These two studies were recently posted elsewhere. (Thanks Philip):
The conclusions are boldfaced and even though one originates from the Netherlands and one originates from France, the outcome is replicated--lower dose radiation near the cochlea.
Gamma knife radiosurgery for vestibular schwannomas:
Results of hearing preservation in relation to the cochlear radiation dose
Ferdinand C. A. Timmer, MD 1 *, Patrick E. J. Hanssens, MD, PhD 2, Anniek E. P. van Haren, MD 1, Jef J. S. Mulder, MD, PhD 1, Cor W. R. J. Cremers, MD, PhD 1, Andy J. Beynon, PhD 1, Jacobus J. van Overbeeke, MD, PhD 2, Kees Graamans, MD, PhD 1
*Correspondence to Ferdinand C. A. Timmer, Radboud University Nijmegen Medical Centre, Donders Institute for Brain, Cognition and Behaviour, Department of Otorhinolaryngology & Head and Neck Surgery, PO BOX 9101, 6500 HB Nijmegen, The Netherlands
Abstract
Objectives/Hypothesis:
This study was designed to evaluate hearing preservation after gamma knife radiosurgery (GKRS) and to determine the relation between hearing preservation and cochlear radiation dose in patients with a sporadic vestibular schwannoma (VS).
Methods:
Prospective study involving patients suffering from VS who received GKRS from June 2003 until November 2007. Pure tone and speech audiometry were conducted before and after GKRS. The thresholds at pure tone audiometry were taken as a measure of hearing. Pure tone average (PTA) was defined as the mean threshold at 0.5 kHz, 1.0 kHz, 2.0 kHz, and 4.0 kHz. Hearing was classified according to the 2003 consensus meeting in Tokyo. Stereotactic surgery was performed using a Leksell 4C Gamma Knife (Elekta, Stockholm, Sweden).
Results:
A total of 69 patients were included in the study. Mean tumor size was 17 mm. Mean marginal dose at the tumor was 11.0 Gy (range, 9.3 Gy-12.3 Gy), mean maximal dose was 19.7 Gy (range, 16 Gy-25.5 Gy). Mean maximal dose at the cochlea was 10.27 Gy (range, 3.1 Gy-16.1 Gy), and mean minimal dose at the cochlea was 2.6 Gy (range, 0.9 Gy-7.4 Gy). Mean PTA before GKRS was 43 dB (standard deviation 20 dB), mean PTA after GKRS was 63 dB (SD 30 dB). Mean interval between pre-GKRS audiometry and GKRS was 8.0 months. Between GKRS and post-GKRS audiometry, mean interval was 14.2 months. Hearing was considered to be preserved (max +1 class, Tokyo classification) in 52 (75%) of 69 patients. However, only 32 patients had class A, B, or C (serviceable hearing) before GKRS. Within this group, only 13 patients (41%) had a hearing class A, B, or C after GKRS. A significant relation was found between the maximal cochlear dose and the difference in PTA before and after GKRS.
Conclusions:
Hearing preservation is correlated to the maximal radiation dose at the cochlea. The purpose of developing GKRS techniques was to avoid collateral damage in healthy tissues. This study emphasizes the need for exact radiation planning to reduce the cochlear radiation dose if the hearing is to be preserved. Laryngoscope, 2009
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Hearing preservation after gamma knife radiosurgery for vestibular schwannomas presenting with high-level hearing.
Tamura M, Carron R, Yomo S, Arkha Y, Muraciolle X, Porcheron D, Thomassin JM, Roche PH, Régis J.
Department of Stereotactic and Functional Neurosurgery, Hôpital de la Timone, Marseille, France.
OBJECTIVE: The aim of this study was to evaluate long-term hearing preservation after gamma knife radiosurgery (GKS) for vestibular schwannomas in patients with initially normal or subnormal hearing (Gardner-Robertson Class 1) and to determine the predictive factors for functional hearing preservation.
METHODS: Since July 1992, more than 2053 vestibular schwannomas have been treated by GKS and followed at the Timone University Hospital, Marseille. A minimum of 3 years of follow-up (range, 3-11 years; median, 48 months) is available for 74 patients (without neurofibromatosis Type 2 or previous surgery) with Gardner-Robertson Class 1 hearing.
RESULTS: The average age of the patients was 47.5 years (range, 17-76 years). The number of tumors in Koos Stage I was 8, the average number in Stage II was 21, the average number in Stage III was 43, and the average number in Stage IV was 2. The median number of isocenters was 8 (range, 2-45), and the median marginal dose was 12 Gy (range, 9-13 Gy). At the time of the last follow-up evaluation, 78.4% of the patients had preserved functional hearing. Tumor control was achieved in 93% of the cases. The probability of preserving functional hearing was higher in patients who had an initial symptom other than hearing decrease (91.1%), in patients younger than 50 years (83.7%), and in those treated with a dose to the cochlea of less than 4 Gy (90.9%).
CONCLUSION: This study shows that the probability of preserving functional hearing in the long term after GKS for patients presenting with unilateral vestibular schwannomas is very high. The positive predictive factors appear to be young age, an initial symptom other than hearing decrease, and a low dose to the cochlea.
PMID: 19057423 [PubMed - indexed for MEDLINE]
