A Dosimetric Comparison Between Gamma Knife And Cyberknife Treatment Plans For Trigeminal Neuralgia

Keywords: gamma knife, trigeminal neuralgia, cyberknife, dose planning, technique

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     Gamma Knife (GK) radiosurgery is the standard non-invasive treatment of trigeminal neuralgia. The CyberKnife (CK) system (Accuray, Inc., Sunnyvale, CA) has also been used to deliver non-isocentric radiosurgical treatments. However, non-isocentric CK treatments may result in higher incidence of toxicity when compared to GK, possibly related to higher integral dose.
     This study investigated the feasibility of using isocentric CK to mimic the dose distribution provided by GK.
     Five patients treated with the GK Perfexion© were selected for this study. Since a CT-based imaging study is required for CK planning, we selected patients with pacemakers who had a CT cisternography procedure to allow identification of the trigeminal nerve.
     The GK plans consisted of either a single isocenter (4 mm diameter), or two coincident isocenters with partial blocking. A maximum dose of 80 Gy was prescribed for the treatment. CK treatments were planned for the same prescription dose, imaging studies and structures using an isocentric technique with a single 5 mm isocenter and conformal weight optimization.
     With CK, the total number of beams generated by the inverse planning algorithm was less than 100 (while 192 beams are provided by the Perfexion unit). Compared to GK, CK plans resulted in similar dose to the nerve root (0.01 cc=V70Gy=0.02 cc), similar sparing of the critical structures (max dose to brainstem < 16 Gy; eye < 2 Gy; chiasm < 3 Gy), and slightly higher beam-on time. However, a significant difference was observed in the average volumes receiving a low dose: V1Gy was 244 cc vs. 4 cc, V8Gy was 2.1 cc vs. 1.2 cc, and V20Gy was 0.5 cc vs. 0.3 cc, for CK and GK, respectively.
     This is a retrospective study.
     For isocentric CK treatment of trigeminal neuralgia, the larger volume of tissue exposed to a low dose is likely a consequence of the lower beam number, although the clinical significance of this effect is unknown.
     Treatment plans that maximize the dose to the trigeminal nerve and reduce radiation to surrounding structures should improve the overall therapeutic ratio. Comparisons of non-isocentric/isocentric CK plans and GK plans will be presented.


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