To explore the choice of fixation options for rectal cancer patients receiving pelvic radiotherapy under the condition of using a 6-DOF (six-degree-of-freedom) bed.

Twenty-seven patients with rectal cancer who underwent pelvic radiotherapy in our department from November 1, 2015, to March 25, 2016, were analyzed of the setup errors to evaluate the fixation effect of low-temperature thermoplastic film and vacuum pad.

There were significant differences in X, Y, Z, RX, RY, and RZ in the heat-shrinkable film mesh group before and after the 6-DOF bed correction (t=0.732, 1.408, -2.335, 5.237, -3.154, -1.130; P＜0.05). There were significant differences in Z, RX, and RY directions in the negative pressure vacuum pad group before and after the correction 6-DOF bed (t=-13.128, 9.945, -5.993; P＜0.05), but there was no significant difference in X, Y and RZ directions (t=1.706, 6.508, -5.34; P＞0.05). The results showed that during the treatment, the thermoplastic membrane mesh group had a proper correction effect. Before the 6-DOF bed correction, the setup errors of the two groups were statistically different only in the direction of RY (t=2.226, P=0.027) and RZ (t=-2.686, P=0.008), and there was no significant difference in other directions (P＞0.05). Moreover, the setup errors in the RZ direction of the vacuum pad fixation group were smaller than that of the thermoplastic film body mesh group, indicating that the vacuum pad fixation effect in the patient's body's long axis is ideal. After 6-DOF bed correction, the setup errors of the two groups were statistically different only in the Z direction (t=3.267, P=0.001), but there was no significant difference in other directions (P＞0.05). Moreover, the thermoplastic film mesh group's setup errors in the Z direction were smaller, which indicated that the thermoplastic film mesh group had better fixation efficiency in Z direction after a 6-DOF bed correction. Before a 6-DOF bed correction, the PTV extension boundary of X, Y, and Z of group 1 was 0.33 cm, 0.61 cm, and 0.90 cm, respectively; The PTV extension boundary of X, Y, and Z of group 2 was 0.39 cm, 0.42 cm, and 0.99 cm, respectively. After a 6-DOF bed correction, the PTV extension boundary of X, Y, and Z of group 1 was 0.09 cm, 0.06 cm, and 0.08 cm, respectively; the PTV extension boundary of X, Y, and Z of group 2 was 0.12 cm, 0.2 cm, and 0.57 cm, respectively. The results showed that the thermoplastic film mesh group had better fixation efficiency.

In this study, we recommend using heat shrinkable mesh for the fixation of rectal cancer patients receiving pelvic radiotherapy, when the 6-DOF bed is used.