The coordinates of the center of gravity of the three fiducial markers were measured at the start of every portal irradiation in intensity-modulated radiotherapy (IMRT) with seven ports. The table position was adjusted to the planned position if the discrepancy was larger than 2.0 mm in the anterior¨Cposterior (AP), cranial¨Ccaudal (CC), or left¨Cright (LR) directions. In total, we analyzed 4,541 observations in 20 patients who received 70 Gy in 30 fractions (7.6 times a day on average).
The incidence of table position adjustment at 10 minutes from the initial setup of each treatment was 14.2 % , 12.3 % , and 5.0 % of the observations in the AP, CC, and LR directions, respectively. The accumulated incidence of the table position adjustment was significantly higher at 10 minutes than at 2 minutes for AP (p?= 0.0033) and CC (p?= 0.0110) but not LR (p?= 0.4296). An adjustment greater than 5 mm was required at least once in the treatment period in 11 (55 % ) patients.
Interportal adjustment of table position was required in more than 10 % of portal irradiations during the 10-minute period after initial setup to maintain treatment accuracy within 2.0 mm.