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Dose Calculation for Tissue Inhomogeneity in Lung Cancer Based on 6 MV Photon Beams

肺組織不均度於6 MV光子的劑量校正

摘要


目的:藉由二度及三度空間放射治療計畫中組織不均勻度的考量與否,來評估肺部放射治療劑量的精確度。 材料與方法:本實驗以治療專用人體假體為人體肺組織參考,全程使用固定模具包覆假體,使其維持一致姿勢再現性,將熱發光劑量計置於腫瘤所在位置作劑量測讀並利用Varian C-Series 600C 6 MV加速器照射,所照射的劑量分別來自於二度、組織空氣比法修正後、及三度空間放射治療計畫,腫瘤的給予劑量為180 cGy。 結果:實驗結果顯示,經三度空間放射治療計畫及組織空氣比法修正後所測出的劑量較接近腫瘤的給予劑量(173 cGy與172 cGy於5×5 cm2照野,175 cGy與174 cGy於10×10 cm2照野),而二度空間放射治療計畫由於未考量組織不均勻度,因此所測出的劑量很明顯的超出腫瘤的給予劑量)212 cGy與216 cGy分別於6×6及10×10 cm2的照野)。 結論:由於肺部組織密度的多元化,不同的計算考量方式將產生不同的照射劑量,雖然一般情況下利用組織空氣比法修正組織不均勻度有一定效果,但此方法並未考量不均勻度組織的形狀及側散射線,而三度空間放射治療計畫較二度空間放射治療計畫更能考量肺部組織密度的不均勻度及整體結構,提供較精確的劑量並減少治療中的負作用。

並列摘要


Purpose : To evaluate the accuracy of dose calculation in lung cancer treatment planning using Two-dimensional radiotherapy (2D-RT) without inhomogeneity correction, with inhomogeneity correction by use of Power Law Tissue-Air Ratio Method (Power Law Method), and Three-dimensional conformal radiotherapy (3D-CRT) with inhomogeneity correction. The latter method takes into account in convolution algorithm. Materials and Methods : Treatment was planned based on Alderson-Rando phantom held in an immobilization device. Doses were calculated on FOCUS release 2.0 planning system both for 2D-RT and 3D-CRT, and irradiation were performed on Varian C-Series 600C linear accelerator, 6MV x-ray. Thermoluminescent dosimeters (TLDs) were placed inside the target to compare the dose based on each method. The prescribed dose was180 cGy in isocenter. Results : The delivered doses by using 3D-CRT planning and 2D with Power Law Method correction were closer to the prescribed dose (173 cGy, 172 cGy in a 5 x 5 cm2 field and 175 cGy, 174 cGy in a 10 x 10 cm2 field). The delivered dose by using 2D-RT planning without heterogeneity correction was significantly higher (212 cGy and 216 cGy). Conclusion : Dose variation between measurement and calculation is due to tissue inhomogenity, especially in the lung. Power Law Method accounts with tissue inhomogeneity but not with the 3D shape of inhomogeneity tissue. 3-D treatment planning with inhomogeneity correction based on CT image, delivers more accurate dose. Use of 3-D planning in lung cancer should reduce the risk of radiation side effects while optimizing delivery to the tumor.

並列關鍵字

Inhomogeneity 2D-RT 3D-CRT Power Law Method Prescribe dose TLDs

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