Ostinelli A, Gelosa S, Frigerio M, Monti A F
Servizio di Fisica Sanitaria, Azienda Ospedaliera S. Anna, Como.
Radiol Med. 1998 Oct;96(4):390-3.
Shaped fields are widely used in radiotherapy to protect critical organs and to avoid unnecessary normal tissue irradiation. The most common system for photon beam shaping consists in a low melting point alloy. We studied the air bubbles which can occur during alloy cooling with both new and remelted alloys and when different cooling techniques are chosen.
Forty cone samples (18 of remelted alloy and 22 of new alloy) were prepared to evaluate the frequence of air bubble recurrence, with reproducible geometric sizes (height = 70 mm, major base surface diameter = 60 mm, minor base surface diameter = 40 mm). Air bubble sizes and dose inhomogeneity were evaluated by reproducing 60Co radiograph of each sample (two orthogonal projections: 6 x 7 cm). The samples were cooled at a constant temperature, following three different modalities: high (25 degrees C), medium (5 degrees C), low (-20 degrees C) temperature. Owing to the small geometrical magnification, air bubble sizes were determined by measuring their surface on samples lateral projections, taking into account the sight detectable bubble edges.
Up to 300 mm2 lateral surface bubbles are always present in all castings. Casting inhomogeneities can produce a film-density inhomogeneity ranging from 9% to 40%. The spatial distribution of bubbles seems to be random.
Bubble recurrence is independent of both the metal alloy (repeatedly used castings) and the different block cooling modalities. The effect of air bubbles on the shielded areas dose inhomogeneity is generally of no relevant importance. However, these inhomogeneities can produce hot spots which must be taken into accurate consideration only in the particular treatments where critical small size organ dose sparing represents a basic issue (i.e. the shielding of eye lens).
成形射野在放射治疗中被广泛用于保护关键器官并避免不必要的正常组织照射。光子束成形最常用的系统由低熔点合金组成。我们研究了在合金冷却过程中，使用新合金和重熔合金以及选择不同冷却技术时可能出现的气泡。
制备了40个锥形样品（18个重熔合金样品和22个新合金样品），以评估气泡重现的频率，其几何尺寸可重现（高度 = 70毫米，大底面直径 = 60毫米，小底面直径 = 40毫米）。通过再现每个样品的60Co射线照片（两个正交投影：6×7厘米）来评估气泡大小和剂量不均匀性。样品按照三种不同方式在恒定温度下冷却：高温（25摄氏度）、中温（5摄氏度）、低温（-20摄氏度）。由于几何放大倍数较小，通过测量样品侧面投影上气泡的表面来确定气泡大小，同时考虑可见的可检测气泡边缘。
在所有铸件中，始终存在面积达300平方毫米的侧面气泡。铸件不均匀性可产生9%至40%的胶片密度不均匀性。气泡的空间分布似乎是随机的。
气泡重现与金属合金（重复使用的铸件）和不同的模块冷却方式均无关。气泡对屏蔽区域剂量不均匀性的影响通常无关紧要。然而，这些不均匀性可产生热点，仅在关键小尺寸器官剂量 sparing 是基本问题的特定治疗中（即晶状体的屏蔽）才必须予以精确考虑。
