Room: AAPM ePoster Library
Purpose: To calculate the rotational positional error corrected setup margin in frameless SRS/SRT.
Methods: total of 79 patients of SRS/SRT each received 3-6 fractions were studied. Two CBCT scans were acquired for each session of treatment, before any patient position correction (TPC) and after TPC using a 6Dcouch. CBCT obtained patient rotational (rolls, pitch, and yaw) positional errors were reduced to equivalent translational shifts (lateral, longitudinal, and vertical) using a mathematical formulation. The post-positional correction setup margin was calculated using the van Hark formula. Further, a PTV_R (PTV with rotational correction) and PTV_NR (PTV without rotational correction) were calculated by applying the rotation corrected and uncorrected setup margins on the GTVs.
Results: total of 380 sessions of pre (190) and post (190) table positional correction CBCT data was analyzed. The pre-table position correction average error for lateral, longitudinal, and vertical translational and rotational shifts were x=0.08±0.09 cm, y=0.04±0.21 cm, z=-0.12±0.2 cm, and a=0.15±1.1°, ß=0.38±0.94 °, ?=-0.05±1.1° respectively. The post TPC error (in same sequence) was -0.01±0.05 cm, -0.02±0.05 cm, 0.0±0.05 cm and 0.04±0.34°, 0.13±0.44°, 0.02±0.44° respectively. The GTV volumes show a range of 0.13 cc to 39.56 cc, with a mean volume of 6.35±8.65 cc. Rotational correction incorporated post-positional correction setup margin the in lateral (x), longitudinal (y), and vertical (z) direction were 0.05 cm, 0.12 cm, and 0.1 cm, respectively. PTV_R ranges from 0.27 cc-44.7 cc, with a mean volume of 7.7±9.8 cc. PTV_NR ranges from 0.32 cc-46.0 cc, with a mean volume of 8.1±10.1 cc.
Conclusion: is possible, for the first time, to incorporate the rotational positional error into the setup margin. The post-correction linear setup margin matches well with the conventional setup margin of 1 mm. Beyond a GTV radius of 2 cm, the difference between PTV_NR and PTV_R is =2.5%, hence it is not significant.
Patient Positioning, Cone-beam CT