What are the guidelines for brainstem contouring in radiation therapy, specifically intensity-modulated radiotherapy (IMRT) or proton therapy?

Brainstem Contouring in Radiation Therapy

The brainstem should be delineated as a critical organ at risk in all IMRT and proton therapy plans, with strict dose constraints applied to prevent radiation necrosis, though the optimal dose-volume parameters remain under investigation and may be more flexible than traditional photon-based constraints. 1

Anatomical Delineation Requirements

Structures requiring mandatory contouring include the brainstem, optic nerves, optic chiasm, retinae, lenses, pituitary, cochleae, and hippocampi when planning radiation therapy for intracranial tumors. 1

• The brainstem should be contoured as a distinct structure separate from other critical neural tissues to enable precise dose-volume analysis 1
• For pediatric cases and posterior fossa tumors, the dorsal vagal complex (DVC) and area postrema may warrant separate delineation, as these structures are associated with nausea and vomiting when irradiated 2
• Contouring should be performed on CT-based planning scans taken in treatment position, ideally with MRI fusion for improved anatomical definition 3

Dose Constraints and Toxicity Thresholds

For IMRT (Photon-Based Therapy)

The definitive criteria based on conventional radiotherapy cannot accurately predict brainstem necrosis after IMRT, necessitating more flexible constraints with strict monitoring. 4

• In a large cohort of 1,544 nasopharyngeal carcinoma patients treated with IMRT, brainstem necrosis occurred in only 0.13% despite 24.9% having Dmax ≥64 Gy, suggesting traditional constraints may be overly conservative 4
• IMRT significantly reduces brainstem dose compared to 3D-CRT, achieving mean dose reductions of 19.8% and maximum dose reductions of 10.7% while maintaining target coverage 5
• Mean brainstem doses can be reduced to 26.5 Gy(RBE) with IMRT compared to 29.5 Gy with conventional techniques 6

For Proton Therapy

Brainstem toxicity including radiation necrosis can occur with proton therapy, and recent guidelines recommend limiting brainstem dose with specific constraints: Dmax <55.8 GyE and V55 <6.0% to achieve brainstem necrosis incidence <2%. 1

• Proton therapy produces significantly lower brainstem doses than IMRT, with mean doses of 166.2 cGy versus 1,553.5 cGy for photons and maximum doses of 1,387.6 cGy versus 3,412.1 cGy 7
• The risk of brainstem necrosis after proton therapy is approximately equivalent to photon therapy when using the same dose constraints, but requires precise calculation-based dosing to avoid uneven dose distribution 1
• More imaging changes in the brainstem have been reported with protons compared to photons, though clinical significance remains under investigation 1

Technical Implementation for IMRT

Modern highly conformal radiation techniques including IMRT should be employed for newly diagnosed tumors to provide superior target coverage and sparing of non-malignant brain tissue, with brainstem as a priority organ at risk. 1

• IMRT achieves improved target conformity (conformity index 1.38 vs 1.52 for 3D-CRT) without increasing integral dose to normal brain tissue 5
• Dose-volume histograms should be generated for the brainstem with attention to mean dose, maximum dose, and volumetric parameters (V50, V55, V60) 6, 4
• IMRT does not increase the 0.5-5 Gy low-dose volume to normal brain and actually reduces total integral dose by 7-10% 5

Critical Pitfalls to Avoid

• Do not apply overly rigid conventional radiotherapy dose constraints to IMRT planning, as these may unnecessarily compromise target coverage without improving safety 4
• Do not assume proton therapy eliminates brainstem toxicity risk—careful dose calculation and monitoring remain essential, particularly for posterior fossa tumors 1
• Do not neglect contouring the dorsal vagal complex in head and neck IMRT cases, as brainstem mean dose correlates with nausea and vomiting, particularly in patients receiving concurrent chemotherapy 2
• Avoid uneven dose distribution in proton therapy, as precise calculation-based brainstem dosing is critical to prevent necrosis 1

Quality Assurance Considerations

• Close interdisciplinary cooperation between radiation oncologists, neuroradiologists, and medical physicists is critical for accurate brainstem delineation 3
• Accurate patient positioning with reproducible immobilization and digital imaging during treatment is required for all highly conformal approaches 1
• Standards for target definition, dose specification, and normal tissue constraints continue to evolve and require institutional experience with complex treatment modalities 3