Risk factors related to interfractional variation in whole pelvic irradiation for locally advanced pelvic malignancies

The goal of the present study was to demonstrate risk factors affecting the interfractional variation in whole pelvic irradiation. Daily image acquisitions of 101 patients with locally advanced pelvic malignancy were undertaken using a kilo-voltage orthogonal on-board imager. The baseline deviation (the shift between the initial treatment and each fraction; Value(Base)) and day-to-day variation (the shift between the previous treatment and each fraction; Value(DD)) were measured. The standard deviations (SD) along the x- (right-left), y- (cranial-caudal), and z- (anterior-posterior) axes (SD[x], SD[y], and SD[z], respectively), the 3D vector of the SD (SD[3D]), and the mean of 3D shift (mean[3D]) were calculated in each patient. Various clinical factors, lumbar pelvic balance and rotation, and the shift of 5 consecutive fractions from the initial treatment (Value(5Fx)) were investigated as risk factors. The prone set-up showed a larger mean(Base)[3D] than in the supine position (p =0 .063). A body mass index (BMI) a parts per thousand yenaEuro parts per thousand 30 kg/m(2) resulted in the largest mean(DD)[3D] (p = 0.078) and SDDD[3D] (p = 0.058). All the SD5Fx along the x-, y-, and z-axes had moderate linear relationships with SDBase and SDDD (p < 0.001). The SD5Fx[3D] also had a moderate linear relationship with the mean(Base)[3D], mean(DD)[3D], SDBase[3D], and SDDD[3D] (p < 0.001). In multivariate analysis, the SD5Fx had the same significant relationship with SDBase and SDDD (p < 0.001). A BMI a parts per thousand yenaEuro parts per thousand 30 kg/m(2) was associated with the largest SDDD[x] (p = 0.003). Close surveillance through high-quality and frequent image guidance is recommended for patients with extensive variations of the initial five consecutive fractions or obesity.