RTHP-26. DOSIMETRIC FEASIBILITY STUDY USING HIPPOCAMPAL AVOIDANCE WITH SIMULTANEOUS INTEGRATED BOOST WHOLE BRAIN RADIOTHERAPY (HA-SIB-WBRT)

Abstract

Abstract BACKGROUND Recently Hippocampal Avoidance (HA-) WBRT has become a recommended treatment option in patients with multiple (≥ 5) brain metastases and good prognosis. We wanted to investigate the dosimetric feasibility of dose painting technique combining HA-WBRT with a simultaneous integrated boost (SIB) to tumours. METHOD 5 patients who had a CT simulation fused with brain MRI with fine cuts, were selected for this study. Volumes were contoured on T1w contrast images. Whole brain prescription dose was 30Gy in 12 fractions. A PTV margin of 2mm was applied to lesions, except when these were ≤5mm from organs at risks (OARs). A simultaneous integrated boost (SIB) of 48Gy and 40.2Gy was prescribed to these volumes respectively. Hippocampal constraints followed RTOG 0933 protocol. For lesions ≤5mm from OARs, the acceptable D0.03cc≤42Gy was allowed. All plans were planned on EclipseTM v.13.6 TPS using 6MV photons, VMAT technique with 3 coplanar and 1 non-coplanar arcs for Varian TrueBeam machine. RESULTS Plans had between 6–24 lesions with GTV and PTV of 3.02–11.32cc and 7.05–31.74cc respectively. 3 of the plans had lesions within/adjacent to brainstem or hippocampus. The achieved PTV_40.2Gy D95% 37.42–39.05Gy with Conformity Index (CI)(95%) 0.63–1.06, PTV_48Gy D95% 44.64–47.04Gy with CI(95%) 0.75–0.97 and GTV_48Gy D95% 47.44–50.16Gy. Whole brain Dmean 31.87–33.15Gy with a Homogeneity Index (D2%-D98%/Dmean) 0.18–0.58. Hippocampal D100% 8.69–10.1Gy, D0.03cc 16.5–40.43Gy and Dmean 12.66–24.68Gy. SUMMARY: There was a steep learning curve when adopting this technique and multiple plan iterations were made to achieve target constraints. To meet acceptable OAR constraints, SIB coverage was compromised. Lesions ≤5mm from hippocampus resulted in higher Hippocampal average Dmean 22.8Gy vs. 12.8Gy. The significance of this value should be tested in clinical trials. Overall, HA-SIB-WBRT seems feasible even with ≥ 5 brain metastases and could result in better brain metastases control then HA-WBRT alone.