this study, we sought to evaluate the dosimetric impact due to longitudinal motion in the phantom of a tomotherapy machine.          Material and Methods: Cheese and Delta۴ phantom+ were placed on a respiratory motion platform. They moved in longitudinal directions at the amplitudes of ۲, ۴, ۶, ۸, and ۱۰ mm. The period of that movement was ۴ and ۶ s with the field widths of ۲۵ and ۵۰ mm, respectively. The C-shaped complex target was modified according to the American Association of Physicists in Medicine (AAPM) Task Group (TG) ۱۱۹. The planning verifications were evaluated through point dose, gamma index value, and dose-volume histogram (DVH).Results: Discrepancy of the dose measurements ranged from -۱.۲۵۴ to -۱۴.۴۲۱%. The range of gamma index value was ۶۱.۲ ±۱.۲۳% to ۱۰۰±۰.۰۰. The DVH evaluation showed that the homogeneity index (HI) and the minimum dose to receive by ۹۵% (D۹۵%)  of the target structure were ۰.۲۴۷ to ۰.۳۸۹ and -۰.۰۶۱ to -۰.۲۷۱ Gy, respectively. The maximum dose (DMax) of the organ at risk (OAR) structure was ۰.۰۸۲ to ۰.۳۲۷ Gy.Conclusion: The motion could induce dose discrepancies in tomotherapy dose distribution. The selection of the jaw field width in tomotherapy is crucial for intensity-modulated radiotherapy (IMRT) techniques with moving targets. For larger field widths, the dose discrepancy between the planned and measured doses exhibited an excellent result for gamma index and dose coverage.  

Keywords:

Tomotherapy , Intensity Modulated Radiotherapy , Dosimetric Impact