依循TG-21議定書比較各型固態假體執行絕對劑量校驗之差異

Comparison of the Reference Dosimetry according TG-21 Protocol among Solid Phantoms

田蕙茹(Hui-Ju Tien) ； 蕭安成(An-Cheng Shiau) ； 葉芯貝(Hsin-Pei Yeh) ； 林其達(Chi-Ta Lin) ； 陳合興(Ho-Hsing Chen) ； 李秀文(Hsiu-Wen Lee) ； 洪照雄(Chao-Hsiung Hung) ； 許仲賢(Chung-Hsien Hsu) ； 熊佩韋(Pei-Wei Shueng)

《放射治療與腫瘤學》 20卷3期 (2013/09) Pp. 199-209

https://doi.org/10.6316/TRO/201320(3)199

摘要

目的：1983年發表的AAPM TG-21高能射束劑量校驗議定書允許使用固態假體量測絕對劑量，但僅提供Acrylic（Acry）與clear polystyrene（Cpoly）兩種固態假體之參數資料。現今臨床使用多種固態水假體校驗絕對劑量，然而卻仍依循TG-21議定書之內容，此操作方式應用在高能光子與電子射束時可能會引起劑量的誤差。本研究主要目的在計算出各種固態假體之假體劑量轉換因子，確保應用固態假體執行絕對劑量校驗時可以得到正確的校驗結果，藉此維持放射治療的品質。材料與方法：本研究依循TG-21議定書之內容執行劑量校驗。研究比較之假體包括：Plastic Water（PSW）、Standard Grade Solid Water（GSW）、Virtual Water^TM（VSW）、RW3 solid water（RW3）與 White polystyrene phantom（Wpoly）等五種固態水假體，以及Acry 與Cpoly 兩種固態假體。射束能量為6、10及15 MV光子射束與6、9、12及15 MeV電子射束，在各射束能量條件下，分別比較各型固態假體與水假體絕對劑量校驗之差異。五種固態水假體之劑量校驗計算皆以水為介質並執行TG-21議定書之參數查表，其中RW3與Wpoly再加以Cpoly為介質執行參數查表。光子射束劑量校驗深度設定於5 cm，而電子射束則於最大劑量深度位置。在完成各型固態假體之TG-21查表後，不同能量射束先後使用水假體與固態假體執行絕對劑量量測，分析比較其劑量差異。此外，透過各型固態假體與水假體的劑量比對，求得各型固態假體的假體劑量轉換因子（phantom dose conversion factor, PDCF），藉以修正固態假體執行劑量校驗時的誤差。除了在劑量校驗點的比對之外，為確認固態水假體在不同深度對於射束衰減及散射等特性與水的差異，本研究測量固態水假體於光子射束5，10及15 cm之百分深度劑量（PDD）與電子射束d50與d80之百分深度游離量（PDI），藉以驗證不同深度下固態水假體於光子及電子射束作用下，相對於水的差異。結果：總計七種固態假體在所有高能光子射束與水假體所量測之絕對劑量差異皆小於1%。PSW與Acry在所有電子射束下與水假體量測之絕對劑量差異小於1%，而GSW 、 VSW 、 RW3 、 Wploy 、 Cpoly之量測差異則介於-0.6 ~ -3.6%之間，對應求得之電子射束固態水假體劑量轉換因子則介於0.994 ~ 0.964之間。五種固態水假體在光子射束下5，10及15 cm之PDD值與水假體所量測之差異皆小於1.1%；而五種固態水假體在電子射束下，d50與d80深度之PDI值與水假體所量測之差異則皆在1.5 mm以下。結論：在光子射束使用PSW 、 GSW 、 VSW 、 RW3、 Wpoly五種固態水假體量測絕對劑量時皆可視為水介質，因其假體材質造成的劑量差異小於1%；且在電子射束使用PSW假體量測絕對劑量時亦可視為水，其因假體材質造成的劑量差異也小於1%。而GSW 、 VSW 、 RW3與Wpoly假體在執行電子射束劑量量測時，則建議先求出電子射束之假體劑量轉換因子，藉以修正固態水假體因材質所造成的劑量差異。

關鍵字

TG-21議定書 ； 固態水假體 ； 假體劑量轉換因子

並列摘要

Purpose: To verify the accuracy of the clinical reference dosimetry according to the TG-21 protocol using seven solid phantoms for high energy photon and electron beams.Materials and Methods: The TG-21 protocol was published in 1983, and then the clinical reference dosimetry can be performed in the solid phantom materials. However, only acrylic and clear polystyrene are supported in TG-21. Reference dosimetry followed by TG-21 but use solid phantoms other than the phantoms supported by this protocol can result uncertainties in absolute dose calibration. Plastic Water® Phantom Material (PSW), Standard Grade Solid Water® (GSW), Virtual Water^TM Phantom Materials (VSW), RW3 solid water phantom (RW3), white polystyrene phantom (Wpoly), acrylic phantom (Acry) and clear polystyrene phantom (Cpoly) are enrolled to compare the responses with water for 6, 10 and 15 MV photon beams and 6, 9, 12 and 15 MeV electron beams. For the RW3 and Wpoly, we compare the results using the water and clear polystyrene parameters for clinical absolute dosimetry calculation. Ionization measurements were taken at water equivalent depths of 5 cm and dmax for photon and electron beams, respectively. The phantom dose conversion factor (PDCF) of the absolute dose between solid water phantoms and water at the identical calibration conditions are obtained. Additionally, comparisons for percent depth doses at water equivalent depths of 5, 10, and 15 cm in photon beams, and d50 and d80 in electron beams were performed to verify the different response at different depths.Results: The variations between five solid water phantoms and water for photon beams are all within 1.0%. For PSW and Acry, the variations in electron beams are within 1.0%. However, the variations are -0.6 to -3.6% for 6 to 15 MeV electron beams for GSW, VSW, RW3, Wploy and Cpoly. The PDCF of 0.994~ 0.964 for electron beams are obtained. The variations of PDD (5 cm), PDD (10 cm), PDD (15 cm) between five solid water phantoms and water for photon beams are within 1.1%. The variations of d50 and d80 between five solid water phantoms and water for photon beams are within 1.5 mm.Conclusion: The PSW、GSW、VSW、RW3、Wpoly solid water phantom can be treated as water for 6, 10 and 15 MV photon beams. And the PSW can also be treated as water for electron beams. For electron beam dose calibration, PDCF should be applied for GSW, VSW, RW3 and Wpoly. Phantom response relative to water should be evaluated carefully when TG-21 protocol is used for clinical dose calibration.

並列關鍵字

TG-21 protocol ； Solid water phantom ； Phantom dose conversion factor