Coutrakon G, Cortese J, Ghebremedhin A, Hubbard J, Johanning J, Koss P, Maudsley G, Slater C R, Zuccarelli C
Department of Radiation Medicine, Loma Linda University Medical Center, California 92354, USA.
Med Phys. 1997 Sep;24(9):1499-506. doi: 10.1118/1.598038.
Protons have long been recognized as low LET radiation in radiotherapy. However, a detailed account of LET (linear energy transfer) and RBE (relative biological effectiveness) changes with incident beam energy and depth in tissue is still unresolved. This issue is particularly important for treatment planning, where the physical dose prescription is calculated from a RBE using cobalt as the reference radiation. Any significant RBE changes with energy or depth will be important to incorporate in treatment planning. In this paper we present microdosimetry spectra for the proton beam at various energies and depths and compare the results to cell survival studies performed at Loma Linda. An empirically determined biological weighting function that depends on lineal energy is used to correlate the microdosimetry spectra with cell survival data. We conclude that the variations in measured RBE with beam energy and depth are small until the distal edge of the beam is reached. On the distal edge, protons achieve stopping powers as high as 100 keV/micron, which is reflected in the lineal energy spectra taken there. Lineal energy spectra 5 cm beyond the distal edge of the Bragg peak also show a high LET component but at a dose rate 600 times smaller than observed inside the proton field.
长期以来，质子在放射治疗中一直被视为低传能线密度（LET）辐射。然而，LET（传能线密度）和相对生物效应（RBE）随入射束能量和组织深度的变化的详细情况仍未得到解决。这个问题对于治疗计划尤为重要，在治疗计划中，物理剂量处方是根据以钴作为参考辐射的RBE计算得出的。RBE随能量或深度的任何显著变化对于纳入治疗计划都很重要。在本文中，我们展示了不同能量和深度下质子束的微剂量谱，并将结果与在洛马林达进行的细胞存活研究进行比较。使用一个根据线能量凭经验确定的生物加权函数，将微剂量谱与细胞存活数据相关联。我们得出结论，在到达束的远端边缘之前，测量到的RBE随束能量和深度的变化很小。在远端边缘，质子达到高达100 keV/微米的阻止本领，这在那里获取的线能量谱中有所体现。布拉格峰远端边缘5厘米处的线能量谱也显示出一个高LET成分，但剂量率比质子场内观察到的小600倍。
