Vianello E A, deAlmeida C E
Laboratorio de Ciências Radiologicas, LCR-DBB-UERJ, Rua São Francisco Xavier 524, CEP 20 500-013, Rio de Janeiro, Brazil.
Med Phys. 2008 Jul;35(7):3389-92. doi: 10.1118/1.2940160.
In brachytherapy, one of the elements to take into account for measurements free in air is the non-uniformity of the photon fluence due to the beam divergence that causes a steep dose gradient near the source. The correction factors for this phenomenon have been usually evaluated by two available theories by Kondo and Randolph [Radiat. Res. 13, 37-60 (1960)] and Bielajew [Phys. Med. Biol. 35, 517-538 (1990)], both conceived for point sources. This work presents the experimental validation of the Monte Carlo calculations made by Rodriguez and deAlmeida [Phys. Med. Biol. 49, 1705-1709 (2004)] for the non-uniformity correction specifically for a Cs-137 linear source measured using a Farmer type ionization chamber. The experimental values agree very well with the Monte Carlo calculations and differ from the results predicted by both theoretical models widely used. This result confirms that for linear sources there are some important differences at short distances from the source and emphasizes that those theories should not be used for linear sources. The data provided in this study confirm the limitations of the mentioned theories when linear sources are used. Considering the difficulties and uncertainties associated with the experimental measurements, it is recommended to use the Monte Carlo data to assess the non-uniformity factors for linear sources in situations that require this knowledge.
在近距离放射治疗中，对于空气中自由测量而言需要考虑的因素之一是，由于束发散导致源附近剂量梯度陡峭，从而造成光子注量的不均匀性。通常通过近藤和伦道夫[《辐射研究》13, 37 - 60(1960)]以及比拉耶夫[《物理医学与生物学》35, 517 - 538(1990)]提出的两种现有理论来评估这种现象的校正因子，这两种理论都是针对点源构想的。这项工作展示了罗德里格斯和德阿尔梅达[《物理医学与生物学》49, 1705 - 1709(2004)]所做的蒙特卡罗计算的实验验证，该计算专门针对使用 Farmer 型电离室测量的 Cs - 137 线性源的不均匀性校正。实验值与蒙特卡罗计算结果非常吻合，并且与广泛使用的两种理论模型所预测的结果不同。这一结果证实，对于线性源，在离源较近的距离处存在一些重要差异，并强调这些理论不适用于线性源。本研究提供的数据证实了使用线性源时上述理论的局限性。考虑到与实验测量相关的困难和不确定性，建议在需要此类知识的情况下，使用蒙特卡罗数据来评估线性源的不均匀性因子。
