2690 - The Impact of Image Guidance on Hippocampal Dose in Hippocampal Avoidance Whole Brain Radiotherapy - A Secondary Analysis of a Randomized Clinical Trial

Purpose/Objective(s):

Hippocampal avoidance whole brain radiotherapy (HA-WBRT) has been shown to reduce neurocognitive decline in patients with multiple brain metastases. In this secondary analysis of our previous clinical trial, we evaluated the impact of couch correction using image-guided radiotherapy (IGRT) with cone-beam CT (CBCT) on hippocampal dosimetry. We aimed to demonstrate that IGRT during HA-WBRT reduces hippocampal dose deviations from the treatment plan.

Materials/Methods:

CBCT data with six-dimensional (6D) correction from patients treated with HA-WBRT in a prior randomized clinical trial were analyzed. Synthetic CT images were generated by rigidly registering CBCT with planning CT, onto which contours and treatment structures were transferred for dosimetric recalculations. Hippocampal dosimetry parameters, including mean dose, maximum dose, and the percentage of volume receiving more than 9 Gy (V9) and 12.5 Gy (V12.5), were documented. Dosimetry outcomes were compared across three correction strategies: 6D correction (original planning CT outcome), 3D correction (X-Y-Z shifts only, without rotation correction), and no correction. Statistical comparisons were conducted using independent t-tests for two-group comparisons and one-way ANOVA for multiple comparisons.

Results:

Between March 2015 and December 2018, 70 patients were enrolled in the clinical trial, with 35 receiving bilateral HA-WBRT (30 Gy in 10 fractions). Of these, 20 underwent daily CBCT with available 6D correction data for this secondary analysis, contributing 200 CBCT datasets for synthetic CT reconstruction and dosimetric evaluation. All 20 patients met hippocampal dose constraints per protocol. The hippocampal maximum dose for the left and right hippocampus was 14.08 Gy and 13.99 Gy with 6D correction, 15.17 Gy and 15.00 Gy with 3D correction, and 19.01 Gy and 19.17 Gy without correction (P < 0.001 for all comparisons). Mean hippocampal dose did not differ between 6D and 3D corrections (left: 10.18Gy vs. 10.23Gy, P = 0.842, right: 10.20Gy vs. 10.23Gy, P = 0.923) but was significantly higher without correction (left 10.79Gy, right 10.91Gy, P < 0.001 for both sides compared to 6D or 3D corrections). Similarly, hippocampal V12.5 increased with no correction compared to either 3D or 6D correction (left: 14.62%, 5.56%, and 3.4%; right: 17.13%, 5.33%, and 3.69%; P < 0.001 for both sides), though differences between 6D and 3D corrections were minor (left: P = 0.048, right: P = 0.153). No significant differences in hippocampal V9 were observed among three groups.

Conclusion:

Patients treated with HA-WBRT without IGRT couch correction had significantly higher hippocampal maximum dose, mean dose, and V12.5. While 3D correction mitigated these effects, the hippocampal maximum dose remained higher than with 6D correction. Given these findings, at least 3D correction should be implemented in daily IGRT during HA-WBRT to minimize hippocampal dose deviations and ensure treatment quality.