Keywords: 
• 重离子治癌 /
• 治疗计划系统 /
• 笔形束模型 /
• 低剂量包络

Abstract: The heavy ion treatment planning system of Institute of Modern Physics(IMP),Chinese Academy of Sciences adopts single Gaussian model to depict the lateral dose distribution of pencil beam currently.Without taking secondary particles produced by heavy-ion pencil beam in medium into full consideration,the single Gaussian model is not accurate enough to describe the lateral dose distribution.In fact,an accurate pencil beam model should include the description of low dose envelope contributed by secondary particles.This work summarized the cause of formation,property,dose contribution,mathematic models and measuring methods of the low dose envelope laterally far away from the incident axis of a pencil beam.Moreover,the lateral dose distribution of heavy ion pencil beams with different energies were calculated by means of the Monte Carlo simulation method,based on the horizontal nozzle of the demonstration facility of heavy ion medical machine in the Wuwei heavy-ion therapy center,and thus the degree of coincidence between the simulated data and different mathematic models was investigated.The results verified that the triple Gaussian model was better than the single and double Gaussian models to depict the lateral dose distribution of heavy ion pencil beams,mentioned from existing papers.Thus,this work provides a basis for further developing practical and accurate pencil beam model for heavy ion cancer therapy.