Cochlear radiation dose does not predict hearing loss after gamma knife for glomus jugulare tumors

Bradley Lega1, James Stephen2, Michael Ruckinstein3, Douglas Bigelow3, Jay Dorsey4, Michele Alonso-Basanta4, John YK Lee5

1Philadelphia, United States 2University of Pennsylvania, Department of Neurosurgery 3University of Pennsylvania, Department of Otorhinolaryngology 4University of Pennsylvania, Department of Radiation Oncology 5University of Pennsylvania

Keywords: glomus tumor, radiosurgery, gamma knife, outcome, cochlea

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     Gamma-knife radiosurgery (GK) has become a mainstay of therapy for glomus jugulare tumors (GJT). Radiosurgery provides excellent tumor control rates (~90%) with a favorable side effect profile. The low incidence of new hearing loss after SRS is surprising given the proximity of GJT to the cochlea (mean cochlear dose often >5.3 Gy). 
     We examined our series of GK for GJT, quantifying radiation dose to auditory structures.
     For patients who underwent GK for GJT at our institution, we compiled data for tumor control rates, symptomatic relief, and incidence of new cranial neuropathies. 9 patients underwent GK (mean dose of 15.8 Gy to the tumor margin); 2 patients had a previous microsurgical resection.
      We measured tumor volumes, marginal dose, cochlear volume receiving 5.3 Gy of radiation, mean cochlear dose, and mean dose to the internal acoustic canal (IAC).
     With a median follow up of 26 months, one patient required required reoperation. In this case, radiation dose to a portion of the tumor was inadequate (<6 Gy). 6/9 patients presented with pulsatile tinnitus, 2 with hearing loss, and 1 with facial weakness. None of these preoperative symptoms improved after GKS. 8/9 patients had clinically serviceable hearing pre-GK; in one patient hearing worsened. 3 of 4 patients who received a mean cochlear dose >8 Gy suffered no hearing loss. Mean radiation received by the cochlear nerve was 4.7 Gy in patients with intact post-GK hearing and 3.5 Gy in the patient who suffered hearing deterioration.
     This was a retrospective study.
     GKS provides control for glomus tumors although follow-up in our series is limited (as it is for others in the literature). The low rate of hearing loss post-GKS we report is in-line with published data. Neither dose to the auditory nerve nor the cochlea predict hearing loss after GKS for glomus tumors. This suggests that thresholds for cochlear dose derived from data for acoustic neuromas should not be extrapolated to planning for other lesions. 
     The existing data suggests that cochlear dose should not limit the dose of radiation given to the superior portion of a glomus tumor.


Project Roles:

B. Lega (), J. Stephen (), M. Ruckinstein (), D. Bigelow (), J. Dorsey (), M. Alonso-Basanta (), J. Lee ()