Author: Makunda Aryal, Gregory Bartlett, David D. Campos, Ke Colin Huang, Sook Kien Ng, Christopher F. Njeh, Oluwaseyi Oderinde, Edwin Quashie, Ashok Tiwari, Yong Yue 👨🔬
Affiliation: Purdue University, Department of Radiation Oncology, Indiana University, Indiana University, Indiana University School of Medicine, Department of Radiation Oncology 🌍
Purpose:
Linear accelerator-based single-isocenter Stereotactic Radiosurgery (SRS) is widely used for treating brain metastases due to its efficiency, reduced treatment time, and lower imaging doses. However, its dosimetric accuracy is compromised by mechanical and rotational uncertainties, particularly for targets further away from the isocenter. According to the AAPM TG142 recommendations, a collimator tolerance angle of ±1.00° is suggested regardless of the delivery technique. This study evaluates the dosimetric variations induced by collimator rotational uncertainties on target dose coverage for single-isocenter multi-lesions brain metastases SRS.
Methods:
Spherical GTVs with 1cm diameter were created at locations 0cm (GTV1), 6cm (GTV2), 8cm (GTV3), and 10cm (GTV4) from isocenter in the brain region in an anonymized CT dataset. PTVs were expanded from GTVs with 2mm margin and named correspondingly. The Isocenter was fixed at PTV1 for all plans. Treatment plans were created in Varian Eclipse 15.6 using HyperArc feature and were planned for VarianEdge equipped with HDMLC, with constant collimator and couch angles and arc numbers for all plans. The goal is to have PTV D95% > 22 Gy. Various differences for collimator angle up to ±1.0° were introduced, and the doses to PTV were recalculated and compared to the original plan.
Results:
The target coverage decreased with the distance increase from isocenter as well as the collimator angle difference. For a 1.0° collimator angle change, minimum dose was reduced by 7.5%, 10.1%, and 13.1% for PTV2, PTV3, and PTV4, respectively. D95 decreased by 2.8%, 4.9%, and 6.5%, while V100 was reduced by 5.2%, 9.9%, and 11% for PTV2, PTV3 and PTV4 respectively.
Conclusion:
Collimator rotational uncertainty significantly reduces dose to distant targets in single-isocenter SRS. These findings highlight the need for tight collimator tolerance in linear accelerators for optimal treatment accuracy in brain SRS metastases cases.