| Summary: | 碩士 === 高雄醫學大學 === 醫學影像暨放射科學系碩士在職專班 === 105 === Purpose:
Hybrid imaging system becomes widely used in nuclear medicine in recent years. This imaging system offers better functional and anatomical information and thus increases the sensitivity and specifity of examinations. However, patients may have higher radiation exposure of SPECT/CT than those with traditional SPECT , this study evaluates effective dose in residul thyroid gland in patients of thyroid cancer status post I-131 treatment between different types of SPECT/CT and assess secondary cancer risks.
Material and methods:
In this study, we used TLD-100H to measure the absorbed dose of thyroid. Before the study, we calibrate the TLDs with Elekta Axesse linac and acquired the calibration curve.
Single absorbed dose under CT exposure were acquired by placing TLDs on thyroid surface of the RT Humanoid phantom to simulate the protocol of examination with SPECT/CBCT and SPECT/spiral CT respectively.
To assess internal dose, 40 patients of differentiated thyroid carcinoma receiving I-131 treatment were selected. Half received the examination by the SPECT/CBCT of Bright view XCT Imaging system (Philips Healthcare, Cleveland, OH) while the other by the SPECT/spiral CT of Discovery NM/CT 670 system(GE Health, USA). The TLDs were placed in the surface of patient''s thyroid gland and measured for 30 minutes. The results (internal dose and effective dose of thyroid) were combined to assess the effective dose and calculate the risk of secondary cancer.
Result:
The patient who undergoing Bright view XCT SPECT / CBCT scanner, the effective dose of thyroid remnant tissue was 0.56 ± 0.08 mSv, the secondary cancer risk was 1.8 10-6 in the whole population and 5.0 10-7 in the working population. The patient who undergoing Discovery NM/CT 670 SPECT/spiral CT scanner, the effective dose of thyroid remnant tissue was 0.33 ± 0.08 mSv, and the risk of secondary cancer was 1.1 10-6 in the whole population and 3.0 10-7 in the working population.
Conclusion:
In these two types of SPECT / CT examinations, the effective dose of CT and I-131 given to patients is lower than that of the Atomic Energy Commission in Taiwan, who can only accepted 1 mSv dose limitation for the general population during one year. However, medical exposure is reasonable to diagnosis and treatment patients’ disease and the advantages outweigh the disadvantages, that we can use these tools reasonable.
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