Brosing J W, Henkelman R M
Phys Med Biol. 1981 Jul;26(4):633-40. doi: 10.1088/0031-9155/26/4/007.
Increased interest in the use of negative pion beams for radiotherapy has led to a requirement for accurate knowledge of the extent and effect of beam contamination. Contaminating muons from the decay of pions upstream of the irradiation position cannot be easily identified by time-of-flight or range measurement techniques. A calculation of the stopping and dose distributions of these muons was performed. A monoenergetic pencil beam of 180 MeV/c pions, normally incident onto a semi-infinite water slab, was assumed to decay into muons over the one metre distance upstream of the water and in the water. Dose from the subsequent decay of muons to electrons was included. Contour maps of the stopping and dose distributions are presented as well as the stopping and depth-dose distributions for an infinite beam. The calculation clearly illustrates that muons from the decay of monoenergetic pions do not have a unique momentum and range, but rather include a wide spectrum of ranges and momenta. Since a muon dose contribution of 16% has been calculated at a depth of 8 cm, these muons could result in unexpected nonuniformities in dose in a clinical pion irradiation if they are ignored in pion treatment planning.
对将负π介子束用于放射治疗的兴趣增加，导致需要准确了解束流污染的程度和影响。通过飞行时间或射程测量技术，难以轻易识别来自照射位置上游π介子衰变产生的污染μ子。对这些μ子的阻止和剂量分布进行了计算。假设一束能量为180 MeV/c的单能笔形π介子束垂直入射到一个半无限水板上，在水上游一米的距离以及水中衰变成μ子。计算中包含了μ子随后衰变成电子产生的剂量。给出了阻止和剂量分布的等高线图以及无限束的阻止和深度剂量分布。计算清楚地表明，单能π介子衰变产生的μ子没有唯一的动量和射程，而是包含了很宽的射程和动量谱。由于在8厘米深度处计算出μ子剂量贡献为16%，如果在π介子治疗计划中忽略这些μ子，它们可能会在临床π介子照射中导致意外的剂量不均匀性。
