What are the recommended dose constraints for a patient with a history of head and neck cancer undergoing reirradiation, considering their previous radiation therapy and potential toxicity to surrounding tissues such as the spinal cord, brainstem, optic nerves, and parotid glands?

Dose Constraints in Head and Neck Reirradiation

Dose constraints for head and neck reirradiation are not clearly established, and the degree of normal tissue recovery from initial radiation is difficult to estimate, requiring accurate reconstruction of previous dose distributions and professional judgment to balance tumor control against life-threatening toxicity. 1

Critical Principle: Accurate Dose Reconstruction is Mandatory

Radiation plans must be based on accurate reconstruction of the previous RT dose distribution, taking into account expected morbidity of additional radiation. 1, 2 Without this reconstruction, reirradiation should not proceed. 2

Decision Algorithm for Reirradiation Feasibility

Step 1: Assess if High-Dose Reirradiation is Possible

• If a new course of high-dose RT can be delivered without exceeding estimated dose constraints on organs at risk, treat with the same intent and approach as a radiation-naïve recurrence. 1
• If adequate target coverage cannot be achieved without exceeding organ constraints, other treatment modalities are preferable. 1
• Low-dose reirradiation with palliative intent is appropriate only if it can be performed with negligible risk of toxicity. 1

Step 2: Consider Advanced Radiation Techniques

• High Linear Energy Transfer (LET) radiation such as carbon ions may be more effective against radio-resistant clones selected by the first treatment. 1
• Proton beam reirradiation can provide effective tumor control with acceptable toxicity profiles due to decreased dose to surrounding previously irradiated tissue. 3

Organ-Specific Cumulative Dose Constraints

Carotid Arteries: HIGHEST PRIORITY

Particular caution is warranted in reirradiating the carotid artery, as severe, life-threatening complications such as carotid blowout syndrome have been reported. 1, 2

• Maximum cumulative dose constraint: 119-120 Gy 4
• This threshold showed AUC = 0.92 for predicting carotid blowout with sensitivity 1.00/specificity 0.89. 4
• Carotid blowout occurred in 4.1% of patients in pooled analyses. 5

Spinal Cord

• Maximum dose should be limited to 50 Gy with conventional fractionation (1.8-2 Gy per fraction) for initial treatment. 2
• Preliminary data are available regarding tolerance to spinal cord reirradiation to help guide decision-making. 1

Mandible and Bone

• Cumulative near-maximum dose constraint: 119 Gy 4
• This threshold showed AUC = 0.74 for predicting osteoradionecrosis with sensitivity 1.00/specificity 0.52. 4

Brain/Temporal Lobes

• Preliminary data are available regarding tolerance to brain reirradiation. 1
• Cumulative dose limit suggestions exist but require careful consideration based on previous exposure. 5

Aorta

• Preliminary data are available regarding tolerance to aortic reirradiation. 1

Target Dose Recommendations for Reirradiation

When Previous RT Was NOT to the Same Area

• Deliver at least 74 GyE using conventional fractionation (1.8-2 GyE) for photon and proton therapy. 1
• For hyperfractionation, high-risk sites require up to 81.6 Gy (1.2 Gy/fraction). 1

When Reirradiating Previously Treated Areas

• No optimal dose and fractionation scheme is established; radiotherapist must exercise professional judgment. 1
• Median reirradiation doses in clinical series ranged from 30 Gy in 3 fractions (stereotactic) to 72 Gy in conventional fractionation. 5
• Median prescribed dose in proton beam series was 60.6 Gy (RBE). 3

Clinical Outcomes and Toxicity Data

Tumor Control

• Five-year locoregional control: 46% for postoperative reirradiation versus 20% for definitive reirradiation. 6
• Twelve-month cumulative incidence of locoregional failure: 25.1% with proton beam reirradiation. 3

Toxicity Rates

• Pooled acute toxicity ≥Grade 3: 32% 5
• Pooled late toxicity ≥Grade 3: 29.3% 5
• Actuarial Grade ≥4 late complications: 28% at 5 years. 6
• Most common Grade 3-4 effects: radionecrosis, dysphagia requiring feeding tube, trismus. 5

Critical Pitfalls to Avoid

Do NOT Assume Complete Tissue Recovery

The degree of recovery from initial radiation is difficult to estimate and varies by organ. 1, 2 Do not use standard organ-at-risk constraints without accounting for previous radiation exposure. 2

Do NOT Proceed Without Dose Reconstruction

If accurate reconstruction of previous dose distribution is impossible, reirradiation should not proceed. 2

Consider Timing Between Treatments

• Median interval time between first course and reirradiation in clinical series: 28 months (range 6-90 months). 5
• Median time between last RT and proton beam reirradiation: 34.4 months. 3
• Time interval between radiation treatments is a key factor for limiting toxicity. 5

Metal Implants Complicate Planning

Metal implants create CT/MRI artifacts that interfere with precise delineation of targets and organs at risk, affecting range calculation for particle therapy. 1 This may be a key factor in deciding not to deliver curative RT or in choosing photons over particles. 1