Evaluation of the Efficacy of Rotational Corrections for Standard-Fractionation Head and Neck Image-Guided Radiotherapy

Abstract

Purpose: Modern linear accelerators are equipped with cone beam computed tomography and robotic couches that can correct for errors in the translational (X, Y, Z) and rotational (α, β, γ) axes prior to treatment delivery. Here, we compared the positional accuracy of 2 cone beam registration approaches: (1) employing translational shifts only in 3 degrees of freedom (X, Y, Z), versus; (2) using translational-rotational shifts in 6 degrees of freedom (X, Y, Z, α, β, γ).

Methods: This retrospective study examined 140 interfraction cone beam images from 20 patients with head and neck cancer treated with standard intensity-modulated radiation therapy. The cone beam images were matched to planning simulation scans in 3, then in 6 degrees of freedom, using the mandible, clivus, and C2 and C7 vertebrae as surrogate volumes. Statistical analyses included a generalized mixed model and was used to assess whether there were significant differences in acceptable registrations between the 2 correction methods.

Results: The rates of improvement with corrections in 6 degrees of freedom for the mandible with a 5-mm expansion margin were 54.55% ( P = .793), for the clivus 85.71% ( P = .222), and for C7 87.50% ( P = .015). There was a 100% increase in acceptability for the C2 vertebra within the 5-mm margin ( P < .001). For the 3-mm expansion margin, the rates of improvement for the mandible, clivus, C2, and C7 were 63.16% ( P = .070), 91.30% ( P = .011), 84.21% ( P = .027), and 76.92% ( P < .001), respectively.

Conclusions: Significant registration improvements with the use of rotational corrections with a 5-mm expansion margin are only seen in the C7 vertebra. At the 3-mm margin, significant improvements are found for the C2, C7, and clivus registrations, suggesting that intensity-modulated radiotherapy treatments for head and neck cancers with 3-mm planning target volume margins may benefit from corrections in 6 degrees of freedom.

Keywords: 6 degrees of freedom; cone beam computed tomography; head and neck cancer; image guidance; intensity-modulated radiation therapy; rotational corrections.

Figure 1.

Bony surrogate structures were contoured in the treatment planning system for image analysis: (A) mandible, (B) clivus, (C) C2 vertebra, and (D) C7 vertebra. An expansion contour of 3 mm and 5 mm was generated for each structure to allow analysis (both margins shown).

Figure 2.

Transverse CBCT image of C7 vertebra. A, Transverse CBCT image of 3-DoF corrections showing unacceptable registration for the 3-mm expansion margin and acceptable registration for the 5-mm expansion margin of the C7 vertebra. B, Cone beam computed tomography image of 6-DoF corrections showing acceptable registrations for the 3-mm and 5-mm expansion margins of the C7 vertebra. CBCT indicates cone beam computed tomography; DoF, degrees of freedom.

Figure 3.

Cone beam computed tomography images from 7 weekly fractions were selected for each of 20 patients. The mandible, clivus, C2, and C7 landmarks were contoured on each patient’s CT simulation scan, and 5-mm expansion margins were generated for each landmark contour. Image registrations in 3-DoF were performed for all fractions, and the acceptability of each landmark was analyzed. Registrations in 6-DoF were performed for the same CBCT images; landmark acceptability was analyzed and compared to that of 3-DoF. The registrations and analyses were then all repeated with 3-mm expansion margins using the same selected CBCT images. CBCT indicates cone beam computed tomography; CT, computed tomography; DoF, degrees of freedom.