SU-E-T-423: Dosimetric Effect of Rotation on Stereotactic Body Radiotherapy (SBRT) of the Spine

To study the dosimetric effects of the residual setup rotation errors in spine SBRT.

We selected 25 patients with 26 lesions from an IRB approved registry. All patients were setup with Exactrac IGRT and treated with Novalis. Excatrac provided 6D shifts (3 translational, 3 rotational). The translational shifts were corrected by moving the couch. The rotational shifts >2 degrees were corrected by repositioning the patient, while smaller shifts were ignored. For each patient, we rotated the simulation CT rigidly around the CTV center by the recorded residual angles, using the MIM 4.22 software. The resulting rotated CT with all contours were imported into the TPS. The clinical plans used for treatment were mapped onto the rotated CT. Dosimetric endpoints were extracted for the rotated CTV (mean, max, D90) and the rotated cord (D0.1cc, D1cc, V10, max). These endpoints were compared to those from the clinical plan.

For the CTV, the average ratio of the mean doses was 1.001 (0.996-1.004), the average ratio of D90 was 1.003 (0.998-1.011), and the average ratio of the maximum doses was 1.005 (0.995-1.029). For the cord, the average ratio of DO.lcc was 1.01 (0.94-1.06), the average ratio of D1cc was 1.036 (0.92-1.23), the average max doses ratio was 1.03 (0.95-1.11), and the average V10 ratio was 1.43 (0.56-2.78).

Rotational errors in patient setup as small as 2 degrees do have dosimetric effects in spine SBRT treatments. The use of a robotic table to eliminate translational and rotational shifts is strongly recommended. It is important to note that, eventhough, the residual rotations resulted in up to 11% variation (cord max dose) from the planned doses, none of these cases violated our clinical treatment plan acceptance criteria (tolerance of the critical organs).