Dosimetric Study in Tomotherapy Based on AAPM TG 119 Structures: A Longitudinal Moving Phantom Case

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.          Cheese and Delta4 phantom+ were placed on a respiratory motion platform. They moved in longitudinal directions at the amplitudes of 2, 4, 6, 8, and 10 mm. The period of that movement was 4 and 6 s with the field widths of 25 and 50 mm, respectively. The C-shaped complex target was modified according to the American Association of Physicists in Medicine (AAPM) Task Group (TG) 119. The planning verifications were evaluated through point dose, gamma index value, and dose-volume histogram (DVH). Discrepancy of the dose measurements ranged from -1.254 to -14.421%. The range of gamma index value was 61.2 ±1.23% to 100±0.00. The DVH evaluation showed that the homogeneity index (HI) and the minimum dose to receive by 95% (D95%)  of the target structure were 0.247 to 0.389 and -0.061 to -0.271 Gy, respectively. The maximum dose (DMax) of the organ at risk (OAR) structure was 0.082 to 0.327 Gy. 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.