Vahc Y W, Chung W K, Park K R, Lee J Y, Lee Y H, Kwon O, Kim S
Department of Physics, Institute of Basic Medical Science, Yonsei University Wonju College of Medicine, Korea.
Med Phys. 2001 Mar;28(3):303-9. doi: 10.1118/1.1350437.
Accurate dosimetry of small-field photon beams tends to be difficult to perform due to the presence of lateral electronic disequilibrium and steep dose gradients. In stereotactic radiosurgery (SRS), small fields of 6-30 mm in diameter are used. Generally thermoluminescence dosimetry chips, Farmer, Thimble ion chamber, and film dosimetry are not adequate to measure dose in SRS beams. These techniques generally do not provide the required precision due to their energy dependence and/or poor resolution. It is necessary to construct a small, accurate detector with high spatial resolution for the small fields used in SRS. The ultramicrocylindrical ionization chamber (UCIC) with a gold wall of 2.2 mm in diameter and 4.0 mm in length has dual sensitive volumes of air (8.0 mm3) and borosilicate (2.6 mm3) cavity. Reproducibility, linearity, and radiation damage with respect to absorbed dose, beam profile of small beam, and independence of dose rate of the UCIC are tested by the dose measurements in high energy photon (5, 15 MV) and electron (9 MeV) beams. The UCIC with a unique supporting system in the polystyrene phantom is demonstrated to be a suitable detector for the dose measurements in a small beam size.
由于存在横向电子不平衡和陡峭的剂量梯度，小射野光子束的精确剂量测定往往难以进行。在立体定向放射外科手术（SRS）中，使用直径为6 - 30毫米的小射野。一般来说，热释光剂量测定芯片、 Farmer剂量仪、指形电离室和胶片剂量测定法不足以测量SRS射野中的剂量。由于这些技术的能量依赖性和/或分辨率差，通常无法提供所需的精度。有必要为SRS中使用的小射野构建一个小型、精确且具有高空间分辨率的探测器。直径为2.2毫米、长度为4.0毫米的金壁超微圆柱形电离室（UCIC）具有空气（8.0立方毫米）和硼硅酸盐（2.6立方毫米）腔的双灵敏体积。通过在高能光子（5、15兆电子伏）和电子（9兆电子伏）束中的剂量测量，测试了UCIC在吸收剂量方面的可重复性、线性和辐射损伤、小射束的射野轮廓以及剂量率独立性。在聚苯乙烯模体中具有独特支撑系统的UCIC被证明是一种适用于小射野尺寸剂量测量的探测器。
