Cheng C W, Das I J
Morristown Memorial Hospital, New Jersey, USA.
Med Phys. 2002 Feb;29(2):226-30. doi: 10.1118/1.1446111.
In the step-and-shoot delivery of an IMRT plan with a Siemens Primus accelerator, radiation is turned off by desynchronizing the injector while the field parameters are being changed. When the machine is ready again a trigger pulse is sent to the injector to start the beam instantaneously. The objective of this study is to investigate the beam characteristics of the machine operating in the IMRT mode and to study the effect of the Initial Pulse Forming Network (IPEN) on the dark current. The central axis (CAX) output for a 10 x 10 cm2 field over the range 1-100 MU was measured with an ion chamber in a polystyrene phantom for both 6 and 15 MV x rays. Beam profiles were also measured over the range of 2-40 MU with the machine operating in the IMRT mode and compared with those in the normal mode. By adjusting the IPFN value, dark current radiation (DCR) was measured using ion chamber measurements. For both the normal and IMRT modes, dose versus MU is nonlinear in the range 1-5 MUs. Above 5 MU, dose varies linearly with MU for both 6 and 15 MV x rays. For stability of dose profiles, the 2 MU-IM group exhibit 20% variation from one subfield to another. The variation is about 5% for the 8 MU-IM group and <5% for 10 MU and higher. The results are similar in the normal treatment mode. With the IPFN at >80% of the PFN value, a spurious radiation associated with dark current at approximately 0.7% of the dose at isocenter for a 10 x 10 cm2 field is detected during the "PAUSE" state of the accelerator for 15 MV x rays. When the IPFN is lowered to <80% of the PFN value, no DCR is detected. For 6 MV x rays, no measurable DCR was detected regardless of the IPFN setting.
在使用西门子Primus加速器进行调强放疗（IMRT）计划的步进-射野放疗过程中，当改变射野参数时，通过使注入器不同步来关闭辐射。当机器再次准备好时，会向注入器发送一个触发脉冲以立即启动束流。本研究的目的是研究机器在IMRT模式下运行时的束流特性，并研究初始脉冲形成网络（IPEN）对暗电流的影响。在聚苯乙烯模体中，使用电离室测量了10×10 cm²射野在1 - 100 MU范围内6和15 MV X射线的中心轴（CAX）输出。还在机器处于IMRT模式下测量了2 - 40 MU范围内的束流轮廓，并与正常模式下的进行了比较。通过调整IPFN值，使用电离室测量来测量暗电流辐射（DCR）。对于正常模式和IMRT模式，在1 - 5 MU范围内剂量与MU均呈非线性关系。对于6和15 MV X射线，在5 MU以上，剂量与MU呈线性变化。为了保证剂量分布的稳定性，2 MU - IM组从一个子野到另一个子野的变化为20%。8 MU - IM组的变化约为5%，10 MU及更高时变化小于5%。正常治疗模式下的结果相似。当IPFN大于PFN值的80%时，在加速器的“暂停”状态下，对于15 MV X射线，在10×10 cm²射野的等中心处检测到与暗电流相关的杂散辐射，其约为剂量的0.7%。当IPFN降低到PFN值的80%以下时，未检测到DCR。对于6 MV X射线，无论IPFN设置如何，均未检测到可测量的DCR。
