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題 名 | Assessment of Dose Perturbation Due to Air Cavity Effects Under Megavoltage Photon Beams=高能X光射線下因空氣腔效應而產生劑量擾動之評估 |
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作 者 | 梁基安; 林芳仁; 蕭安成; 涂振邦; 吳學鼎; 楊世能; 陳尚文; | 書刊名 | 放射治療與腫瘤學 |
卷 期 | 6:1 1999.03[民88.03] |
頁 次 | 頁49-54 |
分類號 | 416.36 |
關鍵詞 | 空氣腔; 劑量擾動; 樹脂; 界面效應; 電子平衡; Air cavity; Dose perturbation; Resin; Interface effect; Electron equilibrium; |
語 文 | 英文(English) |
中文摘要 | 目的:針對在使用百萬伏特光子射線時,因空氣腔與組織間異質性效應而產生之 劑量擾動,加以分析其影響程度及相關因素,並探討樹脂材料學利用於填充器或彌補物之主 成份,在劑量學上之特性。材料與方法:我們定義異質性劑量校正因子 HDCF 等於 Di/Dh, 其中 Di 為異質性材質存在時測出之劑量, 而 Dh 為材質均勻時測出之劑量。利用 6MV 及 10MV 光子射線照射各種不同照野大小,以空氣或樹脂填充之空腔,在腔壁之兩側, 測量其 變化之程度。結果:當照野面積等於或小於空氣腔面積時,在射線遠端界面出現明顯劑量不 足現象(可能多於 30 %), 尤其在使用較高能量射線情況下。 當照野面積增大時,HDCF 也隨之增加,甚至會大於 1,也就是可能出現劑量過高現象,但之後會保持一個固定數值。 在腔後壁測量時,則發現存在有再增建區。至於近端界面產生之劑量不足現象,則隨照野面 積增大而仍然存在,但其程度較不顯著(小於 10 %)。利用樹脂來作為空腔填補物會明顯 降低劑量擾動情形,和材質均勻時比較起來,遠近端差異皆在 2.5 %以內。 結論:當治療 區域內有空氣存在,且腔前介質可達到電子平衡時,在界面及其後組織所測得劑量之影響因 素,有照野及腔徑大小相關情況、腔形狀及射線能量強度等。散射電子減少,及未被衰減之 光子射線會增加,這兩者將有平衡作用。在這些情況下選擇適當能量之射線,經由完善計劃 配合足夠大小之照野,將是必需的。由於樹脂其方便性及在劑量學上之理想特性,我們建議 可以使用於填充空腔內,大幅改善劑量分佈之均勻度。 |
英文摘要 | Purpose:To evaluate the magnitude and corresponding factors of dose perturbation due to air to tissue inhomogeneity for megavoltage photon beams. The dosimetric property of resin, a major component of obturator or prosthesis, is investigated. Materials and Methods:The heterogeneity dose correction factor (HDCF) is introduced and defined as Di/Dh where Di is the dose at heterogeneous condition and Dh is dose of the same point in a homogenous phantom. Dose perturbation for varying sizes of cubic air-filled and resin-filled cavity were examined for 6 and 10 MV photon beams along the beam direction for both proximal side ( entering the cavity ) and distal side ( leaving the cavity ). Results : Significant underdosage ( more than 30% ) can occur on the distal interface when the field size is equal to or less than the air cavity size, especially for higher energy photon beams. The HDCF increases with increase in filed sizes and can be greater than 1 unity, which indicates the possibility of overdosage, but it will remain constant afterwards. A buildup region is observed at the downstream measurements. Underdose phenomenon at the proximal air-phantom interface remains over a wide range of field sizes, but it is less significant ( <10% ). The use of resin as the cavity filling material will dramatically reduce the dose perturbation to within 2.5% on both interfaces, compared with a homogeneous condition. Conclusion: With an air cavity embedded in the treatment volume and under the condition of electron equilibrium of pre-cavity medium, the dose measured at interface and subsequent tissue is influenced by the relationship between the field size and cavity size, and also between the cavity shape and photon beam energy. Loss of scattered electron and increase of unattenuated photon beam balance it. Well designed planning with sufficient field size from proper energy beam is recommended. Resin can be used to fill the cavity for improving the dose distribution. |
本系統中英文摘要資訊取自各篇刊載內容。