Room: AAPM ePoster Library
Purpose: To develop a VMAT based treatment technique to deliver radiotherapy to the entire spinal column.
Methods: At our institution when radiotherapy to the entire spinal column is indicated, extended source surface distance (SSD) parallel opposed pair field geometry has been used. The extended SSD geometry has a field length limitation and is not appropriate for tall patients. To create a dose distribution sufficiently long to encompass the entire spinal column, two full arc VMAT beams were associated with an isocentre superiorly. Similarly, two full arc VMAT beams were associated with an inferior isocentre. Multiple optimization regions of interest were created in the arc intersection region to control the junction dose to a clinically acceptable level. Validation of the junction dose was performed using the ArcCheck phantom. To prevent irradiation of device electronics, the inferior aspect of the two inferior arcs was modified after optimization and calculation. The composite dose distribution of the two isocentre, four full VMAT beam plan was measured using ArcCheck. Other delivery QA methods were explored.
Results: The measured and planned composite distributions were compared using SNC Patient software. Gamma evaluation according to in-house patient specific delivery quality assurance (PSQA) evaluation criteria (2% dose difference, 2 mm distance to agreement, 5% threshold, global normalization) resulted in a pass rate of 99.1% (1177/1188 diodes). A pass rate of 95% using these criteria is required to accept a VMAT plan at our institution.
Conclusion: This work demonstrates a method to deliver radiotherapy to extensive disease spanning the spinal column. The approach uses two isocentres and two full VMAT arcs associated with each isocentre. A PSQA approach has been developed and tested to ensure an acceptable dose distribution in the junction region. Dosimetric results show promise for future clinical implementation for patients benefiting from radiotherapy to entire spine.