71-year-old male with a history of medically refractory trigeminal neuralgia presenting with 3-4 weeks of a severe intermittent shocking pain involving the right V2 and V3 distribution of his face.

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Image:

Rx: 86 Gy (100% IDL) in a single fraction using framed stereotactic radiosurgery (GammaKnife Based) 

Dose: 70-90 Gy

Sim/Planning:

  • Immobilization:
    • Stereotactic head frame (gold standard) or short-frameless immobilization mask.
  • Imaging:
    • Thin slice (1mm) MRI brain with T1 and T2 sequencing (with spin-echo or gradient echo sequencing).
    • CT cisternography in patients who are not candidates for MRI.

Note: Isocenter is visible on image 109

 

Case contributed by Allegheny Health Network Cancer Institute.

Per Tuleasca JNS 2019.

Targeting:

Target Placement

  • Single 4 mm isocenter placed at trigeminal root entry zone (REZ) [2 to 6 mm anterior to the junction of the trigeminal nerve and pons].
    • 30-50% IDL abutting brainstem
    • REZ = histological transition from peripheral myelination (Schwann cells) to central myelination (oligodendrocytes).
  • Alternatively anterior cisternal target (retrogasserian) has been described albeit less common.
    • Pro: further distance from brainstem (lower dose to brainstem and lower rates of hypesthesia).
    • Con: less radiosensitive region of nerve compared to REZ (potentially less responsive to treatment).

Single vs Two Isocenter Treatment

  • Single isocenter treatment favored to reduce volume of treated nerve and associated treatment related toxicity (i.e. hypesthesia).

Therapies

  • First therapeutic line = pharmacologic
    • Refractory symptoms and/or medication intolerance --> 2nd line
  • Second line therapies include:
    • Surgical:
      • Microvascular decompression
      • Rhizotomy (radiofrequency ablation, glycerol injection, balloon decompression)
    • Radiosurgery:
      • Cranial nerve V is optimally visualized on T2 based imaging (Radiopaedia)
  • Re-treatment
    • In refractory/recurrent cases, re-irradiation is feasible and effective (Herman IJROBP 2004).
    • Placement of isocenter anterior/posterior to prior treatment site can minimize region of dose overlap.
    • Re-iradiation more likely to improve pain if good response to initial treatment and long interval between treatments. 

Pain Responnse and Intensity:

  • Pain Response Rates (with medication): 80-90%
  • Pain Response Rates (without medication): 50-60%
  • Median Time to Response:  30-60 days
  • Pain Recurrence Rate: ~20-25%
  • Hypesthesia rates: ~20-25%
  • Higher doses (i.e. >82 Gy) have been associated with improved pain control but higher rates of hypestheia (Kotecha IJROBP 2016).

Barrow Neurological Institute Pain Intensity Score:

  • I - No pain and not on medication
  • II - Occasional pain but not requiring any pain medication
  • IIIa - No pain, with continued medication
  • IIIb - Some pain, controlled with medication
  • IV - Some pain, not controlled with medication
  • V - Severe pain and no pain relief with medication

Facial Hypesthesia Score:

  • I - No facial numbness
  • II - Mild facial numbness, which is not bothersome
  • III - Somewhat bothersome facial numbness
  • IV - Very bothersome facial numbness
    • In rare cases severe numbness can also be painful, a phenomenon known as "anesthesia dolorosa"