Dynamic conformal arc cranial stereotactic radiosurgery: implications of multileaf collimator margin on dose-volume metrics.

Objective: The effect of multileaf collimator (MLC) margin on target and normal tissue dose-volume metrics for intracranial stereotactic radiosurgery (SRS) was assessed.

Methods: 118 intracranial lesions of 83 SRS patients formed the basis of this study. For each planning target volume (PTV), five separate treatment plans were generated with MLC margins of -1, 0, 1, 2 and 3 mm, respectively. Identical treatment planning parameters were employed with a median of five dynamic conformal arcs using the Varian/BrainLab high-definition MLC for beam shaping. Prescription dose (PD) was such that 22 Gy covered at least 95% of the PTV. Dose-volume and dose-response comparative metrics included conformity index, heterogeneity index, dose gradient, tumour control probability (TCP) and normal tissue complication probability (NTCP).

Results: Target dose heterogeneity decreased with increasing MLC margin (p<0.001); mean heterogeneity index decreased from 70.4 ± 12.7 to 10.4 ± 2.2%. TCP decreased with increasing MLC margin (p<0.001); mean TCP decreased from 81.0 ± 2.3 to 62.2 ± 1.8%. Normal tissue dose fall-off increased with MLC margin (p<0.001); mean gradient increased from 3.1 ± 0.9 mm to 5.3 ± 0.7 mm. NTCP was optimal at 1 mm MLC margin. No unambiguous correlation was observed between NTCP and PTV volume. Plan delivery efficiency generally improved with larger margins (p<0.001); mean monitor unit per centigray of the PD decreased from 3.60 ± 1.30 to 1.56 ± 0.13. Conclusion Use of 1 mm MLC margins for dynamic conformal arc-based cranial radiosurgery resulted in optimal tumour control and normal tissue sparing. Clinical significance of these comparative findings warrants further investigation.