Re-irradiation of a Recurrent Head and Neck Tumor Near the Spinal Cord
A 58-year-old woman with a history of laryngeal cancer treated 3 years ago with 70 Gy in 35 fractions to the larynx and bilateral neck (with concurrent cisplatin) now has a biopsy-proven recurrence at the level of the C5-C6 vertebral body, invading the prevertebral space and abutting the spinal cord. She has no distant disease. The prior plan is available and shows the spinal cord received a maximum dose of 46 Gy and the brainstem 50 Gy. She has mild dysphagia and a tracheostomy. The team proposes re-irradiation with 60 Gy in 30 fractions or SBRT. The radiation oncologist must assess cumulative dose to the cord and decide on a strategy.
1. How would you evaluate cumulative spinal cord dose and decide whether re-irradiation is safe?
Read the worked answer
The key is to estimate the cumulative dose to the spinal cord and brainstem using the prior plan and the proposed plan, recognizing that the alpha/beta for the spinal cord is low (~2) and that recovery is incomplete.
Steps:
- Fuse the prior planning CT and dose distribution to the current CT and compute the cumulative dose to the cord surface (or a 0.5-1 cm surrogate) and brainstem.
- Convert to equivalent doses in 2 Gy fractions (EQD2) using the linear-quadratic model with alpha/beta = 2 for the cord. A prior maximum cord dose of 46 Gy in 2 Gy fractions is already near the tolerance of 45-50 Gy; adding 60 Gy in 30 fractions to a region abutting the cord would exceed tolerance unless the cord is excluded from the high-dose volume.
- Consider the time interval (3 years) and the fact that some recovery occurs, but do not rely on it to justify exceeding tolerance.
Decision: re-irradiation is feasible only if the cumulative EQD2 to the cord can be kept below a conservative threshold (often cited as ~60-70 Gy EQD2 cumulative, with the caveat that this is not a hard limit). This likely requires SBRT or highly conformal IMRT with strict cord constraints, not 60 Gy in 30 fractions to a volume abutting the cord.
2. What re-irradiation strategy would you recommend, and what are the radiobiologic and clinical trade-offs?
Read the worked answer
Options:
- SBRT (e.g., 30-40 Gy in 5 fractions) to the recurrent tumor with a strict cord constraint (e.g., cord Dmax < 8-10 Gy per course, cumulative EQD2 kept as low as possible): delivers a high biologic dose to the tumor while sparing the cord, but carries a risk of vertebral body fracture, esophageal toxicity, and carotid blowout if the tumor encases vessels.
- Hypofractionated IMRT (e.g., 45-50 Gy in 15-20 fractions) with cord exclusion: more forgiving than 60 Gy in 30 fractions and allows better normal-tissue repair, but lower BED to the tumor.
- Conventional re-irradiation (60 Gy in 30 fractions): generally not appropriate here because the cord is abutting the target and cumulative dose would be excessive.
Radiobiologically, the tumor is being re-irradiated after a prior high dose, so the effective BED to tumor is already high; the goal is local control with acceptable cord toxicity. The trade-off is between tumor control probability and the risk of radiation myelopathy, which is devastating. The patient's tracheostomy and dysphagia also raise the risk of airway and swallowing toxicity. A multidisciplinary discussion and explicit informed consent about the risk of myelopathy are essential.