Use Of Shielding In Gamma-knife Radiosurgery For Posterior Third Ventricle Lesions- Techniques And Pitfalls

Keywords: brain tumor, outcome, third ventricle, technique, gamma knife

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     GK radiosurgery is especially challenging for posterior third ventricular lesions due to their proximity to vital structures like brain stem, deep venous system and thalami.
     This was a prospective study carried out over a 12 month period in the department of Neurosurgery, All India Institute of Medical Sciences. Patients with posterior third ventricular lesions who underwent GK radiotherapy by one author (DA) were included.
     Shielding was used liberally to optimize marginal dose as well decrease brainstem dose. Efforts were also made to decrease the dose received by the deep venous system. Follow up of the patients was done clinically at one month and six months and radiologically using MRI at six months.
     Fourteen patients with posterior third ventricular lesions were treated with GK radiosurgery in the study period. The pathology was vascular in eight and tumor in six. Shielding was used in 12 (85.7%). Use of shielding helped in increasing the marginal dose by a mean of 6.2 Gy and in decreasing the total brainstem dose by a mean of 4.3 Gy. In one patient shielding paradoxically increased the brainstem dose which necessitated its removal. There were no neurological deficits attributable to radiosurgery in the follow-up period. Four patients (all with tumors) showed a mean reduction of tumor volume of 22% at follow up. In the rest, the lesion was stable with no increase in volume.
     This is a retrospective study.
     Shielding in GK radiosurgery is particulary valuable in posterior third ventricular lesions as it helps in optimizing the marginal dose and minimizing brainstem dose.
     This shielding requires experience and skill and may not be suitable in all cases.


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