Adaptive Replanning for Weight Loss During Head and Neck Chemoradiotherapy
A 58-year-old woman with HPV-positive squamous cell carcinoma of the base of tongue, cT2N2b, starts definitive chemoradiotherapy: 70 Gy in 35 fractions to the primary and involved nodes, 56 Gy to the elective bilateral neck, with weekly cisplatin. At fraction 16 she has lost 8 kg (12% of body weight), her neck circumference has decreased by 3 cm, and daily cone-beam CT shows increasing setup variability. The original plan had parotid mean doses of 25.8 Gy (right) and 26.4 Gy (left), constrictor mean 58 Gy, and PTV70 D95 = 69.8 Gy. She now has grade 2 mucositis, uses a feeding tube, and has a creatinine rise that may limit further cisplatin. The treating team asks whether to continue the same plan, repeat CT and replan, or pause treatment.
1. Should the team continue the original plan, replan now, or pause treatment? Explain the anatomical and dosimetric reasoning.
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Recommendation: repeat CT and replan now; do not simply continue, and do not pause unless toxicity demands it. An 8 kg weight loss and 3 cm neck circumference change alter tissue thickness, parotid position, and setup reproducibility. The original parotid means of 25.8 and 26.4 Gy are already at or above the usual mean-dose objective of <26 Gy; with anatomical shrinkage, the parotids can move into the high-dose region and mean doses may rise by several Gy, increasing permanent xerostomia. Daily cone-beam CT variability also suggests that the immobilization mask no longer fits reproducibly. Replanning should use a new CT in the treatment position, fuse to the original, and re-delimit target volumes using pretreatment MRI/PET and clinical examination. The goal is not to chase the original plan but to maintain PTV70 D95 ≥95% and cord tolerance while keeping at least one parotid mean <26 Gy and constrictors as low as achievable. If cisplatin is stopped for renal toxicity, do not compensate by increasing radiation dose. Pausing radiotherapy for replanning is acceptable for 1–2 days; a prolonged pause reduces local control.
2. What structures and dose metrics must be evaluated on the repeat CT, and how would you resolve a conflict between parotid sparing and PTV coverage?
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On repeat CT, evaluate PTV70/PTV56 coverage (D95, D99, hot spots), spinal cord Dmax, brainstem, mandible, parotid mean and V30, submandibular gland, constrictor mean, larynx, and dose to the feeding-tube site. Also check setup reproducibility and whether the mask needs to be remade. If parotid sparing conflicts with target coverage because the target has shifted into the parotid bed, prioritize target coverage; a cold target is a greater risk than xerostomia in a curable HPV-positive cancer. Use a simultaneous integrated boost with tighter margins only if image guidance supports it; do not expand margins to compensate for setup error. If one parotid mean is 28 Gy and the other is 24 Gy, accept if the lower-dose gland is stable; if both exceed 26 Gy, try beam-angle optimization or a parotid-sparing arc, but do not underdose the PTV. Document the trade-off and monitor renal function.