Adaptive Replanning During Head and Neck VMAT After Weight Loss

Reasoning

The persistent 7 mm inferior and 5 mm posterior shift after re-positioning is unlikely to be a random set-up error. A 7 kg weight loss over 20 fractions, a loose mask, and a systematic shift point to anatomical change and loss of immobilisation. The dose recalculation confirms that the change is clinically relevant: cord maximum 47 Gy exceeds the 44 Gy constraint, parotid mean 29 Gy exceeds 26 Gy, and PTV65 V95 of 93% is below the usual 95% coverage goal.

• Cord: 47 Gy is 3 Gy above constraint. Cord tolerance is close, and the excess may be real if the CBCT dose calculation is reliable.
• Parotid: 29 Gy versus 26 Gy increases xerostomia risk.
• PTV: 93% coverage means underdosage of part of the high-dose target, likely due to the shift and changed anatomy.

The event is both a verification problem and a planning problem. Simply shifting the patient by 7 mm and 5 mm will not correct a changed internal anatomy or a loose mask, and it may not restore cord sparing. It matters because continuing the original plan could overdose the cord and underdose tumour, and because the CBCT dose calculation itself needs validation before being used for a clinical decision.

Reasoning

The next fraction should not proceed unchanged. The cord constraint is exceeded on the day-20 anatomy, and the mask is loose, so set-up reproducibility is compromised. The fact that the consultant is unavailable for 3 hours does not justify treating an unverified plan.

Immediate actions:

• Hold the fraction and inform the on-call oncologist and physics team.
• Repeat the CBCT with careful repositioning and check the mask fit. If the mask is loose, do not force a match to the original plan.
• Verify the dose calculation method on CBCT against the planning CT and check registration accuracy.
• Assess the patient clinically: weight, hydration, mucositis, pain, nutritional intake, and whether cisplatin should continue.
• Consider a repeat planning CT and a new mask. If the anatomy has changed significantly, an adaptive replan is required.
• If the patient is symptomatic or the delay is clinically risky, discuss a short treatment break and ensure urgent replanning rather than delivering a compromised plan.

Judgement: the safest action is to pause and replan. A 3-hour delay is preferable to a cord overdose or a cumulative systematic underdose. Document the hold, the dose concern, and the escalation.

Reasoning

The patient needs a structured adaptive process, not a one-off shift correction.

1. Re-simulation: repeat CT in a new, well-fitting mask, with the same immobilisation and contrast protocol. Document weight and set-up.
2. Replanning: fuse the new CT to the original planning CT, transfer targets and organs at risk, and replan VMAT. Prioritize cord maximum ≤44 Gy, brainstem, parotid mean ≤26 Gy where achievable, and PTV65 V95 ≥95%. If constraints cannot all be met, document the trade-offs and obtain oncologist approval.
3. Dose accumulation: calculate the cumulative dose from fractions 1–20 and the new plan for fractions 21–30. Do not assume the original plan dose still applies.
4. Independent verification: physics check of the new plan, MU calculation, and patient-specific QA as per local policy.
5. Imaging: daily CBCT for the first 3 fractions after replanning, then according to departmental protocol. Use soft-tissue matching and record shifts. Consider weekly MR or CT review if available.
6. Clinical review: dietitian and speech therapy involvement; monitor weight and mucositis; decide on cisplatin continuation.
7. Communication: inform the patient why treatment is being replanned and what the delay means. Document all decisions.

The key QA lesson is that a loose mask plus weight loss is an adaptive-planning trigger, not a routine shift.