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 21, 0, 1, 2 and 3mm, 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 22Gy 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±0.9mm to 5.3±0.7mm. 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 1mm 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.
ASJC Scopus subject areas
- Radiology Nuclear Medicine and imaging