Dosimetric Analysis of Analytic Pencil Beam Algorithm in Head and Neck Tumor Radiotherapy Plan Designing
摘要: 碳离子束治疗因其倒转的深度剂量分布和较高的相对生物学效应，具有对正常组织损伤小、副作用低等优势，有望成为头颈部肿瘤放疗的主力。剂量算法的精确性直接影响临床照射剂量的准确性，减小碳离子束治疗剂量计算误差有助于提高患者生存率和局部控制率。本文采用解析笔形束算法对5例头颈部患者病例进行碳离子束治疗剂量计算，并利用蒙特卡罗模拟方法对治疗计划进行剂量验证，对解析计算和蒙特卡罗模拟结果进行不同评价标准组合下(2%/2mm, 3%/2mm, 3%/3mm, 3%/5mm)的伽马通过率分析、DVH图及参考指标分析、中心轴剂量分布对比。结果显示：笔形束算法在3%/2mm的评价标准下平均伽马通过率为90.441%，满足碳离子束治疗头颈部肿瘤的剂量计算要求。笔形束算法在束流横向上的计算结果较为精准，但在沿束流入射方向上的计算结果受路径上组织非均匀性及空腔的影响，存在射程偏差。为降低剂量计算误差带来的影响，碳离子束治疗头颈部肿瘤时需要采用更保守的射野角度选择。本文对比结果可用于指导基于解析笔形束剂量算法的头颈部肿瘤碳离子束治疗计划设计。
Abstract: Carbon ion radiotherapy is expected to become a workhorse for head and neck tumors radiotherapy due to its inverted depth dose distribution and high relative biological effectiveness, which has the advantages of less damage to normal tissues and low side effects. The accuracy of dose algorithms directly affects the accuracy of clinical irradiation doses, and reducing the dose calculation error of carbon ion radiotherapy could help improve patient’s survival and local control rates. In this paper, the dose distribution of carbon ion radiotherapy for 5 head and neck patients were calculated by the analytic pencil beam algorithm and validated using Monte Carlo method. Three analysis methods were carried out for the calculated and simulated results: (1) Gamma pass rate analysis under multiple evaluation criteria combination (2%/2mm, 3%/2mm, 3%/3mm, 3%/5mm), (2) DVH and reference index analysis, (3) central axis dose curve comparison. The analysis results showed that the average gamma pass rate of the pencil beam algorithm was 90.441% under the evaluation criteria of 3%/2mm, which met the dose calculation requirements of carbon ion radiotherapy for head and neck tumors. The pencil beam algorithm calculated more accurate in the beam transverse direction, and there is a range deviation by which the structure inhomogeneity and cavity exited in the irradiation path. In order to reduce the impact of dose calculation error, a more conservative angle selection to carbon-ion beam treatment of head and neck tumors is recommended. The results herein could be used to guide the design of carbon ion radiotherapy for head and neck tumors based on analytical pencil beam algorithm.