Analysis of clinical target volume positioning errors using cone beam computed tomography for patients with liver tumors with postoperative simplified intensity-modulated radiotherapy

Objective: To evaluate the inter-and intra-fractional clinical target volume (CTV) positioning errors of patients receiving postoperative simplified intensity-modulated radiotherapy (SIMRT) using cone beam computed tomography (CBCT). Methods: Twelve patients with liver tumors underwent postoperative SIMRT. CBCT images were acquired before and after the treatment. The clipbox volume for registration included the fiducial markers in the tumor bed and excluded the ribs and vertebral bodies. If any translational parameter of setup error before treatment exceeded 3 mm or rotational parameter exceeded 3 degree, the treatment couch was adjusted and a verification CBCT was acquired to assess residual setup error. Automatic bone match was used. A total of 214 acquisitions of CBCTs in 111 groups were analyzed. Inter-fractional translational CTV positioning errors in left-right (x), superior-inferior (y) and anterior-posterior (z) axis were calculated in 111 groups, and intra-fractional translational CTV positioning errors in 70 groups. Clinical to planning target volume (PTV) margins were calculated according to the formula:margin =2.0Σ + 0.7σ (Σ is systematic error, σ is random error). Results: Inter-fractional translational CTV positioning errors in x, y and z axis were -0.03 mm, -0.43 mm, 1.02 mm, with systematic error (Σ) of 1.50 mm, 5.89 mm, 1.97 mm, and random error (σ) of 1.76 mm, 4.13 mm, 2.42 mm, respectively. Intra-fractional translational CTV positioning errors in the x, y, z axis were 0.04 mm, 0.86 mm, -0.46 mm, with systematic error (Σ) of 0.46 mm, 1.14 mm, 0.31 mm,and random error (σ) of 0.95 mm, 1.38 mm, 0.91 mm, respectively. The calculate CTV to PTV margins were 4.5 mm, 15.0 mm, 5.8 mm in the x, y, z axis,respectively. Conclusions: The CTV errors were inevitable when patients with liver tumors received SIMRT. Fiducial markers placed in tumor bed during operation were helpful for accurate positioning error analysis. (authors)