RESUMO
BACKGROUND: Set-up errors represent a source of uncertainty in head and neck (H&N) cancer radiotherapy. The present study evaluated set-up accuracy with the use of cone-beam computed tomography (CBCT) in order to establish the proper clinical target volume (CTV) to planning target volume (PTV) margins to be adopted. METHODS: Local set-up accuracy was analysed for 44 H&N cancer patients since the implementation of CBCT. An on-line correction protocol was adopted, with the first 3 scans used to correct systematic errors with a 3-mm action level. The overall mean displacement (M), the population systematic (Σ) and random (σ) errors and the 3D vector length were calculated. PTV margins were calculated according to the van Herk formula (2.5Σ + 0.7σ). RESULTS: A total of 420 CBCT scans were analysed. A systematic correction was needed in 43% of patients. The value of M was <1 mm in all directions; the values of Σ and σ ranged over 1-1.2 and 1.4-1.9 mm, respectively. Pre-correction PTV margins were 3.48, 4.08 and 4.33 mm along the 3 axes. The PTV margins calculated after online correction were <2.5 mm in all directions. CONCLUSIONS: Kilovoltage CBCT is effective in evaluating set-up accuracy in H&N patients. CTV-PTV margins of 5 mm are safe and are currently adopted at our centre; however, some special situations, such as re-irradiation or the close proximity of organs at risk and high-dose regions, could benefit from daily image registration and lower (i.e., 3 mm) margins.