Room: Exhibit Hall | Forum 1
Purpose: The dose of kV and CBCT modalities on head and pelvis protocols, corresponding to C-spine and L-spine, were investigated in SBRT treatment. The cumulative CBCT dose to the spinal cord and the therapeutic dose change induced by setup uncertainty are evaluated, to optimize the workflow for limiting imaging dose while achieving precise setup.
Methods: KV and CBCT dose were measured by IBA MagicMax and CTDI assessment respectively, on 4 Varian TrueBeams, with compliance of TG-142. The CTDI of head and pelvis CBCT protocols were measured with the ACR phantoms and a 10cm chamber. The CTDI was correlated to spinal cord imaging dose, a concern for L-spine SBRT. Dosimetry changes from setup errors were analyzed by shifting/rotating the planning CT by 1mm and 1-degree in all X-Y-Z Pitch-Roll-Rotation directions for 5 patients using MIMVista. Dose differences were evaluated for target coverage and cord sparing by re-calculating the same VMAT plan.
Results: Average kV dose on 4 machines were 0.035cGy (head) and 0.12cGy (pelvis). CBCT showed 0.34cGy and 1.8cGy on CTDIw respectively, around 10 times higher than kV dose with highest CTDI from pelvis scans, as concluded in TG-180. Relating to the Rando-phantom measurement and Monte-Carlo simulation in the literature, pelvis CBCT dose to spinal cord was estimated as 3cGy. The dosimetry deviations with respect to the target D95 showed an average increase of 0.5% prescription dose(n=5). The spinal cord maximum dose (D0.035cc) increase by an average of 97cGy (3.2%Rx) and with the maximum of 295cGy (9.8%Rx) in the worst-case scenario of patient setup error.
Conclusion: CBCT dose from multiple scans contribute to spinal cord dose, especially for L-Spine SBRT treatment. Our patient positioning accuracy, effectively achieved with prioritizing use of multiple 2D-3D kV images and minimizing the number of CBCT verification scans per fraction, does not produce significant dosimetry perturbation.