In this confirmatory study, we tested if a calculation that included the non-uniformity of dose deposition through a voxel-based dosimetric variable Ψ was able to improve the dose–response agreement with respect to the mean absorbed dose D. We performed dosimetry with 99mTc-MAA SPECT/CT and 90Y-PET/CT in 86 patients treated 8 instead of 4 days after the reference date with 2.8 times more 90Y glass microspheres/GBq than in our previous study. The lesion-by-lesion response was assessed with the mRECIST method and with an experimental densitometric criterion. A total of 106 lesions were studied. Considering Ψ as a prognostic response marker, having no Ψ provided a significantly higher AUC than D. The correlation, t-test, and AUC values were statistically significant only with the densitometric method and only with post-therapy dosimetry. In comparison with our previous study, the dose–response correlation and AUC values were poorer (maximum r = 0.43, R2 = 0.14, maximal AUC = 0.71), and the efficacy at a high dose did not reach 100%. The expected advantages of voxel dosimetry were nullified by the correlation between any Ψ and D due to the limited image spatial resolution. The lower AUC and efficacy may be explained by the mega-clustering effect triggered by the higher number of microspheres/GBq injected on day 8.

A small increase in mild late toxicity, but not statistically significant increase in severe late toxicity in the hypo-IGRT group when compared with conventional non-IGRT group was observed. Our study confirmed that IGRT allows for safe moderate hypofractionation, offering a shorter overall treatment time, a good impact in terms of RFS and providing potentially more economic health care.

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