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KMID : 1037420160110010023
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Journal of Korean Skull Base Society
2016 Volume.11 No. 1 p.23 ~ p.26
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MOSFET Measurement of Cochlear Radiation Dose for Rat Model
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Jeong Jun-Hui
Han Su-Jin
Seo Jin-Young
Moon In-Seok
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Abstract
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Objectives : To measure the actual dosages of radiation being delivered to the cochlea is important for hearing preservation in stereotactic radiosurgery for vestibular schwannoma. It is very difficult to quantify because the cochlea is embedded deep within the temporal bone. 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.
Methods : A Norway Brown adult rat was euthanized and placed in a custom restrainer with rigid head fixation; the vertex was located 8cm 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 : 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×Min-0.12, R2 =1) and MOSFET probes placed in the radiated and non-radiated ears(Gy = 5.80×Min+0.01,R2 =0.9999; Gy = 0.08×Min+0.35, R2 =0.06, respectively). MOSFET probes placed on the cochlea promontory in the radiated ear shielded with lead measured from 0.35 to 0.60 Gy of radiation. Standard ion chamber dosimetry probes overestimate cochlear radiation dosages in rats by 10%, when compared to MOSFET probes placed on the cochlear promontory in radiated ears.
Conclusion : MOSFET with small dosimeter probes that can be placed on the cochlear promontory 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. When performing stereotactic radiosurgery for vestibula schwannoma, MOSFET may give information of actual radiation dosages received by the cochlea.
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