Voxel-dosimetry as useful tool for optimized administered activity in radioembolization of liver tumors with 90Y-microspheres

Objectives To show the potentiality of voxel dosimetry in optimizing the treatment planning of liver radioembolisation.

Methods Eleven patients with unresectable liver metastasis were treated with 90Y-resin-microspheres.Therapy was simulated by 99mTc-MAA distribution.The activity administered was based on patient-specific dosimetry using standard-method(MIRD16) on SPECT image, prescribing 40Gy as average dose limit(Dlim) to normal-liver.Retrospectively, voxel-dosimetry was performed.For voxel dose calculation, a software was developed and S-voxel factors for 90Y calculated by MonteCarlo simulation. Dose and Biological-Effective-Dose(BED) distribution were generated for normal-liver. Equivalent-Uniform-Biologically-Effective-Dose(EUBED)and Equivalent-Uniform-Dose (EUD) were assessed.

Results The average dose from standard-method(Dlim=40Gy) differed slightly(within 10%) from the average dose among voxels (Dvoxdos).Conversely, Dlim vs. EUD differences were consistent (median 20 Gy;range 12-23). The EUD/Dvoxdos ratio was indicative of heterogeneous dose distribution (median 0.5;range 0.4-0.7).Consequently, a 40Gy prescription limit to normal-liver set as EUD instead of average dose would have increased the administrable activities.The median EUBED value was 24Gy(range18-39),lower than BED=64Gy from standard-method.These results were consistent with the absence of liver toxicity observed.Expressing the Normal-Tissue-Complication-Probability curves from external radiotherapy vs. BED,the liver injury probability would be 10% for BED=64Gy,negligible for the EUBED values derived.

Conclusions Retrospective dose distribution analysis in treated patient offered useful information for future therapy optimization. The voxel-dosimetry gave evidence of dose inhomogeneities in normal-liver. Radiobiological estimations would suggest to replace the average dose by the EUD