Tissue density heterogeneities influence and simple density correction method for 3D dosimetry in the abdominal region

Objectives The impact of tissue density heterogeneities (TDH) is often neglected in the abdominal region in order to speed up absorbed dose (AD) calculation. We propose a simple density correction and quantify its impact in 3 clinical cases: 1) a NHL treatment with ¹³¹I-Tositumomab; 2) a NET treatment with ¹⁷⁷Lu-Peptide; 3) a HCC treatment with ⁹⁰Y-microspheres.

Methods AD calculations were performed using i) a Monte Carlo (MC) approach accounting for TDH as a reference (3D-RD), ii) the Voxel S-Values (VSV) approach in soft tissues (VoxelDose). A density (ρ) map was generated from the CT data. AD computed with VSV was corrected using the voxel density (VSVcorr). MC and VSV were compared using the relative difference between the mean ADs (ΔAD). At the voxel level, ρ-binned ΔAD was plotted against ρ, and fitted with a linear regression (LR) for AD≥1Gy and ρ≥0.90g.cm^-3.

Results At the tissue level, the results showed a good agreement between MC and VSV. ΔAD<3.5%, except for the tumor of case 1 (5.9%) and the renal cortex of case 2 (5.6%). At the voxel level, ΔAD showed a LR with ρ for case 1 (y=-0.56x+0.62, R²=0.93), case 2 (y=-0.91x+0.96, R²=0.99) and case 3 (y=-0.69x+0.72, R²=0.91). VSVcorr improved the agreement with MC at the tissue level (ΔAD<1.1%), but lesser for the tumor of case 1 (3.1%). At the voxel level, the VSVcorr lowers the LR for case 2 (y=0.04x-0.03, R²=0.13) and 3 (y=0.30x+0.31, R²=0.58) but lesser for case 1 (y=0.41x-0.38, R²=0.88).

Conclusions This study shows a small influence of TDH in the abdominal region for 3 representative clinical cases. A simple density correction is presented which seems efficient for preponderant β^- emissions (⁹⁰Y and ¹⁷⁷Lu)