Reena P, Dayananda S, Pai Rajeshri, Jamema S V, Gupta Tejpal, Deepak D, Rajeev S
Department of Radiation Oncology, ACTREC, TMC, Kharghar, New Mumbai, India.
J Med Phys. 2006 Oct;31(4):269-74. doi: 10.4103/0971-6203.29197.
Implementation of step-and-shoot intensity-modulated radiotherapy (IMRT) needs careful understanding of the accelerator start-up characteristic to ensure accurate and precise delivery of radiation dose to patient. The dosimetric characteristic of a Siemens Primus linear accelerator (LA) which delivers 6 and 18 MV x-rays at the dose rate of 300 and 500 monitor unit (MU) per minutes (min) respectively was studied under the condition of small MU ranging from 1 to 100. Dose monitor linearity was studied at different dose calibration parameter (D1_C0) by measuring ionization at 10 cm depth in a solid water phantom using a 0.6 cc ionization chamber. Monitor unit stability was studied from different intensity modulated (IM) groups comprising various combinations of MU per field and number of fields. Stability of beam flatness and symmetry was investigated under normal and IMRT mode for 20×20 cm(2) field under small MU using a 2D Profiler kept isocentrically at 5 cm depth. Inter segment response was investigated form 1 to 10 MU by measuring the dose per MU from various IM groups, each consisting of four segments with inter-segment separation of 2 cm.In the range 1-4 MU, the dose linearity error was more than 5% (max -32% at 1 MU) for 6 MV x-rays at factory calibrated D1_C0 value of 6000. The dose linearity error was reduced to -10.95% at 1 MU, within -3% for 2 and 3 MU and ±1% for MU ≥4 when the D1_C0 was subsequently tuned at 4500. For 18 MV x-rays, the dose linearity error at factory calibrated D1_C0 value of 4400 was within ±1% for MU ≥3 with maximum of -13.5 observed at 1 MU. For both the beam energies and MU/field ≥4, the stability of monitor unit tested for different IM groups was within ±1% of the dose from the normal treatment field. This variation increases to -2.6% for 6 MV and -2.7% for 18 MV x-rays for 2 MU/field. No significant variation was observed in the stability of beam profile measured from normal and IMRT mode. The beam flatness was within 3% for 6 MV x-rays and more than 3% (Max 3.5%) for 18 MV x-rays at lesser irradiation time ≤3 MU. The beam stability improves with the increase in irradiation time. Both the beam energies show very good symmetry (≤2%) at all irradiation time.For all the three segment sizes studied, the nonlinearity was observed at smaller MU/segment in both the energies. When the MU/segment is ≥4, all segment size shows fairly linear relation with dose/MU. The smaller segment size shows larger nonlinearity at smaller MU/segment and become more linear at larger MU/segment. Based on our study, we conclude that the Primus LA from Siemens installed at our hospital is ideally suited for step-and-shoot IMRT preferably for radiation ON time ≥4MU per segment.
实施步进式调强放射治疗（IMRT）需要仔细了解加速器的启动特性，以确保向患者准确、精确地输送辐射剂量。在1至100的小监测单位（MU）范围内，研究了西门子Primus直线加速器（LA）的剂量学特性，该加速器分别以每分钟300和500监测单位（MU）的剂量率提供6和18兆伏的X射线。通过使用0.6立方厘米电离室在固体水模体中10厘米深度处测量电离，研究了不同剂量校准参数（D1_C0）下的剂量监测线性度。从不同的调强（IM）组研究监测单位稳定性，这些组包括每个射野的MU和射野数量的各种组合。在小MU情况下，使用等中心放置在5厘米深度处的二维轮廓仪，在正常和IMRT模式下研究20×20平方厘米射野的射束平坦度和对称性稳定性。通过测量来自各种IM组的每MU剂量来研究1至10 MU的段间响应，每个IM组由四个段组成，段间间距为2厘米。在1 - 4 MU范围内，对于6兆伏X射线，在工厂校准的D1_C0值为6000时，剂量线性误差超过5%（1 MU时最大为-32%）。当D1_C0随后调整为4500时，1 MU时的剂量线性误差降至-10.95%，2和3 MU时在-3%以内，MU≥4时在±1%以内。对于18兆伏X射线，在工厂校准的D1_C0值为4400时，MU≥3时剂量线性误差在±1%以内，1 MU时最大观察到-13.5%。对于两种射束能量且MU/射野≥4，针对不同IM组测试的监测单位稳定性在正常治疗射野剂量的±1%以内。对于2 MU/射野，6兆伏X射线的这种变化增加到-2.6%，18兆伏X射线增加到-2.7%。从正常和IMRT模式测量的射束轮廓稳定性未观察到显著变化。在较小照射时间≤3 MU时，6兆伏X射线的射束平坦度在3%以内，18兆伏X射线超过3%（最大3.5%）。射束稳定性随照射时间增加而改善。两种射束能量在所有照射时间都显示出非常好的对称性（≤2%）。对于所研究的所有三种段尺寸，在两种能量下较小的MU/段处都观察到非线性。当MU/段≥4时，所有段尺寸与每MU剂量显示出相当线性的关系。较小的段尺寸在较小的MU/段处显示出较大的非线性，在较大的MU/段处变得更线性。基于我们的研究，我们得出结论，安装在我们医院的西门子Primus LA非常适合步进式IMRT，最好用于每段辐射开启时间≥4 MU。
