Anatomical change during radiotherapy for head and neck cancer, and its effect on delivered dose to the spinal cord

BACKGROUND AND PURPOSE: The impact of weight loss and anatomical change during head and neck (H&N) radiotherapy on spinal cord dosimetry is poorly understood, limiting evidence-based adaptive management strategies. MATERIALS AND METHODS: 133 H&N patients treated with daily mega-voltage CT image-guidance (MVCT-IG) on TomoTherapy, were selected. Elastix software was used to deform planning scan SC contours to MVCT-IG scans, and accumulate dose. Planned (D(P)) and delivered (D(A)) spinal cord D(2%) (SCD(2%)) were compared. Univariate relationships between neck irradiation strategy (unilateral vs bilateral), T-stage, N-stage, weight loss, and changes in lateral separation (LND) and CT slice surface area (SSA) at C1 and the superior thyroid notch (TN), and ΔSCD(2%) [(D(A) – D(P)) D(2%)] were examined. RESULTS: The mean value for (D(A) – D(P)) D(2%) was −0.07 Gy (95%CI −0.28 to 0.14, range −5.7 Gy to 3.8 Gy), and the mean absolute difference between D(P) and D(A) (independent of difference direction) was 0.9 Gy (95%CI 0.76–1.04 Gy). Neck treatment strategy (p = 0.39) and T-stage (p = 0.56) did not affect ΔSCD(2%). Borderline significance (p = 0.09) was seen for higher N-stage (N2-3) and higher ΔSCD(2%). Mean reductions in anatomical metrics were substantial: weight loss 6.8 kg; C1LND 12.9 mm; C1SSA 12.1 cm(2); TNLND 5.3 mm; TNSSA 11.2 cm(2), but no relationship between weight loss or anatomical change and ΔSCD(2%) was observed (all r(2) < 0.1). CONCLUSIONS: Differences between delivered and planned spinal cord D(2%) are small in patients treated with daily IG. Even patients experiencing substantial weight loss or anatomical change during treatment do not require adaptive replanning for spinal cord safety.