Systematic investigation on the validity of partition model dosimetry for ⁹⁰Y radioembolization using Monte Carlo simulation

We aimed to investigate the validity of the partition model (PM) in estimating the absorbed doses to liver tumour ($D_T$), normal liver tissue ($D_{NL}$) and lungs ($D_L$), when cross-fire irradiations between these compartments are being considered. MIRD-5 phantom incorporated with various treatment parameters, i.e. tumour involvement (TI), tumour-to-normal liver uptake ratio (T/N) and lung shunting (LS), were simulated using the Geant4 Monte Carlo (MC) toolkit. 10⁸ track histories were generated for each combination of the three parameters to obtain the absorbed dose per activity uptake in each compartment ($D_T^{A_T}$, $D_{NL}^{A_{NL}}$, and $D_L^{A_L}$). The administered activities, A were estimated using PM, so as to achieve either limiting doses to normal liver, $D_{NL}^{lim}$ or lungs, $D_L^{lim}$ (70 or 30 Gy, respectively). Using these administered activities, the activity uptake in each compartment ($A_T$, $A_{NL}$, and $A_L$) was estimated and multiplied with the absorbed dose per activity uptake attained using the MC simulations, to obtain the actual dose received by each compartment. PM overestimated $D_L$ by 11.7% in all cases, due to the escaped particles from the lungs. $D_T$ and $D_{NL}$ by MC were largely affected by T/N, which were not considered by PM due to cross-fire exclusion at the tumour-normal liver boundary. These have resulted in the overestimation of $D_T$ by up to 8% and underestimation of $D_{NL}$ by as high as  −78%, by PM. When $D_{NL}^{lim}$ was estimated via PM, the MC simulations showed significantly higher $D_{NL}$ for cases with higher T/N, and LS  ⩽  10%. All $D_L$ and $D_T$ by MC were overestimated by PM, thus $D_L^{lim}$ were never exceeded. PM leads to inaccurate dose estimations due to the exclusion of cross-fire irradiation, i.e. between the tumour and normal liver tissue. Caution should be taken for cases with higher TI and T/N, and lower LS, as they contribute to major underestimation of $D_{NL}$. For $D_L$, a different correction factor for dose calculation may be used for improved accuracy. (paper)