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Wireless measurement of ionizing radiation in close proximity or/and within an irradiated solid tumor is extremely valuable for dose verification and quality control in radiation oncology. For such applications, it is preferable to manufacture such sensors using passive components since high levels of ionizing radiation can damage active electronics. In addition, passive implementation can reduce the cost associated with fabrication and assembly. This paper reports on the development of an implantable micro- machined passive LC transponder for in situ radiation measurement. Dose measurement is performed by monitoring the resonance frequency change associated with the decay of surface change of an electret upon exposure to radiation. This is achieved through a micromachned capacitor with a movable plate that is partially filled with a Teflon electret and connected in parallel with an inductor, thus forming a passive LC tank circuit. For an implantable prototype encapsulated in a glass capsule (2.5 mm in diameter, 2.8 cm in length), test results show that a dose of 30 Gy (from a Co60 source) can produce 1.46 MHz frequency shift resulting in a sensitivity of 49 kHz/Gy.