Poster — Thur Eve — 34: Estimation of true cumulative dose to the spinal canal in retreated spine SBRT

Abstract

Decisions on repeat spine SBRT are based on maximum dose constraints to the spinal cord region. The total dose that is assumed to be tolerable is typically the cumulative maximum biological effective dose (BED) calculated irrespective of location of this dose in the two plans. We explore the true spinal cord region BED taking into account BED position in the two plans.Treatment plans of 16 repeat spine SBRT patients were retrospectively evaluated. TBEDmax and TBED0.35cc (sum from two plans) of spinal cord planning organ‐at‐risk volume (SCPRV) and thecal sac (TS) were calculated using α/β of 2Gy. To assess the ‘true’ dose, the first CT was deformably‐ or rigidly‐registered to the second CT. Image registration integrity was visually tested using the modified volumes from the first plan on the recent CT. Using the acquired registration parameters, the voxel‐based BED(VBEDmax, VBED0.35cc) from the two plans were added together and compared to the TBEDmax and TBED0.35cc.The median(range) SCPRV TBEDmax was 98.7(72.5–135.7)Gy and VBEDmax was 76.6(57.3–129.6)Gy (p=0.01). The median(range) TS TBEDmax was 123.2(108.7–261.6)Gy compared to VBEDmax, which was 93.2(59.6–161.2)Gy (p=0.002). The median(range) SCPRV TBED0.35cc was 72.5(53.1–112.1)Gy and VBED0.35cc is 62.2(37.9–93.3)Gy (p=0.01). The median(range) TS TBED0.35cc was 80.3(64.7–120.6)Gy compared to VBED0.35cc was 68.2(45.2–92.1)Gy (p=0.01).When taking location into account, true additive BED to the SCPRV/TS is lower than the current method of estimating BED for repeat spinal SBRT. By incorporating the dose location information, increase in dose to target volume adjacent to SC may be achievable with accurate and precise image registration.