What are the indications and restrictions for radiation therapy in a patient with a subtotal resection of a meningioma, particularly with regards to proximity to the brainstem and optic structures?

Radiation Therapy for Subtotally Resected Meningiomas: Indications and Restrictions

Radiation therapy should be administered after subtotal resection of WHO grade 2 or 3 meningiomas, while for WHO grade 1 meningiomas, radiation is only indicated if the residual tumor is incompletely resected and symptomatic. 1

Indications for Radiation Therapy After Subtotal Resection

Based on WHO Grade

• WHO Grade 1 (Benign) Meningiomas:

  • Consider radiation therapy if incomplete resection 1
  • Radiation significantly improves progression-free survival from 48% to 88% at 8 years after initial incomplete surgery 2
  • For asymptomatic patients with small residual tumor, observation may be appropriate

• WHO Grade 2 (Atypical) Meningiomas:

  • Radiation therapy strongly recommended after subtotal resection
  • Reduces recurrence rates from 91% to 20% after subtotal resection 3
  • Conventionally fractionated RT to at least 59.4 Gy is considered standard of care 4

• WHO Grade 3 (Anaplastic) Meningiomas:

  • Postoperative radiotherapy with doses of 45-54 Gy is recommended regardless of extent of resection 1

Based on Tumor Characteristics

• Tumor Size and Location:

  • Large tumors (≥30mm) with incomplete resection 1
  • Tumors with proximity to critical structures that prevented complete resection
  • Symptomatic residual disease

• Pathological Features:

  • Presence of necrosis (spontaneous or embolization-related) 5
  • Brain invasion 5
  • High mitotic index 3
  • Sheeting pattern on histology 3
  • Molecular markers: 1p deletion with concurrent monosomy of 22q and/or NF2 oncogenic variants 1

Restrictions for Radiation Near Brainstem and Optic Structures

Dose Constraints

• Optic Structures:

  • Radiation dose to optic nerves and chiasm should be carefully limited
  • Stereotactic radiosurgery (SRS) is unsuitable if it would result in excess dose to optic nerves and/or optic chiasm 1

• Brainstem:

  • Caution with radiation near brainstem due to risk of edema
  • Meningioma with mass effect on the brainstem requires careful multidisciplinary discussion due to risk of edema following radiation therapy 1

Treatment Approach for Tumors Near Critical Structures

1. Stereotactic Options:

   • Consider hypofractionated stereotactic RT (HSRT) for tumors near critical structures 1
   • SRS may be appropriate for smaller residual tumors (<2-3 cm) not adjacent to critical structures 4

2. Conventional Fractionation:

   • For tumors adjacent to brainstem or optic structures, conventional fractionation (45-54 Gy) is preferred over SRS 1, 4
   • Allows for better protection of adjacent critical structures

3. Surgical Considerations:

   • Debulking surgery may be appropriate to separate critical structures from residual tumor, allowing for safer radiation delivery 1
   • This approach is particularly valuable when proximity to critical structures precludes adequate radiation coverage

4. Alternative Approaches:

   • For tumors with somatostatin receptor expression, consider SSTR-directed PET imaging to support delineation of tumor tissue for radiation planning 1
   • In cases where radiation poses high risk to critical structures, consider systemic therapy options or observation with close monitoring

Treatment Algorithm for Radiation After Subtotal Resection

1. Assess WHO Grade and Molecular Profile:

   • Grade 1: Consider observation if asymptomatic; radiation if symptomatic
   • Grade 2-3: Recommend radiation therapy
   • Check for molecular markers (1p deletion, 22q monosomy) that may upgrade risk classification

2. Evaluate Tumor Location and Proximity to Critical Structures:

   • Far from critical structures: Consider conventional RT (45-60 Gy) or SRS based on size
   • Near optic pathway/brainstem: Use conventional fractionation or HSRT

3. Consider Tumor Size:

   • <2-3 cm: SRS may be appropriate if not adjacent to critical structures

   • 3-4 cm: Conventional fractionation recommended

4. Evaluate Patient Factors:

   • Age >55 years with high mitotic index: Higher risk of recurrence, radiation strongly recommended 3
   • Poor performance status: Consider hypofractionated approaches

Follow-up Recommendations

• Perform post-treatment MRI to establish new baseline
• Continue long-term follow-up with enhanced MRI due to risk of late recurrence 1
• Within first 3 months after radiation, be aware that pseudoprogression may occur and can be indistinguishable from true progression 1

Pitfalls and Caveats

• Radiation near brainstem can cause edema; consider prophylactic steroids
• Optic structures typically have lower radiation tolerance than brainstem
• Surgical complications are dramatically greater when operating near prior high-dose radiation areas 1
• Consider the cumulative dose tolerance when re-irradiating previously treated areas
• The risk of radiation necrosis increases with higher doses and larger treatment volumes

By following these guidelines, radiation therapy can be effectively and safely administered after subtotal resection of meningiomas, even when located near critical structures like the brainstem and optic pathways.