Mosfets for Cochlear Dosimetry in Animal Irradiation

Objectives: Actual dosages of radiation received by the cochlea are challenging to quantify in humans and animal models since the cochlea is a small organ embedded deep within the temporal bone. It is particularly important to measure the actual dosages of radiation being delivered to the cochlea when studying hearing preservation after stereotactic radiosurgery for vestibular schwannoma. The objective of this study is to compare cochlear irradiation dosages as measured by small-sized metal-oxide semiconductor-field-effect transistor (MOSFET) probes placed directly on the cochlear promontory through a myringotomy incision to larger standard ion chamber dosimeter (gold standard) probes positioned at the level of the cochlea or on the mastoid surface in an adult rat phantom.

Methods: Two MOSFET probes were calibrated in a small animal irradiator against a standard ion chamber dosimeter probe to obtain the calibration coefficient. A Norway Brown adult rat was euthanized and placed in a custom restrainer with rigid head fixation; the vertex was located ~8 cm from the X-ray irradiation source. One ear was exposed to radiation, while the contralateral ear and remainder of the body were shielded by 6 mm of lead. MOSFET probes were then placed on bilateral cochlear promontories (via myringotomies) and the standard dosimeter probe was placed alongside the mastoid at the level of the cochlea. Various dosages of radiation (160 kV, 25 mA; 0 to ~20 Gy) were delivered to the cochlea at ~5.35 Gy/minute, and dosages of cochlear radiation were measured by the MOSFET and standard dosimeter probes.

Results: Radiation sensitivities of MOSFET probes were consistent; the calibration coefficient of MOSFETs to standard ion chamber dosimetry probes ranged from 384 to 406 mV/Gy. There were linear dose-dependent relationships between the total time of delivered radiation and total Gy measured by the standard ion chamber dosimetry (Gy = 6.39 x Min – 0.12; R²=1) and MOSFET probes placed in the radiated and non-radiated ears (Gy = 5.80 x Min + 0.01, R²=0.9999; Gy = 0.08 x Min + 0.35, R²=0.06, respectively). MOSFET probes placed on the cochlear promontory in the radiated ear shielded with lead measured from 0.35 to 0.60 Gy of radiation. Standard ion chamber dosimetry probes over-estimate cochlear radiation dosages in rats by 10%, when compared to MOSFET probes placed on the cochlear promontory in radiated ears.

Conclusions: MOSFETs are small dosimeter probes that can be placed on the cochlear promontory through myringotomies in rats. In designing radiation protocols in vivo, MOSFETs can more accurately measure the amount of radiation received by the cochlea compared to large standard ion chamber dosimeter probes that are positioned at the approximate height of the cochlea or mastoid surface. Delivering accurate dosages of radiation to the cochlea will help researchers better understand how various radiation regimens can affect the molecular mechanisms and hearing outcomes in radiation-induced hearing loss in vivo. Placement of MOSFET probes along the tympanic membrane in humans when performing stereotactic radiosurgery for vestibular schwannoma may give insight into actual radiation dosages received by the cochlea.