The ratio of weight loss to planning target volume significantly impacts setup errors in nasopharyngeal cancer patients undergoing helical tomotherapy with daily megavoltage computed tomography

Background. Changes in head and neck anatomy during radiation therapy (RT) produce setup uncertainties of nasopharyngeal cancer (NPC) irradiation. We retrospectively analyzed image guidance data to identify clinical predictors of setup errors. Patients and methods. The data of 217 NPC patients undergoing definitive RT on a helical tomotherapy (HT) unit were analyzed. Factors including tumor stage, body mass index, weight loss, and planning target volume (PTV) were assessed as predictors of daily megavoltage computed tomography (MVCT) setup displacements, which were automatically registered using software. Results. Mean daily setup displacements (in mm) were 1.2 +/- 0.6, 1.8 +/- 0.8, 3.4 +/- 1.4 in the medial-lateral (ML), superiorinferior (SI), and anterior-posterior (AP) directions, respectively. Mean weight loss was 4.6 +/- 3.3 kg (6.8 +/- 4.9%). Patients with weight loss > 5% had significantly larger setup displacements in the AP (3.6 +/- 1.5 vs. 2.9 +/- 1.1 mm, p < 0.001) and SI (1.6 +/- 0.7 vs. 1.9 +/- 0.9 mm, p = 0.01) direction, but not in the ML direction (p = 0.279). The AP setup error increased 0.06 mm (y = 0.055x + 2.927, x: percentage of weight loss/PTV, y: AP displacement) per one percent increase in weight loss normalized to PTV. Conclusions. Patients with weight loss > 5% and smaller PTVs, possibly because of small body frame or neck girth, were more likely to have increased setup errors in the AP direction.