碘-131治療甲狀腺癌症病患執行SPECT/Cone Beam CT 與 SPECT/spiral CT之殘存甲狀腺之劑量評估

背景與目的: 近年來隨著醫療科技的發展，核子醫學科的造影儀器由傳統的加瑪攝影機(γ-camera)發展為結合電腦斷層掃描儀(computed tomography scanner , CT )的單光子發射電腦斷層攝影/電腦斷層攝影掃描儀(single photo emission computed tomography scanner/Computed tomography scanner , SPECT/CT)。在影像的呈現方面會有γ-camera提供的功能性影像結合CT提供的解剖影像，提升了異常區域的靈敏度與特異度。但病患在執行SPECT/CT掃描時除了接受原本進入體內之放射性核種之體內輻射劑量之外，亦會接受CT之體外照射的輻射劑量。因此本研究評估碘-131治療甲狀腺癌病患接受不同型態的SPECT/CT檢查同時CT與放射藥物碘-131對甲狀腺殘存組織所造成的有效劑量，並估算其二次癌症的風險度。材料與方法: 本研究使用熱發光劑量計(thermoluminescence ,TLD)型號為TLD-100H，進行劑量量測實驗。實驗前先使用Elekta Axesse醫用直線加速器校正TLD，得到劑量校正曲線後開始進行實驗。將TLD置於擬人假體甲狀腺表面位置，分別使用Bright view XCT Imaging system (Philips Healthcare, Cleveland, OH) 之SPECT/cone beam CT與Discovery NM/CT 670 system(GE Health, USA) SPECT/spiral CT 仿照實際檢查情況求得單一次曝露於CT 下的吸收劑量，後續體內劑量評估則選用40位碘-131廓清治療後的病患分別進行SPECT/cone beam CT檢查與SPECT/spiral CT同時，將TLD置放於病患甲狀腺體表處量測約30分鐘後續將CT劑量與甲狀腺體內劑量合併評估有效劑量後計算二次癌症的風險度。結果: 碘-131治療後病患執行不同型態的SPECT/CT掃描來自CT的體外暴露輻射量加上來自碘-131所造成的甲狀腺體內劑量分別為接受Bright view XCT SPECT/CBCT scaner 檢查的病患其甲狀腺殘存組織的有效劑量為0.56±0.08 mSv，二次癌症危險度於全人口為1.8ず10-6，於工作人口為5.0ず10-7。接受Discovery NM/CT 670 SPECT/spiral CT scanner檢查病患其甲狀腺殘存組織的有效劑量為0.33±0.08 mSv，二次癌症危險度於全人口為1.1ず10-6，於工作人口為3.0ず10-7。結論: 以上兩種型態的SPECT/CT檢查中，CT和碘-131給予病患的有效劑量低於我國原子能委員會對一般民眾一年僅能接受1 mSv的劑量規範，然而醫療曝露目的是為診療病患之病情而使用，在利大於弊的前提之下是可以合理使用這些檢查工具的。

關鍵字

SPECT/CT ；碘-131 ；甲狀腺癌；有效劑量；體內輻射劑量；二次癌症風險度

並列摘要

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 ず 10-6 in the whole population and 5.0ず 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ず 10-6 in the whole population and 3.0 ず 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.

並列關鍵字

SPECT/CT ； I-131 ； Thyroid cancer ； Effective dose ； Internal dose ； Secondary cancer risk

參考文獻

1. Hay, I. D. (2006). Selective use of radioactive iodine in the postoperative management of patients with papillary and follicular thyroid carcinoma. J Surg Oncol, 94(8)