Session Item

With the introduction of a second system for SGRT(3) on 9 linacs (next to the previous SGRT(1) system for SBRT applications on a Varian TrueBeam STX platform), we investigated the positioning residual error obtained by Cone Beam CT matching after surface guided positioning for lung patients. Further, we compared 2 different surface scanning systems and 2 different immobilization devices for the thorax. In addition, a comparison with conventional lung treatments with external markings was performed.

A search on our ARIA database yielded the online matched parameters of all the lung patients who were irradiated in the first 10 months of 2022. Further a historical dataset was retrieved one year earlier for patients positioned on device A[2] with SGRT[1] to compare both SGRT systems. Patients with arms positioned next to the thorax were excluded. Further, the data were sorted according to the positioning technique. This resulted in 4 groups:

1)    123 patients positioned (2420 fractions) on device A[2] without surface guidance

2)    55 patients positioned (1053 fractions) on device A[2] with SGRT[3]

3)    68 patients (394 fractions) positioned on device A[2] with SGRT1[1]

4)    55 patients (346 fractions) positioned on device B[4] with SGRT[1]

All feet and knees were indexed to the couch or base plate by cushions. For 301 patients (4213 fractions) 3D vectors were calculated and the residual translations after imaging were analyzed for all groups. ANOVA was performed with p 0,05.

The average magnitude of the translations for group 1 were -0,23 (±0,46) cm; -0,11 (±0,55) cm; -0,02 (±0,66) cm; 0,71 (±0,49) , group 2 -0,22 (±0,33) cm ;-0,04 (±0,39) cm; -0,04 (±0,23) cm; 0,57 (±0,32) , group 3 -0,24 (±0,37) cm, -0,02 (±0,67) cm, -0,01 (±0,22) cm, 0,60 (±0,58) and group 4 were -0,22 (±0,37) cm; 0,01(±0,56) cm and 0,00 (±0,31) cm, 0,5 (±0,27) for vertical, longitudinal, lateral online matched values and 3D error respectively. All F values were > F critical threshold and all the p values were below the 0,05 threshold except between group 2&3 (for all residual translational error and calculated 3D values).

This study shows that the overall 3D shift corrections for lung patients initially aligned with surface guidance were significantly smaller than those aligned without surface guidance.Surface imaging systems can be considered a good option for initial patient setup and are preferable to external marks for lung cancer patients.  There is no significant difference between SGRT systems for lung patients treated on the same immobilization device. The positioning of SBRT patients on the Orfit base plate ™ resulted in a significantly smaller residual translational error compared to the Thorax Support ™.