Assessment of dosimetric impact of interfractional 6D setup error in tongue cancer treated with IMRT and VMAT using daily kV-CBCT

BACKGROUND: This study aimed to evaluate the dosimetric influence of 6-dimensional (6D) interfractional setup error in tongue cancer treated with intensity-modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) using daily kilovoltage cone-beam computed tomography (kV-CBCT). MATERIALS AND METHODS: This retrospective study included 20 tongue cancer patients treated with IMRT (10), VMAT (10), and daily kV-CBCT image guidance. Interfraction 6D setup errors along the lateral, longitudinal, vertical, pitch, roll, and yaw axes were evaluated for 600 CBCTs. Structures in the planning CT were deformed to the CBCT using deformable registration. For each fraction, a reference CBCT structure set with no rotation error was created. The treatment plan was recalculated on the CBCTs with the rotation error (R(Error)), translation error (T(Error)), and translation plus rotation error (T+R(Error)). For targets and organs at risk (OARs), the dosimetric impacts of R(Error), T(Error), and T+R(Error) were evaluated without and with moderate correction of setup errors. RESULTS: The maximum dose variation ΔD (%) for D(98%) in clinical target volumes (CTV): CTV-60, CTV-54, planning target volumes (PTV): PTV-60, and PTV-54 was −1.2%, −1.9%, −12.0%, and −12.3%, respectively, in the T+R(Error) without setup error correction. The maximum ΔD (%) for D(98%) in CTV-60, CTV-54, PTV-60, and PTV-54 was −1.0%, −1.7%, −9.2%, and −9.5%, respectively, in the T+R(Error) with moderate setup error correction. The dosimetric impact of interfractional 6D setup errors was statistically significant (p < 0.05) for D(98%) in CTV-60, CTV-54, PTV-60, and PTV-54. CONCLUSIONS: The uncorrected interfractional 6D setup errors could significantly impact the delivered dose to targets and OARs in tongue cancer. That emphasized the importance of daily 6D setup error correction in IMRT and VMAT.