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

Abstract

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.