Istituto di Fisica, Università Cattolica del Sacro Cuore, Rome, Italy.
Phys Med. 2011 Jan;27(1):30-8. doi: 10.1016/j.ejmp.2010.02.002. Epub 2010 Mar 2.
Many researchers are studying new in vivo dosimetry methods based on the use of Elelctronic portal imaging devices (EPIDs) that are simple and efficient in their daily use. However the need of time consuming implementation measurements with solid water phantoms for the in vivo dosimetry implementation can discourage someone in their use. In this paper a procedure has been proposed to calibrate aSi EPIDs for in vivo transit dosimetry. The dosimetric equivalence of three aSi Varian EPIDs has been investigated in terms of signal reproducibility and long term stability, signal linearity with MU and dose per pulse and signal dependence on the field dimensions. The signal reproducibility was within ± 0.5% (2SD), while the long term signal stability has been maintained well within ± 2%. The signal linearity with the monitor units (MU) was within ± 2% and within ± 0.5% for the EPIDs controlled by the IAS 2, and IAS 3 respectively. In particular it was verified that the correction factor for the signal linearity with the monitor units, k(lin), is independent of the beam quality, and the dose per pulse absorbed by the EPID. For 6, 10 and 15 MV photon beams, a generalized set of correlation functions F(TPR,w,L) and empirical factors f(TPR,d,L) as a function of the Tissue Phantom Ratio (TPR), the phantom thickness, w, the square field side, L, and the distance, d, between the phantom mid-plane and the isocentre were determined to reconstruct the isocenter dose. The tolerance levels of the present in vivo dosimetry method ranged between ± 5% and ± 6% depending on the tumor body location. In conclusion, the procedure proposed, that use generalized correlation functions, reduces the effort for the in vivo dosimetry method implementation for those photon beams with TPR within ± 0.3% as respect those here used.
许多研究人员正在研究新的基于电子射野影像装置（EPID）的体内剂量学方法，这些方法在日常使用中简单高效。然而，由于需要使用固体水模体进行耗时的实施测量，这可能会使一些人对其使用望而却步。本文提出了一种校准 aSi EPID 进行体内瞬态剂量学的方法。研究了三种瓦里安 aSi EPID 在信号重现性和长期稳定性、信号与 MU 的线性关系、剂量与脉冲以及信号与射野尺寸的依赖性方面的剂量学等效性。信号重现性在±0.5%（2SD）范围内，而长期信号稳定性保持在±2%范围内。信号与 MU 的线性关系在±2%范围内，IAS2 和 IAS3 分别控制的 EPID 在±0.5%范围内。特别是验证了信号与 MU 的线性关系的校正因子 k(lin)独立于束质和 EPID 吸收的剂量脉冲。对于 6、10 和 15 MV 光子束，确定了一组广义相关函数 F(TPR,w,L)和经验因子 f(TPR,d,L)，作为组织体模比（TPR）、模体厚度 w、射野边长 L 和模体中平面与等中心之间距离 d 的函数，以重建等中心剂量。本体内剂量学方法的容差范围在±5%和±6%之间，具体取决于肿瘤体的位置。总之，所提出的使用广义相关函数的方法，减少了对于 TPR 在±0.3%范围内的那些光子束的体内剂量学方法实施的工作量，相对于本文中使用的那些光子束。
