顱內腫瘤立體定位放射手術/放射治療之周邊劑量評估

Evaluation of Peripheral Doses for Intracranial Tumors Treating with Cyberknife Stereotactic Radiosurgery

王美淑(Mei-Shu Wang)

指導教授 ： 張寶樹

高雄醫學大學/健康科學院/醫學影像暨放射科學系碩士班/碩士(2010年)

摘要

目的：本研究的主要目的係針對顱內腫瘤之腦下垂體腫瘤病灶為研究對象，利用熱發光劑量計在不同體積的腫瘤下及不同大小之準直儀條件下，分別量測評估病患身體各部位器官接受的輻射劑量，探討電腦刀立體定位放射手術治療所產生的周邊劑量，進而評估該周邊劑量是否會有誘發續發性癌症(secondary malignancy) 的隨機效應(stochastic effect)發生率以及其他可能產生的延遲生物效應的機率，藉以提供顱外劑量相關的資訊以及影響這些劑量的因子。
材料與方法：實驗係在電腦刀系統下執行，治療計畫運算方法為反算式治療計劃非等中心方式照射( non-isocenter )，治療模組使用skull 6-D tracking mode。三組實驗之處方劑量均為32.50 Gy，以5個分次投予，毎分次之劑量為6.5 Gy。實驗過程係藉由Alderson–Rando phantom人形假體進行臨床實驗，標靶位置為頭顱中央蝶鞍處，A組腫瘤體積0.45㏄，使用7.7 mm準直儀；B組腫瘤體積3.32 ㏄，使用12.5 mm準直儀；C組腫瘤體積10.14 ㏄，使用15 mm準直儀。以TLD-100H熱發光劑量計進行不同器官所接受劑量值之量测。所使用TLD量測之不確定性约為3％。另實驗所量测的器官計有：水晶體、甲狀腺、心臟、乳房與性腺（卵巢、睪丸）等。不同器官所接受到的劑量值繼而參酌ICRP-103 號報告，求得個別組織或器官的致死癌病機率。
結果：電腦刀治療總劑量32.5 Gy，5個分次(fraction)放射治療療程中使用7.5 mm準直儀之個別器官劑量值：水晶體劑量86.72 ±6.6 cGy，甲狀腺29.82 ±0.17 cGy，心臟24.63 ±0.25 cGy，乳房18.41 ±0.31 cGy，卵巢11.82 ±0.20 cGy，睪丸13.24 ±0.13 cGy。使用12.5 mm準直儀之個別實驗器官劑量值：水晶體劑量為79.27 ±1.97 cGy，甲狀腺37.59 ±0.07 cGy，心臟28.73 ±0.53 cGy，乳房23.7 ±0.40 cGy，卵巢18.25 ±0.16 cGy，睪丸17.71 ±0.17 cGy。使用15 mm準直儀之個別實驗器官劑量值：其水晶體劑量為 93.66 ±1.66 cGy，甲狀腺69.68 ±4.15 cGy，心臟 33.28 ±0.30 cGy，乳房 25.82 ±0.36 cGy，卵巢15.79 ±0.16 cGy，睪丸17.66 ±0.29 cGy。器官劑量視其與治療照野位置之距離以及所使用之準直儀而有不同的劑量。
結論：電腦刀系統設備具有獨特機械手臂與多自由度的射束方向，針對腫瘤附近對輻射較敏感之器官或正常組織，運用電腦治療計畫計算範圍內能有效限制且降低其劑量是其儀器特色。然因洩漏散及射輻射之劑量導致距離照野遠處周圍劑量有昇高之疑慮。本實驗結果發現周邊之劑量為很小量，且正常器官組織誘發續發性癌症的危險度不高 (約0.01%∼0.289％)，但值得注意及追蹤觀察的是放射治療後，長期存活之良性腫瘤患者，尤其是年輕族群且為女性或懷孕婦女者，其乳房與腹部劑量是應值得注意的。

關鍵字

立體定位放射手術 ； 放射治療 ； 腦下垂體腺瘤

並列摘要

Stereotactic radiosurgery (SRS) for intracranial tumors with Cyberknife does generate certain peripheral radiation doses over the body. We use TLD to evaluate the radiation doses over the normal organs while treating pituitary tumors with Cyberknife and the dose variances resulted from different tumor volumes and the size of collimators. The stochastic effect of secondary malignancy and the late biologic effect can be further estimated based on the information provided in this study.

Cyberknife SRS is performed on the Alderson-Rando phantom using skull 6-D tracking mode and the non-isocenter, inverse treatment plan derived from CYRIS MultiPlan, version 1.5.2. The targets are located in the pituitary fossa and the sellar region. SRS is performed with 3 different collimators: 7.5 mm for tumor volume 0.45 cc, 12.5 mm for tumor volume 3.32 cc, and 15 mm for tumor volume 10.14 cc. The prescribed dose is 32.5 Gy in 5 fractions, equals to 6.5 Gy per fraction. TLD-100H is placed at the associated location in the phantom representing the normal organ and thus obtaining the absorbed dose. The following organs are evaluated: lens, thyroid gland, heart, female breast and gonads. The risk of fatal cancer developing is further estimated according to ICRP Publication No. 103.

After Cyberknife SRS with 32.5 Gy in 5 fractions, the absorbed doses of the normal organs are stated as follows. Among the group with 7.5 mm collimators: lens 86.72 ±6.6 cGy, thyroid gland 29.82 ±0.17 cGy, heart 24.63 ±0.25 cGy, female breast 18.41 ±0.31 cGy, ovary 11.82 ±0.20 cGy and testis 13.24 ±0.13 cGy. Among the group with 12.5 mm collimators: lens 79.27 ±1.97 cGy, thyroid gland 37.59 ±0.07 cGy, heart 28.73 ±0.53 cGy, female breast 23.7 ±0.40 cGy, ovary 18.25 ±0.16 cGy and testis 17.71 ±0.17 cGy. Among the group with 15 mm collimators: lens 93.66 ±1.66 cGy, thyroid gland 69.68 ±4.15 cGy, heart 33.28 ±0.30 cGy, female breast 25.82 ±0.36 cGy, ovary 15.79 ±0.16 cGy and testis 17.66 ±0.29 cGy.

Cyberknife system is unique in the high freedom of robotic arm movement and beam arrangement, achieving the maximal conformal dose without excess of normal tissue tolerance. Peripheral doses derived from leakage and scattering remain a topic of risk. Data from our study revealed that only trace peripheral dose is induced by Cyberknife SRS, and the risk of secondary malignancy is relatively low (approximately 0.01%~0.289%). Nevertheless, the unexpected doses over the breast, abdomen and pelvis after SRS among patients with benign tumors deserve extra consideration, especially in young females or pregnant women.

並列關鍵字

Stereotactic Radiosurgery ； Radiotherapy ； Pituitary adenoma

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