In the last decades, selective internal radiation therapy (SIRT) has become a real alternative in the treatment of unresectable hepatic cancers. In practice, the activity prescription is limited by the irradiation of organs at risk (OAR), such as the lungs and nontumoral liver (NTL). Its clinical implementation is therefore highly dependent on dosimetry. In that context, a 3-dimensional personalized dosimetry technique—personalized Monte Carlo dosimetry (PMCD)—based on patient-specific data and Monte Carlo calculations was developed and evaluated retrospectively on clinical data.
Methods. The PMCD method was evaluated with data from technetium human albumin macroaggregates (99mTc-MAA) evaluations of 10 patients treated for hepatic metastases. Region-of-interest outlines were drawn on CT images to create patient-specific voxel phantoms using the OEDIPE software. Normalized 3-dimensional matrices of cumulated activity were generated from 99mTc-SPECT data. Absorbed doses at the voxel scale were then obtained with the MCNPX Monte Carlo code. The maximum-injectable activity (MIA) for tolerance criteria based on either OAR mean absorbed doses (Dmean) or OAR dose-volume histograms (DVHs) was determined using OEDIPE. Those MIAs were compared with the one recommended by the partition model (PM) with Dmean tolerance criteria. Finally, OEDIPE was used to evaluate the absorbed doses delivered if those activities were injected to the patient and to generate the corresponding isodose curves and DVHs.
Results. The MIA recommended using Dmean tolerance criteria is, in average, 27% higher with the PMCD method than with the PM. If tolerance criteria based on DVHs are used along with the PMCD, an increase of at least 40% of the MIA is conceivable, compared with the PM. For MIAs calculated with the PMCD, Dmean delivered to tumoral liver (TL) ranged from 19.5 to 118 Gy for Dmean tolerance criteria whereas they ranged from 26.6 to 918 Gy with DVH tolerance criteria. Thus, using the PMCD method, which accounts for fixation heterogeneities, higher doses can be delivered to TL. Finally, absorbed doses to the lungs are not the limiting criterion for activity prescription. However, Dmean to the lungs can reach 15.0 Gy.
Conclusion. Besides its feasibility and applicability in clinical routine, the interest for treatment optimization of a personalized Monte Carlo dosimetry in the context of SIRT was confirmed in this study.