Chang Jenghwa, Yenice Kamil M, Jiang Kailiu, Hunt Margie, Narayana Ashwatha
Medical Physics Department, Memorial Sloan-Kettering Cancer Center, New York, NY 10021, USA.
Med Dosim. 2009 Summer;34(2):110-6. doi: 10.1016/j.meddos.2008.06.002. Epub 2008 Jul 1.
We studied the effect of MLC (multileaf collimator) leaf width and PTV (planning target volume) margin on treatment planning of intensity modulated stereotactic radiosurgery (IMSRS) or radiotherapy (IMSRT). Twelve patients previously treated with IMSRS/IMSRT were retrospectively planned with 5- and 3-mm MLC leaf widths and 3- and 2-mm PTV margins using the already contoured clinical target volume and critical structures. The same beam arrangement, planning parameters, and optimization method were used in each of the 4 plans for a given patient. Each plan was normalized so that the prescription dose covered at least 99% of the PTV. Plan indices--D(mean) (mean dose), conformity index (CI), V(70) (volume receiving >or= 70% of the prescription dose), and V(50) (volume receiving >or= 50% of the prescription dose)--were calculated from the dose-volume histograms (DVHs) of the PTV, normal tissue, and organs at risk (OARs). Hypothesis testing was performed on the mean ratios of plan indices to determine the statistical significance of the relative differences. The PTV was well covered for all plans, as no significant differences were observed for D(95), V(95), D(max), D(min), and D(mean) of the PTV. The irradiated volume was approximately 23% smaller when 2-mm instead of 3-mm PTV margin was used, but it was only reduced by approximately 6% when the MLC leaf width was reduced from 5 mm to 3 mm. For normal tissue and brainstem, V(70), V(50), and D(mean) were reduced more effectively by a decrease in MLC width, while D(mean) of optic nerve and chiasm were more sensitive to a change in PTV margin. The DVH statistics for the PTV and normal structures from the treatment plan with 5-mm MLC and 2-mm PTV margin were equal to those with 3-mm MLC and 3-mm PTV margin. PTV margin reduction is more effective in sparing the normal tissue and OARs than a reduction in MLC leaf width. For IMSRS, where highly accurate setup and small PTV margins are routinely employed, the use of 5-mm MLC is therefore less desirable.
我们研究了多叶准直器（MLC）叶片宽度和计划靶区（PTV）边界对调强立体定向放射外科治疗（IMSRS）或调强放射治疗（IMSRT）治疗计划的影响。对12例先前接受过IMSRS/IMSRT治疗的患者，利用已勾画的临床靶区体积和关键结构，回顾性地制定了MLC叶片宽度为5毫米和3毫米、PTV边界为3毫米和2毫米的治疗计划。对于给定患者的4个计划中的每一个，均采用相同的射野布置、计划参数和优化方法。每个计划均进行归一化处理，以使处方剂量覆盖至少99%的PTV。从PTV、正常组织和危及器官（OAR）的剂量体积直方图（DVH）中计算计划指标——D(mean)（平均剂量）、适形指数（CI）、V(70)（接受≥70%处方剂量的体积）和V(50)（接受≥50%处方剂量的体积）。对计划指标的平均比值进行假设检验，以确定相对差异的统计学显著性。所有计划对PTV的覆盖均良好，因为PTV的D(95)、V(95)、D(max)、D(min)和D(mean)未观察到显著差异。当使用2毫米而非3毫米的PTV边界时，照射体积大约减小23%，但当MLC叶片宽度从5毫米减小到3毫米时，照射体积仅减小约6%。对于正常组织和脑干，V(70)、V(50)和D(mean)通过减小MLC宽度能更有效地降低，而视神经和视交叉的D(mean)对PTV边界的变化更敏感。MLC为5毫米、PTV边界为2毫米的治疗计划中PTV和正常结构的DVH统计数据与MLC为3毫米、PTV边界为3毫米的治疗计划相等。与减小MLC叶片宽度相比，减小PTV边界在保护正常组织和OAR方面更有效。因此，对于通常采用高精度摆位和小PTV边界的IMSRS，使用5毫米的MLC不太理想。
