Aperture effect for LINAC-based SRS in small target treatment†

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiosurgery Techniques

Background:

• Stereotactic Radiosurgery (SRS) employs various techniques for precise radiation delivery.
• The choice of aperture (cone or Multi-Leaf Collimator/MLC) significantly impacts treatment for small targets in SRS.
• Understanding dosimetric differences between MLC and cone techniques is crucial for optimizing small target SRS.

Purpose of the Study:

• To compare the dosimetric characteristics of cone-based and MLC-based techniques for Stereotactic Radiosurgery (SRS) of small cranial targets.
• To evaluate how varying aperture sizes influence dose distribution and conformity in small target SRS.

Main Methods:

• Utilized an anthropomorphic phantom with a 4mm spherical cranial target.
• Generated single 360 ARC plans using Varian Eclipse system for both cone and MLC techniques.
• Compared plans based on varying aperture sizes (MLC margins: 2-6mm; cone diameters: 5-16mm).
• Assessed dosimetric parameters including 12Gy Volume (V12Gy), Max Dose to Prescription Dose ratio (MDPD), Prescription Isodose Volume to Target Volume ratio (PITV), and conformity indices (RCI, ICI).

Main Results:

• MLC plans exhibited larger variations in V12Gy (1.0-5.3cc) and PITV (average 0.80) compared to cone plans (V12Gy: 0.1-3.2cc, PITV average 0.54).
• Integrated Conformity Index (ICI) showed higher variability (up to 45%) with MLC plans compared to cone plans (7%).
• Maximum Dose to Prescription Dose (MDPD) variations were comparable between MLC (9%) and cone (12%) plans.

Conclusions:

• MLC-based SRS plans for small targets demonstrate greater variability in V12Gy, PITV, and ICI than cone-based plans.
• MDPD differences were not significant between the two techniques at the selected aperture sizes.
• Findings can inform treatment planning decisions for small target SRS, pending further clinical outcome data and investigation into advanced planning algorithms.