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SU-F-T-626: Intracranial SRS Re-Treatment Without Acquisition of New CT Images.

PURPOSE
Linear accelerator based stereotactic radiosurgery (SRS) for multiple intracranial lesions with frequent surveillance is becoming a popular treatment option. This strategy leads to retreatment with SRS as new lesions arise. Currently, each course of treatment uses magnetic resonance (MR) and computed tomography (CT) images for treatment planning. We propose that new MR images, with course 1 CT images, may be used for future treatment plans with negligible loss of dosimetric accuracy.


METHODS
Ten patients that received multiple courses of SRS were retrospectively reviewed. The treatment plans and contours from non-initial courses were copied to the initial CTs and recalculated. Doses metrics for the plans calculated on the initial CTs and later CTs were compared. All CT scans were acquired on a Philips CT scanner with a 600 mm field of view and 1 mm slice thickness (Philips Healthcare, Andover, MA). All targets were planned to 20 Gy and calculated in Eclipse V. 13.6 (Varian, Palo Alto, CA) using analytic anisotropic algorithm with 1 mm calculation grid.


RESULTS
Sixteen lesions were evaluated. The mean time between courses was 250 +/- 215 days (range 103-979). The mean target volume was 2.0 +/- 2.9 cc (range 0.1-10.1). The average difference in mean target dose between the two calculations was 0.2 +/- 0.3 Gy (range 0.0 - 1.0). The mean conformity index (CI) was 1.28 +/- 0.14 (range 1.07 - 1.82). The average difference in CI was 0.03 +/- 0.16 (range 0.00 - 0.44). Targets volumes < 0.5 cc showed the largest changes in both metrics.


CONCLUSION
Continued treatment based on initial CT images is feasible. Dose calculation on the initial CT for future treatments provides reasonable dosimetric accuracy. Changes in dose metrics are largest for small volumes, and are likely dominated by partial volume effects in target definition.