基于Trilogy加速器的周边剂量学研究

目的 研究不同条件下Trilogy加速器的周边剂量及Diode半导体电离室测量的可行性。方法 在固体水测量模体中使用CC13空气电离室和Diode半导体电离室测量不同距离(1~31 cm共13个测量点)、深度(3、10、15 cm)、射野大小(10、20、30 cm)、楔形板(W15、W45、VW15、VW45)、射线能量(6、18 MV)下的周边剂量分布。移除散射模体，测量其周边剂量与漏射剂量D_leakage和模体散射剂量D_scatter之间关系。模拟宫颈癌放疗患者使用VMAT、IMRTstepshoot、IMRTsliding window照射CRIS仿真模体，测量乳腺、甲状腺及晶体周边剂量。剂量归一于等中心点处。结果 周边剂量随测量距离增加逐渐减小(由距射野边缘1 cm处的13.41%降至31 cm处0.25%)，射野边缘相同距离处随深度增加基本无差异，30 cm射野约为10 cm射野的2倍。随物理楔形板角度增加逐渐增大，与开放野相比略增加1%;随虚拟楔形板角度增加而减小，与开放野相比降低2%~3%。6、18 MV X线下分别由1 cm处13.35%、11.06%衰减至31 cm处0.23%、0.20%。近射野边缘处D_scatter占主导地位，随距离由1 cm增加至25 cm，D_scatter所占比例约从62.45%降至5.71%。6 MV X线下所有测量结果中CC13电离室与Diode电离室的最大百分比偏差<1%。VMAT、IMRTstepshoot、IMRTsliding window模式下乳腺、甲状腺、晶体的分别为6.72、2.90、2.37 mGy，7.39、4.05、2.48 mGy，9.17、4.61、3.21 mGy。结论 CC13空气电离室和6 MV半导体电离室测量周边剂量具有较好一致性和可行性。临床治疗中了解周边剂量与不同照射条件关系有助于减少照射野外OAR剂量，采用屏蔽防护技术可进一步减少剂量沉积。

Peripheral dosimetry of a Trilogy accelerator

Objective To determine the peripheral dose (PD) of a Trilogy accelerator under different conditions and the feasibility of PD measurement using the semiconductor diode ionization chamber. Methods In a solid water phantom, a CC13 air-filled ionization chamber and a semiconductor diode ionization chamber were used for PD measurements with different distances (13 measurement locations within 1-31 cm), depth (3, 5, 15 cm), field sizes (10, 20, 30 cm), wedge (W15, W45, VW15, VW45), and beam energy (6, 18 MV). The relationship of PD with PD_leakage and PD_scatter was determined by removing the scatter phantom. Simulating the patients with cervical cancer undergoing radiotherapy, a CIRS phantom received volumetric modulated arc therapy (VMAT), step-shoot intensity-modulated radiotherapy (IMRT), and sliding-window IMRT to measure PDs of the breast, thyroid, and lens. All the data were normalized to the isocenter. Results PD was gradually reduced with the increase in distance (13.41% at 1 cm from the edge to 0.25% at 31 cm from the edge). With a fixed distance from the edge of the radiation field, there was no significant difference in PD between different depths. A radiation field with a size of 30 cm had a PD about two-fold higher than that with a size of 10 cm. PD increased with the increase in the physical wedge angle and increased by 1% compared with the open field;PD decreased with the increase in the virtual wedge angle and decreased by 2-3% compared with the open field. PD decayed from 13.35% at 1 cm to 0.23% at 31 cm under 6 MV X-ray and from 11.06% at 1 cm to 0.20% at 31 cm under 18 MV X-ray. D_scatter was dominant in the regions close to the edge of radiation field and decreased from 62.45% at 1 cm to 5.71% at 25 cm. In all measurements under 6 MV X-ray, the maximum proportion difference between CC13 ionization chamber and diode ionization chamber was less than 1%. PDs of the breast, thyroid, and lens were 6.72, 2.90, and 2.37 mGy in VMAT mode, 7.39, 4.05, and 2.48 mGy in step-shoot IMRT mode, and 9.17, 4.61, and 3.21 mGy in sliding-window IMRT mode, respectively. Conclusions For the measurement of PDs, the CC13 air-filled ionization chamber and semiconductor diode ionization chamber have good consistency and feasibility under 6 MV X-ray. In clinical practice, the understanding of the relationship of PD with different radiation conditions helps to reduce the doses to organs at risk. Shielding and protective techniques can further reduce dose deposition.