Comparison of treatment planning for Stereotactic Radiosurgery(SRS) and Stereotactic Body Radiation Therapy(SBRT) techniques with 2.5mm and 5mm multileaf collimator(MLC)- a Pilot Study

Summary: The study compares two different types of multi-leaf collimators (MLCs) for treatment planning in Stereotactic Radiosurgery (SRS) and Stereotactic Body Radiotherapy (SBRT) using the Elekta Versa HD machine. The APEX micro-MLC (mMLC) offers excellent dose distribution but has drawbacks like longer treatment duration and technical errors. The aim is to compare treatment plans using a 2.5mm APEX mMLC and a 5mm MLC (Agility) for brain and lung targets. Methods: The study involves 10 patients with brain and lung targets. Two treatment plans are created for each case using the MONACO 5.11.03 treatment planning system, with 2.5mm and 5mm MLCs. A 6FFF x-ray photon beam is utilized with various gantry, couch, and collimator setups. Plans are compared using target coverage, conformity index, homogeneity index, gradient index, and organ at risk doses. Results: No significant differences are observed in target coverage, conformity index, homogeneity index, and organ at risk doses between the two MLC designs. The 5mm MLC plan requires more monitor units but provides comparable tumor coverage. For brain targets, organ at risk doses are similar for both MLC widths, while lung targets show slightly lower organ at risk doses with the 2.5mm mMLC. The gradient index is better with the 2.5mm mMLC, indicating steeper dose falloff (p=0.158). Conclusion: The study concludes that both 2.5mm and 5mm MLCs yield similar results in terms of target coverage, conformity index, homogeneity index, and organ at risk doses in VMAT plans for SRS/SBRT. The 2.5mm mMLC offers an advantage in gradient index, resulting in sharper dose gradients and reduced lower isodose volumes. However, the 5mm MLC can still be used effectively with further reduction in gradient index. The study suggests the need for more extensive research with larger sample sizes and diverse parameters for comparison.

摘要：本研究依托医科达（Elekta）Versa HD直线加速器，对比两种多叶准直器（multi-leaf collimators, MLCs）在立体定向放射外科（Stereotactic Radiosurgery, SRS）与立体定向体部放射治疗（Stereotactic Body Radiotherapy, SBRT）治疗计划中的应用表现。APEX微型多叶准直器（APEX micro-MLC, mMLC）虽具备优异的剂量分布特性，但存在治疗时长较长、易产生技术误差等不足。本研究旨在对比2.5mm叶片间距的APEX mMLC与5mm叶片间距的Agility MLC分别针对脑部及肺部靶区的治疗计划效果。

方法：本研究纳入10例存在脑部或肺部靶区的患者。采用MONACO 5.11.03治疗计划系统，为每例患者分别构建基于2.5mm叶片间距MLC与5mm叶片间距MLC的两套治疗计划。照射采用6FFF X射线光子束，搭配多种机架、治疗床及准直器参数组合。通过靶区覆盖率、适形指数、均匀性指数、梯度指数及危及器官剂量对两套计划开展对比分析。

结果：两种MLC设计在靶区覆盖率、适形指数、均匀性指数及危及器官剂量方面均无显著差异。5mm叶片间距MLC计划所需机器跳数更多，但肿瘤靶区覆盖率与2.5mm组相当。针对脑部靶区，两种MLC叶片宽度对应的危及器官剂量无明显差异；而针对肺部靶区，2.5mm mMLC计划的危及器官剂量略低。2.5mm mMLC的梯度指数更优，提示剂量跌落更为陡峭（p=0.158）。

结论：本研究证实，在针对SRS/SBRT的VMAT计划中，2.5mm与5mm两种MLC在靶区覆盖率、适形指数、均匀性指数及危及器官剂量方面表现相近。2.5mm mMLC在梯度指数上具备优势，可实现更锐利的剂量梯度与更小的低剂量照射体积。不过5mm MLC仍可有效应用，仅梯度指数相对更低。本研究建议后续开展更大样本量、涵盖更多参数类型的对比研究，以进一步验证相关结论。