Background: L'Hermitte's sign (LS) after chemoradiotherapy for head and neck cancer appears related to higher spinal cord doses. IMRT plans limit spinal cord dose, but the incidence of LS remains high. Methods: One hundred seventeen patients treated with TomoTherapy™ between 2008 and 2015 prospectively completed a side-effect questionnaire (VoxTox Trial Registration: UK CRN ID 13716). Baseline patient and treatment data were collected. Radiotherapy plans were analysed; mean and maximum spinal cord dose and volumes receiving 10, 20, 30 and 40 Gy were recorded. Dose variation across the cord was examined. These data were included in a logistic regression model. Results: Forty two patients (35.9%) reported LS symptoms. Concurrent weekly cisplatin did not increase LS risk (p=0.70, OR=1.23 (95% CI 0.51-2.34)). Of 13 diabetic participants (9 taking metformin), only 1 developed LS (p=0.025, OR=0.13 (95% CI 0.051-3.27)). A refined binary logistic regression model showed that patients receiving unilateral radiation (p=0.019, OR=2.06 (95% CI 0.15-0.84)) were more likely to develop LS. Higher V40Gy (p=0.047, OR=1.06 (95% CI 1.00-1.12)), and younger age (mean age 56.6 vs 59.7, p=0.060, OR=0.96 (95% CI 0.92-1.00)) were associated with elevated risk of LS, with borderline significance. Conclusions: In this cohort, concomitant cisplatin did not increase risk, and LS incidence was lower in diabetic patients. Patient age and dose gradients across the spinal cord may be important factors.