LMAP-14 EFFECT OF TRANSLATIONAL VARIATION ON DOSE CHANGE ADOPTED FOR CONE-BEAM CT GUIDED FRAME BASED STEREOTACTIC RADIOSURGERY OF BRAIN TUMORS

INTRODUCTION: Gamma knife (GK) is a highly precise form of stereotactic radiosurgery (SRS) that delivers high doses of radiation to small, well-defined targets in a variety of tumor and vascular abnormalities. In some frame-based cases, a small translation can occur between the planning MRI and the cone-beam CT acquired at the time of treatment. Measuring the effect of patient translation on the dose distribution may be useful for informing clinical decisions. Materials/ METHODS: This retrospective study of 113 planning target volumes (PTV) from 33 patients investigates the effect of translational variation on dose metrics recalculated based on cone-beam CT guided for frame based GK radiosurgery. The approved dose adjustments after the original treatment plan in terms of change in mean dose and dose coverage were compared with corresponding translation variation and the volume of PTV. The magnitude of translational variation was compared with the change in dose distribution. RESULTS: We observed about 53% of cases have at least 0.1 Gy to max 1.2 Gy change in mean dose approved for the translation variation of patients while about 20% of cases have dose coverage fraction change in the range of 2-10%. Furthermore, the change in mean dose and coverage fraction is not linear with the translation variation, but a significant number of dose changes is observed when the variation magnitude is >0.5 mm. PTV volume varied between 0.04 to 22 cm(3) and the dose change has been most often adopted when the volume is <2.5 cm(3) and the change increases for the decreasing PTV volume. CONCLUSION: This work provides new insights identifying the suitable tolerance level for translational variations of patients and the resulting impact on the prescribed dose for frame-based SRS.