Kan M W, Wong T P, Young E C, Chan C L, Yu P K
Department of Physics and Materials Science, City University of Hong Kong.
Australas Phys Eng Sci Med. 1995 Jun;18(2):95-103.
We report results of a comparison of two photon planning algorithms, the Clarkson Scatter Integration algorithm and the Equivalent Tissue-air Ratio algorithm, using a simple lung phantom for 8 MV and 25 MV X-ray beams of field sizes 5 cm x 5cm and 10 cm x 10 cm. Central axis depth-dose distributions were measured with a thimble chamber or a Markus parallel-plate chamber. Dose profile distributions were measured with TLD rods and films. Measured dose distributions were then compared to predicted dose distributions. Both agorithms overestimate the dose at mid-lung as they do not account for the effect of electronic disequilibrium. The Clarkson algorithm consistently shows less accurate results in comparison with the ETAR algorithm. There is additional error in the case of the Clarkson algorithm because of the assumption of a unit density medium in calculating scatter, which gives an overestimate in the effective scatter-air ratios in lung. For a 5 cm x 5 cm field, the error of dose prediction (Dpredicted-Dmeasured) for 25 MV x-ray beam at mid-lung is 15.8% and 12.8% for Clarkson and ETAR algorithm respectively. At 8 MV the error is 9.3% and 5.1% respectively. In addition, both algorithms underestimate the penumbral width at mid-lung as they do not account for the penumbral flaring effect in low density medium. It is very important for medical physicists, radiation therapists and clinicians to be aware of the limitation of their radiotherapy treatment planning systems.
我们报告了使用简单肺部模体，对两种光子治疗计划算法（克拉克森散射积分算法和等效组织空气比算法）进行比较的结果，该模体用于5 cm×5 cm和10 cm×10 cm射野尺寸的8 MV和25 MV X射线束。中心轴深度剂量分布用指形电离室或马库斯平行板电离室进行测量。剂量剖面分布用热释光剂量计棒和胶片进行测量。然后将测量的剂量分布与预测的剂量分布进行比较。两种算法都高估了肺中部的剂量，因为它们没有考虑电子不平衡的影响。与等效组织空气比算法相比，克拉克森算法始终显示出不太准确的结果。对于克拉克森算法，由于在计算散射时假设介质密度为单位密度，导致在肺中有效散射空气比被高估，从而存在额外误差。对于5 cm×5 cm的射野，在肺中部，25 MV X射线束的剂量预测误差（预测剂量-测量剂量），克拉克森算法和等效组织空气比算法分别为15.8%和12.8%。在8 MV时，误差分别为9.3%和5.1%。此外，两种算法都低估了肺中部的半值层宽度，因为它们没有考虑低密度介质中的半值层扩展效应。对于医学物理学家、放射治疗师和临床医生来说，了解他们的放射治疗治疗计划系统的局限性非常重要。
