Session Item

The use of SGRT on pelvis region has been a controversial discussion for the past few years. Bladder inconsistency and topography of pelvis region could be the root cause of the uncertainties including accuracy and reproducibility. Hence, we would like to study further if a good bladder management could improve the use of SGRT on cervical cancer patients. 

11 locally advanced cervical cancer patients who received concurrent chemoradiotherapy were enrolled and analyzed in this study. All patients were positioned in supine position. Patient’s bladder volume was measured with AvantSonic Z5 Bladder Scanner (AvantSonic Technology Co., Ltd) before treatment. The difference between the measured bladder volume and CT-sim bladder volume should be within ±15%. Then, patient was positioned based on isocenter skin markings (1 anterior and 2 laterals). The couch vertical value was kept the same as the latest CBCT couch acquired value. CBCT was taken and registration was performed accordingly. 3D translational and rotational shifts were recorded and applied. Next, SGRT system (OSMS, Varian, Palo Alto, CA) was activated to start monitoring. A standard recommended region of interest (ROI) was used across all patients, as shown in Figure 1. The 6DoF deltas shown on the SGRT system were recorded. The deltas represented the discrepancies between CBCT and the reconstructed 3D surface. The average, standard deviation and the error distributions were analyzed using ANOVA test and chi-square test with SPSS software (IBM corp.). A difference of P < 0.05 was considered statistically significant.

Figure 1: Pelvis ROI

A total of 101 CBCT images were analyzed. The average and standard deviations (Table 1) between skin marking-based and SGRT-based group were 0.026±0.500cm, 0.199±0.366cm, 0.049±0.209cm and 0.012±0.126cm, 0.002±0.301cm, -0.003±0.307cm in x(lateral), y(longitudinal) and z(vertical) translational directions; 0.462±1.279°,-0.048±0.942°, -0.281±0.882°, and -0.151±1.629°, 0.082±0.625°, -0.053±0.813° in Rx(pitch), Ry(roll), and Rz(rotation) rotational direction. The results showed that the setup error in longitudinal and pitch could be improved with SGRT system. The percentage of positioning accuracy within 5mm and 2° were shown in Table 2. The positioning accuracy based on SGRT was comparable to skin marking-based, and SGRT was notably beneficial in lateral,  longitudinal and roll direction. 

Incorporating a good bladder management can improve the accuracy of SGRT application on cervical cancer radiotherapy patients. It is feasible, reliable, and potentially mark-free. We will continue to study for bigger sample size for more concrete conclusion.