Interfractional variability in intensity-modulated radiotherapy of prostate cancer with or without thermoplastic pelvic immobilization

Abstract

To determine the variability of patient positioning errors associated with intensity-modulated radiotherapy (IMRT) for prostate cancer and to assess the impact of thermoplastic pelvic immobilization on these errors using kilovoltage (kV) cone-beam computed tomography (CBCT). From February 2012 to June 2012, the records of 314 IMRT sessions in 19 patients with prostate cancer, performed with or without immobilization at two different facilities in the Korea University Hospital were analyzed. The kV CBCT images were matched to simulation computed tomography (CT) images to determine the simulation-to-treatment variability. The shifts along the x (lateral)-, y (longitudinal)- and z (vertical)-axes were measured, as was the shift in the three dimensional (3D) vector. The measured systematic errors in the immobilized group during treatment were 0.46 +/- 1.75 mm along the x-axis, -aEuro parts per thousand 0.35 +/- 3.83 mm along the y-axis, 0.20 +/- 2.75 mm along the z-axis and 4.05 +/- 3.02 mm in the 3D vector. Those of nonimmobilized group were -aEuro parts per thousand 1.45 +/- 7.50 mm along the x-axis, 1.89 +/- 5.07 mm along the y-axis, 0.28 +/- 3.81 mm along the z-axis and 8.90 +/- 4.79 mm in the 3D vector. The group immobilized with pelvic thermoplastics showed reduced interfractional variability along the x- and y-axes and in the 3D vector compared to the nonimmobilized group (p < 0.05). IMRT with thermoplastic pelvic immobilization in patients with prostate cancer appears to be useful in stabilizing interfractional variability during the planned treatment course.