Department of Radiation Oncology, Vanderbilt University School of Medicine, Vanderbilt-Ingram Cancer Center, B-902, TVC, 1301 Medical Center Drive, Nashville, TN 37232-5671, USA.
Phys Med Biol. 2012 Sep 7;57(17):5509-21. doi: 10.1088/0031-9155/57/17/5509. Epub 2012 Aug 8.
Small megavoltage (MV) photon fields of dimensions less than 3 × 3 cm(2) are increasingly being used in modern radiation therapy. To our knowledge, small beam characteristics and dosimetric parameters, such as the energy spectra, particle fluence, and water-to-air stopping-power ratios (SPRs) directly affect the accuracy of small field dosimetry. This study presents the characteristics of small photon beams and investigates the variations of energy spectra of photons and electrons as a function of field size and their effects on the water-to-air SPRs for field sizes ranging from a small 4 mm diameter circular field to a 10 × 10 cm(2) field. It sheds light on the differences between small fields collimated by the cone accessory and X- and Y-jaws and on beam characteristics outside the primary radiation fields. In addition, we also investigated the use of an 'intermediate machine-specific-reference field' (Alfonso et al 2008 Med. Phys. 35 5179-86) to determine if the variations between a small and a reference field can be eased by introducing an intermediate 4 × 4 cm(2) field instead of a standard 10 × 10 cm(2) reference field. The Monte Carlo simulation codes BEAMnrc, DOSXYZnrc and SPRRZnrc were used in this study. The accelerator head and circular cone accessory were simulated in detail including two designs of flattening filters: one for a standard-dose rate (100-600 MU min(-1)) and the other for a high-dose rate (1000 MU min(-1)) 6 MV beam. The mean energy of photons at depths (1.5-30 cm) in water are 1.72-2.36 MeV, 1.55-1.97 MeV, and 1.44-1.74 MeV for field sizes of 4 mm diameter, 4 × 4 cm(2), and 10 × 10 cm(2), respectively. The mean energy also varies significantly for electrons at depths (1.5-30 cm): 0.99-1.25 MeV, 0.94-1.09 MeV, and 0.93-1.04 MeV for field sizes of 4 mm, 4 × 4 cm(2), and 10 × 10 cm(2), respectively. The calculated water-to-air SPRs at depths (1.5-30 cm) are 1.120-1.113, 1.121-1.117, and 1.122-1.119 for field sizes of 4 mm, 4 × 4 cm(2) and 10 × 10 cm(2), respectively. Although the differences in mean energy are > 20% for photons and > 5% for electrons between 4 mm field and 10 × 10 cm(2), the effects on the water-to-air SPRs are small (<0.5%). For detectors with responses to energy changes that are not negligible, significant energy variations between small fields and the reference 10 × 10 cm(2) field may have a significant impact on the dosimetry accuracy. However, the use of an intermediate field is capable of greatly reducing these energy variations. This study also found negligible differences in dosimetric parameters between beams with different flattening filters and different incident electron energies on the target when each has the same beam quality k(Q) values specified by %dd(10)(x).
小尺寸的兆伏（MV）光子束尺寸小于 3×3cm²，在现代放射治疗中越来越多地被使用。据我们所知，小射束特性和剂量学参数，如能谱、粒子注量和水-空气比阻止本领比（SPRs），直接影响小射束剂量测量的准确性。本研究介绍了小光子束的特性，并研究了光子和电子的能谱随射束尺寸的变化及其对水-空气 SPRs 的影响，射束尺寸范围从 4mm 直径的小圆形射束到 10×10cm²的射束。它揭示了锥形附件和 X、Y 型准直器准直的小射束之间的差异，以及主射束外的射束特性。此外，我们还研究了使用“中间机器特定参考射束”（Alfonso 等人，2008 年，《医学物理学》，第 35 卷，第 5179-5186 页），以确定通过引入一个 4×4cm²的中间射束，而不是标准的 10×10cm²的参考射束，是否可以缓解小射束和参考射束之间的差异。本研究使用了 BEAMnrc、DOSXYZnrc 和 SPRRZnrc 蒙特卡罗模拟代码。在详细模拟了加速器头和圆形锥形附件，包括两种类型的平坦化滤波器：一种用于标准剂量率（100-600MUmin⁻¹），另一种用于高剂量率（1000MUmin⁻¹）6MV 射束。在水的深度（1.5-30cm）处，光子的平均能量为 1.72-2.36MeV、1.55-1.97MeV 和 1.44-1.74MeV，分别对应于 4mm 直径、4×4cm²和 10×10cm²的射束尺寸。在水的深度（1.5-30cm）处，电子的平均能量也有很大变化：0.99-1.25MeV、0.94-1.09MeV 和 0.93-1.04MeV，分别对应于 4mm、4×4cm²和 10×10cm²的射束尺寸。在水的深度（1.5-30cm）处计算出的水-空气 SPRs 为 1.120-1.113、1.121-1.117 和 1.122-1.119，分别对应于 4mm、4×4cm²和 10×10cm²的射束尺寸。尽管 4mm 射束和 10×10cm²射束之间的光子平均能量差异大于 20%，电子平均能量差异大于 5%，但对水-空气 SPRs 的影响很小（<0.5%）。对于能量变化响应不可忽略的探测器，小射束和参考 10×10cm²射束之间的能量变化可能对剂量测量的准确性有重大影响。然而，使用中间射束可以大大减少这些能量变化。本研究还发现，当每个射束都具有相同的束质 k(Q)值（由%dd(10)(x)指定）时，使用不同平坦化滤波器和不同入射电子能量的射束之间的剂量学参数几乎没有差异。
